Efficient Use of Air-source Heat Pumps

[Dave EV]

That's contrary to conventional wisdom when it comes to heat pumps. If your system is right sized, then on cold mornings, it's not going to have a lot of extra capacity to quickly raise the temperature from 58F back to 64F. More of an issue when the outdoor design temperature is 20F or 30F than when it's 40F.

Cheers, Wayne

This is a good point - but depending on the outdoor temperature and the design temp, you probably do have enough capacity most of the time to catch back up, even with right sized equipment. And it's true that your typical furnace is massively oversized - my house came with a 80-90,000 BTU furnace - but a 36,000 BTU heat pump is more than enough to keep the house at reasonable temps. It actually has more of an issue with cooling, fighting 25F temp differentials with humidity to pull out compared to

There's a lot of variables here - outdoor temp, forecasted outdoor temps, tolerance to temperature below the set-point, etc.

Not to mention how the system is set up - what are the backup-heat settings - do you have heat strips which might kick on and kill any energy savings, etc.

From a pure physics point of view, it's always going to be more efficient to use a set back if you don't go back to heat strips - assuming that the temp differential when you turn the heat back on is smaller and outdoor temps are not any cooler and as the indoor/outdoor temp differential drops, so does the rate of heat loss.

With the latest generation variable speed scroll-type heat pumps which tend to have more capacity at colder temps, and thanks to their variable speed lend themselves to some degree over over-sizing compared to 1-stage systems.

 

[wwhitney]

From a pure physics point of view, it's always going to be more efficient to use a set back if you don't go back to heat strips

That's probably true in practice, but from a pure physics point of view all you can say is that the number of BTUs lost by the house, and hence that the heat pump has to move back into the house, will be less with the night-time setback during heating season. Depending on the heat pump's efficiency curve, and the temperature delta when the setpoint goes back up in the morning, it might not save any electricity, or even take more electricity, to move that lower number of BTUs. Since dawn is often the coldest time of day, as I understand it.

Also, if you want to specify that at the outdoor design temperature the heat pump can raise the house from say 58F to 64F within say 1 hour, now you have a capacity constraint that depends on the thermal mass of the house. As opposed to the typical unit sizing algorithm of just looking at need to maintain equilibrium at the outdoor design temperature. Which make sizing the equipment more complicated.

Cheers, Wayne

 

[RKCRLR]

That's a function of the thermostat, not the heat pump. The thermostat decides when to kick in the auxiliary heat. If your thermostat is kicking on the auxiliary when you don't want it to, and there's no way to reconfigure it, it's time for a new thermostat.

I also have a dual fuel system with a natural gas furnace as the backup. I have the changeover temperature set to 35°F (last year, if I had this same system, I would have set the changeover temperature to around 45°F but PG&E has raised the gas prices so high as of the beginning of this month that the economic crossover point is actually below freezing...at least until they raise the electric rates again). The thermostat (Emerson 1F95-1277 with optional outdoor sensor F145-1378) kept kicking into auxiliary heat mode after 25-30 minutes, either because it didn't like how long it was taking to meet the set point or because it didn't like the rate of temperature rise. In any case, I configured the thermostat's Auxiliary Off setting to 36°F, meaning never use auxiliary heat if the outdoor temperature is at or above 36°F, and now it uses the heat pump exclusively if it's >= 36°F outside (except when the heat pump goes into defrost mode and activates the furnace). If I really want to use auxiliary heat above 35°F outside, I can always put the thermostat into emergency heat mode and force it to do this manually.

I agree that the thermostat controls the auxiliary heat but many systems don't have the option to adjust the tolerances and if you have a proprietary communicating system you loose too much functionality by going with a non-OEM thermostat. If hh9oolu has 5 zones it is likely he has a communicating system and possibly a variable speed outside unit. But my point to Ahh9oolu was that he may need to investigate how his system works to see if he is inadvertently kicking in the auxiliary heat.

My Carrier Infinity system has what it calls "Smart" setback. Basically, this means that it starts to ramp up to the new temperature 90 minutes before the time of the new temperature. If the heat pump starts falling behind during the ramp up it will call for auxiliary heat. And the 90 minute ramp isn't adjustable. I found out by trial and error the heat pump could only recover from a 4 degree setback during the colder days of the year before it made a call for auxiliary heat. And once a call for auxiliary heat is made it continues using auxiliary heat until the cycle is complete. So I had to take this into account if wanted to use a setback temperature above my furnace lockout temperature. And if my wife turned up the room temperature by more than 2 degrees it would bump it into auxiliary heating mode. This wasn't adjustable either.

