Efficient Use of Air-source Heat Pumps

[STS-134]

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.

Yeah, I was just going by the numbers on the thermostat itself. But there can be an error of +/- 0.5°F on those readings. Looking at the readings from my Aranet4, which does readings to 0.1°C, it was about 2.34°F colder last morning than it was this morning. So that probably accounts for the bigger than expected difference in run time. Although the Aranet4 is not next to the thermostat so there could have been some difference in the temperature each was measuring.

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.

Not if the entire connector is bundled into one cable. An Ethernet cable has 1 connector with 4 twisted pairs in it, and 8 wires across it.

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

Yes, but what my area lacks is any time of year where there is an advantage to having a variable speed or even a 2 stage system. In Arizona or Florida in the summer time or Minnesota in the winter time, a HVAC system can run more or less continuously, 24/7. This isn't the case during any time of the year in the SF Bay Area.

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.

Could just get two separate systems and be done with it. That at least provides you with the ability to have a backup in case one system fails and then you aren't scrambling to get it fixed if it happens in the middle of a heat wave or cold snap.

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.

I actually use an Aranet4 to monitor temperature. It samples the CO2 level, temperature, humidity, and air pressure every 5 minutes and you can connect to it via Bluetooth and pull the logs for the last week.

 

[David_Cary]

I fell for Carrier Infinity on my last house. Didn't realize how little control I would have and how expensive and unreliable the compressors and fans were.
New house - much cheaper non staged, non zoned Tranes. Get to use Ecobee with fine control. And Ecobee logs indoor and outdoor temps.

The lack of controls on aux and temp tolerances was horrible on the Carrier and really no option other than throwing away all the controllers. I had dual fuel with NG so aux was less of a big deal but it was still annoying. We had off peak (5 cents) from 1-4 pm so I would try to force the compressor to run at high stage for those 3 hours. Had to use a separate system to send it a signal forcing it to low stage the rest of the time. It was a simple signal designed for utility-level control but I used it to maximize the off-peak. The house would typically heat up from 68 to 72 - and sometimes I could coast until 1pm the next day. Certainly didn't want that happening from aux but there was an outdoor temp cutoff for aux so most of the time it worked.

So much for peak rates not effecting usage.....

Peak rates matter a heck of a lot for EV charging. Hot water is something else that can be timed - part of the challenge is that you typically use NG for HW in CA where most of the TOU is. You are also pushed to things like NG dryers. But outside of CA, we mostly use electricity for dryers which is a very large load than is very easy to control the timing of.

The other issue with peak pricing is that a tight house (like hopefully most modern houses) won't heat up or cool down that quickly. I can easily just run a/c at night and the house stays cool all day - even at 90 and sunny. Similarly, the only time the house gets really cold is when we shut off the heat for days. It didn't run last night and we went to bed at 67 and it is 65 this am at 32 outside. So it is pretty easy to run the heat only in off peak and be reasonably comfortable. If you live in a 50 year old house, that doesn't work very well.

In my last house with TOU, it went to peak at 6 am in the winter. So I always tried to heat the house up starting at 4 am. Again - a behavior change. Imagine that. Didn't hurt that lowest outdoor temp is usually 6-8 am so it helped to avoid that window.

Medically, you should sleep at colder temps. That is influenced by personal preference but it is also well established medical fact. In Nordic countries, they tend to have babies nap outside and probably have done so for hundreds of years.

 

[STS-134]

I fell for Carrier Infinity on my last house. Didn't realize how little control I would have and how expensive and unreliable the compressors and fans were.

That would be my fear for any multi stage and particularly variable speed system. Higher chance of something going wrong and very high repair costs if something does break.

The lack of controls on aux and temp tolerances was horrible on the Carrier and really no option other than throwing away all the controllers. I had dual fuel with NG so aux was less of a big deal but it was still annoying. We had off peak (5 cents) from 1-4 pm so I would try to force the compressor to run at high stage for those 3 hours. Had to use a separate system to send it a signal forcing it to low stage the rest of the time. It was a simple signal designed for utility-level control but I used it to maximize the off-peak. The house would typically heat up from 68 to 72 - and sometimes I could coast until 1pm the next day. Certainly didn't want that happening from aux but there was an outdoor temp cutoff for aux so most of the time it worked.

Hmm...that's something I didn't really consider before. If you're on a TOU plan and you then want to run your compressor at 100% during a certain time period when electricity is cheap and then shut it down when electricity becomes expensive, that basically negates the purpose of having a variable speed system in the first place, even in climates where it would otherwise be a good thing to have.

The other issue with peak pricing is that a tight house (like hopefully most modern houses) won't heat up or cool down that quickly. I can easily just run a/c at night and the house stays cool all day - even at 90 and sunny. Similarly, the only time the house gets really cold is when we shut off the heat for days. It didn't run last night and we went to bed at 67 and it is 65 this am at 32 outside. So it is pretty easy to run the heat only in off peak and be reasonably comfortable.

Do you have a heat recovery or energy recovery ventilator? If not, I'm curious what your indoor CO2 levels are. Houses built up through the 1980s and probably into the 1990s generally have enough natural ventilation to not need mechanical ventilation but modern houses are so well sealed that CO2 levels can spike if you have the windows closed. We stayed in a VRBO that was a modern construction (less than 5 years old) and I took my Aranet4 with me on that trip. It was 2000 square feet and we only had 4 people in there even though the place had enough sleeping space for 8. And I saw CO2 levels exceed 2000 ppm in there and rarely drop below 1800. Even when we went out for the entire day, the levels would drop to "only" 800-1000 ppm. I'm not sure why building codes don't require CO2 sensors and mechanical ventilation if you're going to try to insulate and seal every nook and cranny because you can't just seal up a house like it's the International Space Station and expect indoor air quality to remain good. With modern houses, the natural balance point where CO2 diffuses out at the same rate that people inside are creating it is so high that it's way beyond the recommended limits. Above 1000 ppm is listed as "drowsiness" and "cognitive impairment" on most charts.

 

[RayB]

tl;dr 6-10 kWh per day to conservatively heat a mid-sized, well insulated house with a heat pump, estimated 15 to 20 to heat to 72.

I have a 7.2 kW system in the Sacramento region with south facing panels, but some late afternoon shading from trees. The house is recently remodeled so good insulation in the walls and attic. We went with a 5 zone heat pump, and I severely underestimated how much electricity it would take to heat the house. With the outside in the 30s overnight to 60s during the day, it is taking 6-10 kWh per day to heat the public areas to 70. And that is turning the heat off from 11pm to 9am, and mostly not heating the bedrooms. At a rough estimate, it would take 15 to 20 kWh to heat the entire house to 72 for all 24 hours.

I would say put in as much as you can fit and afford.

Did you install a Mitsubishi heat pump and AC?

 

[Dave EV]

It didn't run last night and we went to bed at 67 and it is 65 this am at 32 outside.

Wow, that's really good! My house is not all that well sealed or insulated (early 70s, cheap construction + vaulted ceilings), and my house fell from 70F at 9PM to 65F at 8 AM while outdoor temps were 50F at 9 PM falling gradually to 45F around 6 AM and then hitting 65F at 9 AM. So quite a bit of heat loss despite a much smaller temp differential.

Do you have a heat recovery or energy recovery ventilator?

One thing that we have not learned as a country despite COVID is that air ventilation and filtering is super important to keep indoor air healthy.

I'm very tempted to get an ARANET4 or other CO2 monitor, but given how leaky my house is, I'm sure that CO2 levels are OK most of the time. I do run the HVAC 20 min / hour to help with air circulation and filtration, but this doesn't do anything for CO2...

 

[STS-134]

Wow, that's really good! My house is not all that well sealed or insulated (early 70s, cheap construction + vaulted ceilings), and my house fell from 70F at 9PM to 65F at 8 AM while outdoor temps were 50F at 9 PM falling gradually to 45F around 6 AM and then hitting 65F at 9 AM. So quite a bit of heat loss despite a much smaller temp differential.

I have seen temperature falls of > 6F° between the hours of 10pm to midnight and 7am if it gets cold enough outside. For heating, the thermostat sets back from 64°F to 58°F overnight and then sets back up to 64°F in the morning. Occasionally (and I'm talking like maybe a handful of days a year, probably less than 10), I'll notice that the heat has kicked in because it's dropped to 57°F and before the thermostat tries to hit the 64°F target. It needs to go to near or below freezing outside before this happens though. But most winter nights I'd say a 2-4F° fall overnight is normal.

One thing that we have not learned as a country despite COVID is that air ventilation and filtering is super important to keep indoor air healthy.

I'm very tempted to get an ARANET4 or other CO2 monitor, but given how leaky my house is, I'm sure that CO2 levels are OK most of the time. I do run the HVAC 20 min / hour to help with air circulation and filtration, but this doesn't do anything for CO2...

I generally see CO2 levels hover in the 600-900 ppm range at home. So there's just enough natural air intrusion that mechanical ventilation isn't needed. But the issue with the VRBO we stayed at was that it had an underpowered AC system (actually it was one of those mini split systems but they only had air handlers in the bedrooms) and I had to keep the bedrooms set at 68°F just so the common areas could stay at 78-80°F. 78°F is actually my preferred set temperature for home but it was in Hawaii so opening the windows to lower the CO2 levels would have created humidity issues.