 

[Dave EV]

That's probably true in practice, but from a pure physics point of view all you can say is that the number of BTUs lost by the house, and hence that the heat pump has to move back into the house, will be less with the night-time setback during heating season. Depending on the heat pump's efficiency curve, and the temperature delta when the setpoint goes back up in the morning, it might not save any electricity, or even take more electricity, to move that lower number of BTUs. Since dawn is often the coldest time of day, as I understand it.

Yeah - there's definitely a ton of variables here, and what's sad is that it probably wouldn't take a lot of compute power to figure out what is the optimal way to run the system for maximum efficiency. Knowing the equipment's efficiency curves and running some heating / cooling tests would be enough for the equipment to learn how much house it's conditioning and combined with some basic weather data it would know what's the most efficient way to hit it's temperature set points.

Also, if you want to specify that at the outdoor design temperature the heat pump can raise the house from say 58F to 64F within say 1 hour, now you have a capacity constraint that depends on the thermal mass of the house. As opposed to the typical unit sizing algorithm of just looking at need to maintain equilibrium at the outdoor design temperature. Which make sizing the equipment more complicated.

And that is the reason why most HVAC installers don't bother with proper load calculations and simply to apply a rule of thumb that tends to oversize equipment by 50-100%, despite the fact that those systems provide less comfort than the right sized system. That said, I'm hopeful that new variable speed systems get you the best of both worlds - ability to dial down the power during periods of low heating/cooling requirements, but still able to really move the BTUs if necessary.

I agree that the thermostat controls the auxiliary heat but many systems don't have the option to adjust the tolerances and if you have a proprietary communicating system you loose too much functionality by going with a non-OEM thermostat. If hh9oolu has 5 zones it is likely he has a communicating system and possibly a variable speed outside unit. But my point to Ahh9oolu was that he may need to investigate how his system works to see if he is inadvertently kicking in the auxiliary heat.

My Carrier Infinity system has what it calls "Smart" setback. Basically, this means that it starts to ramp up to the new temperature 90 minutes before the time of the new temperature. If the heat pump starts falling behind during the ramp up it will call for auxiliary heat. And the 90 minute ramp isn't adjustable. I found out by trial and error the heat pump could only recover from a 4 degree setback during the colder days of the year before it made a call for auxiliary heat. And once a call for auxiliary heat is made it continues using auxiliary heat until the cycle is complete. So I had to take this into account if wanted to use a setback temperature above my furnace lockout temperature. And if my wife turned up the room temperature by more than 2 degrees it would bump it into auxiliary heating mode. This wasn't adjustable either.

It's really sad what passes for "Smart" these days. If my heat pump had heat-strip aux heating, I would lock that out for sure to eliminate most of the issues you describe above. One roadblock in the HVAC arena are the proliferation of proprietary communications protocols. A universal standard should be established so that a standard thermostat can have fine-grained control over the equipment - this would help avoid vendor lock-in. And this standard needs to be a bit more flexible than the typical single or dual stage compressor "communication" relay-switch system.

There's a lot of low-hanging fruit here for the optimization of HVAC in your typical house / building...

 

[Ahh9oolu]

But my point to Ahh9oolu was that he may need to investigate how his system works to see if he is inadvertently kicking in the auxiliary heat.

An excellent question. I do not remember the installer talking about auxiliary heat, and the best I can figure, it does not have it, and none of the Mitsubishi thermostats mention auxiliary heat. My outdoor unit is a MXZ-8C48NA2 and I have a 1.5 ton ducted system (MSVZKP18NA) and 4x MMLZKP09NAU1.

Power draw with just the ducted (central) unit enabled for the last 24 hours is graphed below. I tend to leave it off until the outside temps rise into the 50s, figuring it is more efficient to move heat inside with the lower differential. If I turn on 4 of the 5 zones power draw peaks closer to 3,500 watts. Again, I am in the Sacramento region, so winter is pretty mild by many standards.

 

[STS-134]

That's contrary to conventional wisdom when it comes to heat pumps. If your system is right sized, then on cold mornings, it's not going to have a lot of extra capacity to quickly raise the temperature from 58F back to 64F. More of an issue when the outdoor design temperature is 20F or 30F than when it's 40F.