 

[Dave EV]

actually it was one of those mini split systems but they only had air handlers in the bedrooms

One of the issues with mini-splits is that any ventilation system has to be separate from the rest of the HVAC - that and they don't generally have have the capability to install decent filtration if you want to.

 

[STS-134]

One of the issues with mini-splits is that any ventilation system has to be separate from the rest of the HVAC - that and they don't generally have have the capability to install decent filtration if you want to.

I never really understood the appeal of mini splits. The right way to do things is to run ducts, and if you really want to control things one room at a time, do a variable speed system with one zone per room, each with its own thermostat, supply register and damper, and return register. That way you can still easily do whole house air filtration, humidification, and ventilation by just attaching the proper device to the system.

 

[Dave EV]

I never really understood the appeal of mini splits. The right way to do things is to run ducts, and if you really want to control things one room at a time, do a variable speed system with one zone per room, each with its own thermostat, supply register and damper, and return register. That way you can still easily do whole house air filtration, humidification, and ventilation by just attaching the proper device to the system.

Well, you do avoid the efficiency loss of pushing air through ducts, which is nice, so they tend to be more efficiency overall.

And they also have some nice multi-zone features where you can get something like up 4-5 zones with a single outdoor compressor which is handy, too.

 

[STS-134]

Well, you do avoid the efficiency loss of pushing air through ducts, which is nice, so they tend to be more efficiency overall.

And they also have some nice multi-zone features where you can get something like up 4-5 zones with a single outdoor compressor which is handy, too.

Yeah I dunno about that. Ducts should be insulated. And you're instead running refrigerant through potentially long runs instead of air through ducts, so it just exchanges one type of inefficiency for another. Of course, the refrigerant lines should be insulated too but we're talking about what leaks through the insulation here. And the problem there is that when the system needs to be replaced, you may have to rerun all of the refrigerant lines (if the new system uses a new type of refrigerant that uses different diameter lines) as opposed to only having to run one set of new refrigerant lines and being able to reuse all of the ducts.

 

[stopcrazypp]

Yeah I dunno about that. Ducts should be insulated. And you're instead running refrigerant through potentially long runs instead of air through ducts, so it just exchanges one type of inefficiency for another. Of course, the refrigerant lines should be insulated too but we're talking about what leaks through the insulation here. And the problem there is that when the system needs to be replaced, you may have to rerun all of the refrigerant lines (if the new system uses a new type of refrigerant that uses different diameter lines) as opposed to only having to run one set of new refrigerant lines and being able to reuse all of the ducts.

I think mini-splits were mainly made popular in the Asian countries in apartments where they typically only cool the bedrooms, which many times are much better insulated than the rest of the house, so saves a lot of energy and they can make do with a much smaller outdoor unit (usually hanging out a window or on a balcony). They don't cool the whole apartment unit because that would be way more expensive for them, plus it is not practical to install the huge central units we see here in the US.

 

[OzarkEV]

One appeal of mini split systems is the ease of installation in a retrofit. My house is an early 1900s bungalow. They didn't really think about hvac back then the way we do now. It was a wood stove in the middle for heat, and windows for cooling. It is a lot easier to poke 3" holes in the wall for refrigerant lines than to install an adequate duct system.

In my case, someone did put in a ducted system, but they had to make a lot of performance sacrifices to make it work. Reducers to pass between floors, undersized cold air returns. Stuff I could fix, but taking a Sawzall to old growth oak joists gives me pause.

I find the system works best on one setpoint. I didn't see a big change when I used scheduled setpoints.

 

[Big Earl]

One of the issues with mini-splits is that any ventilation system has to be separate from the rest of the HVAC - that and they don't generally have have the capability to install decent filtration if you want to.

Many mini-split heads have an optional knock-out for up to 15% outside air ventilation.

 

[David_Cary]

Moving refrigerant is a whole lot more efficient than moving air. Doesn't matter if it is insulated. And you still need to move refrigerant with conventional HVAC - but often less distance.
Refrigerant really doesn't leak unless there is a problem. Ducts always leak - something about gas being harder to prevent leaks....
There are many efficiency advantages to mini-splits. But they have their issues for sure. We had the issue of not being able to set the temp below 67 and attempts to fool it didn't work as there were lockouts at 67 at the unit also.

 

[Ahh9oolu]

Did you install a Mitsubishi heat pump and AC?