Cheers, Wayne

So yesterday morning, the inside temperature dropped to 59°F and the outside temperature was 37-38°F and the heat pump took about 80 minutes to bring the temperature up to the set point of 64°F. Today, the inside temperature was 61°F and the outside temperature was 48°F and the heat pump took only about 35 minutes to bring the temperature up. Per the performance charts I have on my model, the unit has a capacity of about 18-20% more when it's 48°F out compared to 37-38°F.

I agree that the thermostat controls the auxiliary heat but many systems don't have the option to adjust the tolerances and if you have a proprietary communicating system you loose too much functionality by going with a non-OEM thermostat. If hh9oolu has 5 zones it is likely he has a communicating system and possibly a variable speed outside unit. But my point to Ahh9oolu was that he may need to investigate how his system works to see if he is inadvertently kicking in the auxiliary heat.

My Carrier Infinity system has what it calls "Smart" setback. Basically, this means that it starts to ramp up to the new temperature 90 minutes before the time of the new temperature. If the heat pump starts falling behind during the ramp up it will call for auxiliary heat. And the 90 minute ramp isn't adjustable. I found out by trial and error the heat pump could only recover from a 4 degree setback during the colder days of the year before it made a call for auxiliary heat. And once a call for auxiliary heat is made it continues using auxiliary heat until the cycle is complete. So I had to take this into account if wanted to use a setback temperature above my furnace lockout temperature. And if my wife turned up the room temperature by more than 2 degrees it would bump it into auxiliary heating mode. This wasn't adjustable either.

That's pretty bad, and what it sounds like you need is a new thermostat that knows how to "speak" the Carrier Infinity protocol and doesn't do idiotic calls for auxiliary heat when it's not required. But in the long run, there needs to be a standard interface for communicating with variable speed systems. Instead of using a mess of wires that you screw in individually, they should use a 16 pin connector like this and have all of the signals put in standard places on it. The standard could allow up to 100 different compressor speeds by using just 7 Y wires, Y0-Y6, by signaling the desired compressor operating speed in binary, i.e. 1 = 1% is binary 0000001 so power Y0 and leave Y1-Y6 floating. 50 = 50% = binary 0110010 so power Y5, Y4, and Y1. 100 = 100% = binary 1100100 so power Y7, Y6, and Y2. You could use an 8th pin as an error check/parity bit to make sure that all of the wires are connected and the thermostat/HVAC system should throw an error if it gets a nonsensical value. Since all of the signals run through the air handler, the air handler could intercept the signals coming from the thermostat and set its fan accordingly. Or you could have separate signals for the fan speed (for dehumidification purposes) encoded in the same manner.

And that is the reason why most HVAC installers don't bother with proper load calculations and simply to apply a rule of thumb that tends to oversize equipment by 50-100%, despite the fact that those systems provide less comfort than the right sized system.

Part of the problem is that if you size a system appropriately for winter in Tahoe, or Minnesota, or any extremely cold place, it's going to be grotesquely oversized for cooling especially in those climates. If you size a system appropriately for cooling in Phoenix, it's going to be grotesquely oversized for heating.

That said, I'm hopeful that new variable speed systems get you the best of both worlds - ability to dial down the power during periods of low heating/cooling requirements, but still able to really move the BTUs if necessary.

I don't really have a desire to get a variable speed system or even a multi stage system. These systems are supposed to not cycle which supposedly increases comfort but here in the SF Bay Area, the outdoor temperature often crosses through my indoor set point. Actually if you look at the ASHRAE design temperatures in my area, the 1% temperature for cooling is only 31°C and if you actually designed the system for that, you'd be uncomfortable during heat waves (which is why you turn on the AC to begin with around here). There are often days in July and August where I don't need to use the AC at all. And even during the worst heat waves, the temperature usually goes below my indoor set point of 78°F after around midnight. So after around 9pm or so, even a variable speed system is going to start cycling because a variable speed system can only go down to around 20-30% capacity, and then you're back to where you would have been with a single stage system, only with added complexity and increased maintenance costs if anything goes wrong.

 

[RKCRLR]

An excellent question. I do not remember the installer talking about auxiliary heat, and the best I can figure, it does not have it, and none of the Mitsubishi thermostats mention auxiliary heat. My outdoor unit is a MXZ-8C48NA2 and I have a 1.5 ton ducted system (MSVZKP18NA) and 4x MMLZKP09NAU1.