I did. It's a 5 zone system with a ceiling cassettes in each of the 4 bedrooms, and a 1.5 ton ducted unit that covers all the public areas. I love being able to heat/cool only a single room as required, but there are some downsides, as mentioned by others above. One really interesting thing we have observed during the heating season, is how little the air mixes between the bedrooms and the public areas. We generally leave all the bedroom units off and turn the public areas ducted unit on during the day. With a set point of 70 in the public areas, the adjacent bedrooms still stay in the low 60s.

Overall I would give it a B+, would install again.

 

[Ahh9oolu]

Refrigerant really doesn't leak unless there is a problem. Ducts always leak - something about gas being harder to prevent leaks....

I don't have stats on the refrigerant in my 5 zone mini-split system, but I have a 1.5 ton ducted unit in the public areas. In the summer, when the attic was around 110°F, I discovered that in fan-only mode, the air coming out of the vents was 2°F warmer than the air going in. So not only does air leak out, but as expected by physics, heat leaks in.

 

[STS-134]

I did. It's a 5 zone system with a ceiling cassettes in each of the 4 bedrooms, and a 1.5 ton ducted unit that covers all the public areas. I love being able to heat/cool only a single room as required, but there are some downsides, as mentioned by others above. One really interesting thing we have observed during the heating season, is how little the air mixes between the bedrooms and the public areas. We generally leave all the bedroom units off and turn the public areas ducted unit on during the day. With a set point of 70 in the public areas, the adjacent bedrooms still stay in the low 60s.

Overall I would give it a B+, would install again.

Would be more interesting to see if there's any noticeable difference in energy consumption between the scenario where you keep the bedrooms at the same temperature as the public areas vs. only conditioning the public areas. There's generally not much insulation (if any) in interior walls, so what's likely to happen is that your ducted air handler is just going to operate longer and it's probably not as much of a savings as you'd think.

I don't have stats on the refrigerant in my 5 zone mini-split system, but I have a 1.5 ton ducted unit in the public areas. In the summer, when the attic was around 110°F, I discovered that in fan-only mode, the air coming out of the vents was 2°F warmer than the air going in. So not only does air leak out, but as expected by physics, heat leaks in.

I take it that your return and supply registers are all in the attic? Or do you have one set of ducts in the attic and another in the crawlspace? Of course, you can eliminate these sorts of losses by just running the ducts directly through the occupied space, so that any leakage doesn't really hurt you from an energy usage perspective. It's ugly as hell but some office buildings do this.

 

[wwhitney]

Wow, that's really good! My house is not all that well sealed or insulated (early 70s, cheap construction + vaulted ceilings), and my house fell from 70F at 9PM to 65F at 8 AM while outdoor temps were 50F at 9 PM falling gradually to 45F around 6 AM and then hitting 65F at 9 AM. So quite a bit of heat loss despite a much smaller temp differential.

You can't actually compare heat losses just by comparing temperature changes, as the thermal mass matters. So you have 5F of temperature change over ll hours with a roughly constant 20F delta. Whereas Dave Cary had 2F of temperature change over 8? (unspecified) hours with a 35F? delta. Which if my guesses and constant approximations are accurate enough, means that Dave Cary's building has a (heat loss / thermal mass) compared to your building of 2 / (8 * 35) vs 5 / (11 * 20), or roughly one third yours.

Now I agree that if the buildings are similar sizes and similar construction types, then the thermal masses may be similar, which would let you make conclusions about heat loss rates.

Cheers, Wayne

 

[Mtns]

This thread is extremely helpful as we’re in our first heating season with our Mitsubishi hyper heat heat pump, ducted install.

We have heat strips as backup. Is there any harm to shutting off the breaker for the strips to lock them out? We only set back 3-4 degrees at night and don’t mind if it takes a bit for the heat pump to increase temps in the morning. Since I can’t figure out how to lock out aux heat from our thermostat, I’ve just shut off the breaker.

 

[RKCRLR]

This thread is extremely help as we’re in our first heating season with our Mitsubishi hyper heat heat pump, ducted install.

We have heat strips as backup. Is there any harm to shutting off the breaker for the strips to lock them out? We only set back 3-4 degrees at night and don’t mind if it takes a bit for the heat pump to increase temps in the morning. Since I can’t figure out how to lock out aux heat from our thermostat, I’ve just shut off the breaker.

The main problem with shutting off the heat strips breaker would be the defrost cycle. Best case is it will blow cold air during the cycle and take longer to defrost. Worst case is the outside unit will ice up solid or perhaps throw a code. Hopefully it has internal protections to prevent this. It may be worth a try to call your installer to see what they say.