Power draw with just the ducted (central) unit enabled for the last 24 hours is graphed below. I tend to leave it off until the outside temps rise into the 50s, figuring it is more efficient to move heat inside with the lower differential. If I turn on 4 of the 5 zones power draw peaks closer to 3,500 watts. Again, I am in the Sacramento region, so winter is pretty mild by many standards.View attachment 875758

I didn't realize you had a mini-split system, I'm not familiar with their details and how (or if) they have auxiliary heat. But all outdoor units need to go through a defrost cycle. During this cycle the heat pump runs in AC mode and ducted systems usually use heat strips to defrost quicker and lessen the impact of the cold air blowing out the ducts. They also use these heat strips as auxiliary heat when the heat pump can't keep up. You would need to dig find out from your installer if the system has auxiliary heat and how it operates.

 

[STS-134]

They also use these heat strips as auxiliary heat when the heat pump can't keep up. You would need to dig find out from your installer if the system has auxiliary heat and how it operates.

That's not necessarily true. You can use a gas furnace as your auxiliary heat in which case the system will turn on the gas when it's defrosting. The difference with a dual fuel system is that the coils are downstream of the heat exchanger instead of upstream of the heat strips, and you'd better not energize the heat pump in heating mode simultaneously with the auxiliary heat or you can damage your heat pump (although modern heat pumps have thermal cutoff switches that would detect this and shut off the power before any damage occurs). In any case, a dual fuel system is either in heat pump mode OR it's in auxiliary mode but never both.

 

[RKCRLR]

That's pretty bad, and what it sounds like you need is a new thermostat that knows how to "speak" the Carrier Infinity protocol and doesn't do idiotic calls for auxiliary heat when it's not required. But in the long run, there needs to be a standard interface for communicating with variable speed systems. Instead of using a mess of wires that you screw in individually, they should use a 16 pin connector like this and have all of the signals put in standard places on it. The standard could allow up to 100 different compressor speeds by using just 7 Y wires, Y0-Y6, by signaling the desired compressor operating speed in binary, i.e. 1 = 1% is binary 0000001 so power Y0 and leave Y1-Y6 floating. 50 = 50% = binary 0110010 so power Y5, Y4, and Y1. 100 = 100% = binary 1100100 so power Y7, Y6, and Y2. You could use an 8th pin as an error check/parity bit to make sure that all of the wires are connected and the thermostat/HVAC system should throw an error if it gets a nonsensical value. Since all of the signals run through the air handler, the air handler could intercept the signals coming from the thermostat and set its fan accordingly. Or you could have separate signals for the fan speed (for dehumidification purposes) encoded in the same manner.

When I was looking at options to replace my failed outside unit I considered using a different brand and looked at non-Carrier thermostats. They would require replacing my zone controller with their zone controller, replacing all the other zone thermostats with their zone thermostats, and configuring my variable speed furnace blower as "dumb" blower. None of them could control a variable speed outside unit. If I went with a variable speed outside unit it would need to be configured as a 2 speed unit which defeats the advantage of a variable speed unit. And, even then, the outside unit wouldn't be able to throttle down enough to handle the smallest zone if it was the only one calling.

I understand the logic of why the OEM thermostats work the way they do. If people increase the temperature on the thermostat they expect the room to get up to temperature fairly soon. Most people wouldn't be satisfied waiting for hours for the room to get up to temperature on a cold day. And my system operated as expected by most people when recovering from a large setback. If I locked out the furnace then instead of using auxiliary heat it would not be able to keep up when recovering. And I can manually control the heat source from the thermostat.

Actually, with my system, a standard communications interface wouldn't be necessary. I can change the temperature, heat source, turn the system off or on, etc. remotely. So the proper setbacks and ramp up rates could be controlled by a computer program. There is already an Alexis Skill for controlling the system. What is missing is a sophisticated algorithm that takes the weather forecast, fuel prices, TOU rates, current temperature, etc. into account and actively adjusts the fuel source, setback temperatures, recovery times, etc., (both for heating and AC).

 

[RKCRLR]

That's not necessarily true. You can use a gas furnace as your auxiliary heat in which case the system will turn on the gas when it's defrosting. The difference with a dual fuel system is that the coils are downstream of the heat exchanger instead of upstream of the heat strips, and you'd better not energize the heat pump in heating mode simultaneously with the auxiliary heat or you can damage your heat pump (although modern heat pumps have thermal cutoff switches that would detect this and shut off the power before any damage occurs). In any case, a dual fuel system is either in heat pump mode OR it's in auxiliary mode but never both.

Yes, I use my furnace for the defrost cycle (my controller allows that option) and won't allow me to use both at the same time for heating. But the OP doesn't sound like he has a dual fuel system.

 

[STS-134]

I understand the logic of why the OEM thermostats work the way they do. If people increase the temperature on the thermostat they expect the room to get up to temperature fairly soon. Most people wouldn't be satisfied waiting for hours for the room to get up to temperature on a cold day.

Don't those things have touch screens? All they have to do is pop up a notification that says "Disable auxiliary heat?" when you raise the set point more than few degrees above the current temperature. If you ignore it, it kicks in auxiliary heat. If you say "yes", it locks it out.

 

[RKCRLR]

Don't those things have touch screens? All they have to do is pop up a notification that says "Disable auxiliary heat?" when you raise the set point more than few degrees above the current temperature. If you ignore it, it kicks in auxiliary heat. If you say "yes", it locks it out.

Sort of. It has a touch screen and shows me when it is using auxiliary heat. I can then lock out auxiliary heat if I want or lock it out initially if I know I will be exceeding its limits. But that doesn't help when it is recovering from a large setback on a cold morning while I'm asleep. And expecting my wife to switch sources isn't a conversation I want to have.

 

[STS-134]

Sort of. It has a touch screen and shows me when it is using auxiliary heat. I can then lock out auxiliary heat if I want or lock it out initially if I know I will be exceeding its limits.

That's kind of dumb. You mean it has to first kick in auxiliary heat and then you have to be around and actively paying attention to what it's doing in order to tell it to stop?

 

[RKCRLR]

That's kind of dumb. You mean it has to first kick in auxiliary heat and then you have to be around and actively paying attention to what it's doing in order to tell it to stop?

Yep, unless you lock it out initially because you know you are asking it to do something that it can't without using auxiliary heat. But I do wish it had some kind of option to allow a longer heating time vs reaching its set point in what it considers a reasonable time.

As I said earlier, this isn't a problem now that I'm a net producer. I have the furnace locked out down to 30F. The only time it was a problem is when I was trying to establish an economic balance point based on the price of propane vs electricity. And my workaround for the 4 degree recovery over 90 minutes was to stack 2 setbacks so I could recover from 60F to 68F over a 3 hour period.

The other feature it has is Smart Recovery. This is supposed to override a lower setback temperature if it recognizes that it wouldn't be able to recover from the lower setback due to temperature conditions. But the feature isn't very documented and I never did get it to always avoid using auxiliary heat for recovery on the coldest days.

 

[Dave EV]

So yesterday morning, the inside temperature dropped to 59°F and the outside temperature was 37-38°F and the heat pump took about 80 minutes to bring the temperature up to the set point of 64°F. Today, the inside temperature was 61°F and the outside temperature was 48°F and the heat pump took only about 35 minutes to bring the temperature up. Per the performance charts I have on my model, the unit has a capacity of about 18-20% more when it's 48°F out compared to 37-38°F.

Pretty good! Unfortunately, I don't have good indoor temperature logging - my weather station is mounted on an outside wall in between two windows so it seems that it takes a while for the thermostat to pick up air temperature settings. But I do have a "smart" thermostat waiting for me to install (have a Trane Communicating, 2-stage heat-pump hybrid w/gas system, which I wish was just a standard system given the expense of "smart" thermostats and limited selection.

Your climate is similar to mine but generally a few degrees cooler. Low temp was 44F last night, high of 72F. Actually turned the heat on for the first time today, but only to bump indoor temps up from 65F to 69F after outdoor temps had hit 60F. 2nd stage ran about 20 min to catch up, then the 1st stage ran another 20 min, then 1st stage would cycle on/off every 15 min or so.

That's pretty bad, and what it sounds like you need is a new thermostat that knows how to "speak" the Carrier Infinity protocol and doesn't do idiotic calls for auxiliary heat when it's not required. But in the long run, there needs to be a standard interface for communicating with variable speed systems. Instead of using a mess of wires that you screw in individually, they should use a 16 pin connector like this and have all of the signals put in standard places on it. The standard could allow up to 100 different compressor speeds by using just 7 Y wires, Y0-Y6, by signaling the desired compressor operating speed in binary, i.e. 1 = 1% is binary 0000001 so power Y0 and leave Y1-Y6 floating. 50 = 50% = binary 0110010 so power Y5, Y4, and Y1. 100 = 100% = binary 1100100 so power Y7, Y6, and Y2. You could use an 8th pin as an error check/parity bit to make sure that all of the wires are connected and the thermostat/HVAC system should throw an error if it gets a nonsensical value. Since all of the signals run through the air handler, the air handler could intercept the signals coming from the thermostat and set its fan accordingly. Or you could have separate signals for the fan speed (for dehumidification purposes) encoded in the same manner.

16 wires is a lot and 1% control over compressor power is also overkill. Could probably get away with 8 power levels (3-bits) or if you really wanted more, use 4-bits for power levels and have 16 power levels. Still, 2 wires with RS485 comms and a standard protocol would be a lot easier to wire up. 16 wires is a bit of a pain to string longer distances. Make all systems understand both a basic 2-stage heat/cool setup, but also capable of an advanced 2-wire RS485 communication standard for more control and hook up one or the other.

Part of the problem is that if you size a system appropriately for winter in Tahoe, or Minnesota, or any extremely cold place, it's going to be grotesquely oversized for cooling especially in those climates. If you size a system appropriately for cooling in Phoenix, it's going to be grotesquely oversized for heating.

Well, to be fair, isn't that true of all places for parts of the year where you need heating or cooling?

I don't really have a desire to get a variable speed system or even a multi stage system. These systems are supposed to not cycle which supposedly increases comfort but here in the SF Bay Area, the outdoor temperature often crosses through my indoor set point. Actually if you look at the ASHRAE design temperatures in my area, the 1% temperature for cooling is only 31°C and if you actually designed the system for that, you'd be uncomfortable during heat waves (which is why you turn on the AC to begin with around here). There are often days in July and August where I don't need to use the AC at all. And even during the worst heat waves, the temperature usually goes below my indoor set point of 78°F after around midnight. So after around 9pm or so, even a variable speed system is going to start cycling because a variable speed system can only go down to around 20-30% capacity, and then you're back to where you would have been with a single stage system, only with added complexity and increased maintenance costs if anything goes wrong.

Yeah, where I live we only need cooling during heat waves and get by with 80-84F temps indoors at the worst. But when we want the AC on it's usually 90-100F out, humid and we want it around 75F - depending on how much the house has heat soaked it can take some time to hit target temps.

I really need a zoning system as I have two stories - a lot the year really only need cooling upstairs and heating downstairs and without zoning upstairs is always a bit warmer than we like and downstairs is a bit cooler than we like. Could be done if I had a thermostat upstairs and could control a couple dampers and keep the system in stage 1 when conditioning only up or downstairs.

Actually, with my system, a standard communications interface wouldn't be necessary. I can change the temperature, heat source, turn the system off or on, etc. remotely. So the proper setbacks and ramp up rates could be controlled by a computer program. There is already an Alexis Skill for controlling the system. What is missing is a sophisticated algorithm that takes the weather forecast, fuel prices, TOU rates, current temperature, etc. into account and actively adjusts the fuel source, setback temperatures, recovery times, etc., (both for heating and AC).

But that's all manual and as you say, your wife won't bother with it. These systems need to be automatic to really be beneficial so everyone uses them or at least as simple to use as possible. Otherwise the average person won't bother.

 

[Big Earl]

Pretty good! Unfortunately, I don't have good indoor temperature logging - my weather station is mounted on an outside wall in between two windows so it seems that it takes a while for the thermostat to pick up air temperature settings. But I do have a "smart" thermostat waiting for me to install (have a Trane Communicating, 2-stage heat-pump hybrid w/gas system, which I wish was just a standard system given the expense of "smart" thermostats and limited selection.

Your climate is similar to mine but generally a few degrees cooler. Low temp was 44F last night, high of 72F. Actually turned the heat on for the first time today, but only to bump indoor temps up from 65F to 69F after outdoor temps had hit 60F. 2nd stage ran about 20 min to catch up, then the 1st stage ran another 20 min, then 1st stage would cycle on/off every 15 min or so.


16 wires is a lot and 1% control over compressor power is also overkill. Could probably get away with 8 power levels (3-bits) or if you really wanted more, use 4-bits for power levels and have 16 power levels. Still, 2 wires with RS485 comms and a standard protocol would be a lot easier to wire up. 16 wires is a bit of a pain to string longer distances. Make all systems understand both a basic 2-stage heat/cool setup, but also capable of an advanced 2-wire RS485 communication standard for more control and hook up one or the other.


Well, to be fair, isn't that true of all places for parts of the year where you need heating or cooling?


Yeah, where I live we only need cooling during heat waves and get by with 80-84F temps indoors at the worst. But when we want the AC on it's usually 90-100F out, humid and we want it around 75F - depending on how much the house has heat soaked it can take some time to hit target temps.

I really need a zoning system as I have two stories - a lot the year really only need cooling upstairs and heating downstairs and without zoning upstairs is always a bit warmer than we like and downstairs is a bit cooler than we like. Could be done if I had a thermostat upstairs and could control a couple dampers and keep the system in stage 1 when conditioning only up or downstairs.


But that's all manual and as you say, your wife won't bother with it. These systems need to be automatic to really be beneficial so everyone uses them or at least as simple to use as possible. Otherwise the average person won't bother.

If you're looking for good indoor temperature logging, check out GoVee products on Amazon. I have one of their interior temperature sensors and it's fantastic... looking to order a few more soon.

Here is my house this past week. Outside temps have been freezing overnight and highs in the mid to high 40s during the day. Our heat has been set at 64° the entire time. This is what good southern exposure gets you - highs around 80 inside. This weather is pretty unusual for the PNW - ten straight days of no precipitation in what is normally the wettest month of the year.

 

[RKCRLR]

But that's all manual and as you say, your wife won't bother with it. These systems need to be automatic to really be beneficial so everyone uses them or at least as simple to use as possible. Otherwise the average person won't bother.

It is automatic if you don't lockout your auxiliary heat at a low temperature. If you raise the temperature in a room it will ramp up and provide that temperature. If you want to have large setback temperature at night and have the the system recover to the desired temperature in the morning it will do that. It just may not do this in the most cost effective manner since it may have to us auxiliary heat to meet the demand.

If you want the system to operate in the most cost effective manner then you lockout the auxiliary heat at your economic balance point. But it may take a long time for a room to heat up and the house may not be able to reach the desired temperature in the morning unless you reduce the setback temperature.

 

[Dave EV]

It is automatic if you don't lockout your auxiliary heat at a low temperature. If you raise the temperature in a room it will ramp up and provide that temperature. If you want to have large setback temperature at night and have the the system recover to the desired temperature in the morning it will do that. It just may not do this in the most cost effective manner since it may have to us auxiliary heat to meet the demand.

If you want the system to operate in the most cost effective manner then you lockout the auxiliary heat at your economic balance point. But it may take a long time for a room to heat up and the house may not be able to reach the desired temperature in the morning unless you reduce the setback temperature.

I know in my Trane thermostat there's some secret installer settings to tweak the setback temp to trigger aux-heat along with the run time limit, too. Maybe your Carrier thermostat has them hidden away somewhere?

 

[RKCRLR]

I know in my Trane thermostat there's some secret installer settings to tweak the setback temp to trigger aux-heat along with the run time limit, too. Maybe your Carrier thermostat has them hidden away somewhere?

There is an extended Service menu that you access by pressing and holding the Service icon for 10 seconds. That is where you put in lockout temperatures and all sorts of other stuff. But there is no setting to change the tolerance on how much above the current temperature the demand temperature needs to be before it decides auxiliary heat is necessary. It is probably part of their proprietary algorithm. There is also no field to change the 90 minute setback recovery ramp up time. But there are workarounds.
There is no limit on the auxiliary heat run time. Once the system intersects the place it should be on the heat up ramp time it will shut down. It will then try heating again without auxiliary heat when it falls below the ramp up temperature. But most of the time when the auxiliary heat is called for during the ramp up it is so far behind it continues until it has reached the new set temperature and then turn off. After that the heat pump is fine at maintaining the set temperature.

 

[mspohr]

I have this Chiltrix aux heat unit which is fairly sophisticated to take into account the exact heat deficit ramp rather than just turning on the aux heat full blast.

Air To Water Heat Pump Backup Heater | Tankless Inline Backup Heat For Radiant Heating Hydronic Heat Pump