Efficient Use of Air-source Heat Pumps

[Dave EV]

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.

We have one at work (An "AC PRO" model) that locks out the heat if the outdoor unit gets too warm. Which is a problem this time of year as the outdoor unit sits on the roof in the sun and thinks it's warmer than it is, while the area we'd like to warm up has concrete floors and walls so doesn't warm up nearly as fast as it does outdoors. Might need some shade for it to prevent this issue. Not sure if other more popular brands of mini-splits have similar issues.

 

[David_Cary]

Yep - thermal mass can be a big variable. Internal gains also. Smaller house, internal gains make a bigger difference.

I suspect most people have stone countertops. The amount of brick in a fireplace is probably somewhat of a variable.

But I think envelope characteristics is still the dominate variable. I have R-98 in the ceiling which crushes average housing stock and an ACH of 1.9 and average housing stock is probably 8 or so. So I would expect my losses to be about 1/3 average housing stock. 2x6 walls with blown fiberglass and some a bit of foam in the sheathing.

But I suspect my thermal mass is pretty typical. The only tile is showers - all floors are wood. But larger than average brick fireplace. Probably a wash.

 

[STS-134]

Did a test this morning and forced my system into auxiliary heat mode. Seems that auxiliary heat mode, with the furnace operating in the second stage (maximum heat), generally takes about half the time to bring the temperature up compared to the heat pump. With an inlet air temperature of 15-18°C and the furnace on high, the air temperature coming out of the supply registers is around 42-45°C. With the heat pump running, it's only about 33-34°C. The furnace is also moving more air when it runs on high; with the heat pump running, it's set to use 3 tons worth of airflow but when it's using high heat from gas, it seems to be pushing even more air than that.

Yep - thermal mass can be a big variable. Internal gains also. Smaller house, internal gains make a bigger difference.

I suspect most people have stone countertops. The amount of brick in a fireplace is probably somewhat of a variable.

But I think envelope characteristics is still the dominate variable. I have R-98 in the ceiling which crushes average housing stock and an ACH of 1.9 and average housing stock is probably 8 or so. So I would expect my losses to be about 1/3 average housing stock. 2x6 walls with blown fiberglass and some a bit of foam in the sheathing.

What's your CO2 level when your house is occupied? Do you have a ventilator?

But I suspect my thermal mass is pretty typical. The only tile is showers - all floors are wood. But larger than average brick fireplace. Probably a wash.

Thermal mass really only matters when determining how much the temperature falls or rises during setback and how long it takes to pull back to the set point when the time comes. If you have a single stage system, the system will probably keep operating with shorter downtime for 2-3 cycles as that thermal mass will keep absorbing or releasing heat. But once everything in the house reaches the desired temperature, the system will go into steady state.

 

[Ampster]

My wife just sets it to whatever she wants.

WAF (Wife Acceptance Factor) is important. We found a compromise when I found a half sized electric pad for her side of the bed.

 

[Ahh9oolu]

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

Good idea. I took a day and heated the whole house rather than just the public areas. The difference was around 40%, 14kWh vs 10kWh. Of course, the inside did not start at the exact same temperatures and the outside was not exactly the same, but they were close.

Interestingly enough, we did actually fully insulate all the interior walls with Rockwool for the sound deadening. Additionally, every interior door is actually a fiberglass, foam core door, again for the sound reduction.

I take it that your return and supply registers are all in the attic?

Correct, everything is in the attic. Give how much discussion we had about where to place the return (in the hall ceiling, not easily visible), there is no way SWMBO would allow the ducts to be visible.

 

[David_Cary]

I think it would take longer than a day to check for true differences in consumption. Obviously, thermal mass and insulation and all that - but it really takes a while to reach steady state. As the temperature changes become more significant - the thermal mass in the framing comes into play.

You need a week I would think to get real data.

The big issue for interior spaces is the air movement around doors. Air infiltration that high is a huge leak and dominates the walls because drywall has some R-value as does the air inside it.

I do not have a CO2 monitor but I was thinking of getting one with VOx monitor. We have a ventilation fan mostly because Energy Star wanted one. This is all double edged and you want control. Opened windows last night and the smell was terrible. We had a water main break about 1/4 mile away and I think it was repairing the road. People burn things near me - firepits and whatever - at times and I appreciate the isolation. Generally we crack a window when the outside air is fine and not too cold. Near the cat litter box..... We have lived her for 4 years and never noticed headaches or anything. Our volume of air relative to occupants is pretty high compared to HCOLA. 3700 sqft, 10/9 ft ceilings. 3 occupants.

 

[STS-134]

I think it would take longer than a day to check for true differences in consumption. Obviously, thermal mass and insulation and all that - but it really takes a while to reach steady state. As the temperature changes become more significant - the thermal mass in the framing comes into play.

You need a week I would think to get real data.

The big issue for interior spaces is the air movement around doors. Air infiltration that high is a huge leak and dominates the walls because drywall has some R-value as does the air inside it.

Assuming you set the temperatures back at night and let the entire house equalize, I'm not sure it would take longer than a couple days to measure. After day 2 or 3, the thermal mass of the bedrooms in is going to be roughly at matching what you're doing on the HVAC.

I do not have a CO2 monitor but I was thinking of getting one with VOx monitor. We have a ventilation fan mostly because Energy Star wanted one. This is all double edged and you want control. Opened windows last night and the smell was terrible. We had a water main break about 1/4 mile away and I think it was repairing the road. People burn things near me - firepits and whatever - at times and I appreciate the isolation.

You can't manually override and disable the ventilator?

 

[David_Cary]

Assuming you set the temperatures back at night and let the entire house equalize, I'm not sure it would take longer than a couple days to measure. After day 2 or 3, the thermal mass of the bedrooms in is going to be roughly at matching what you're doing on the HVAC.

You can't manually override and disable the ventilator?

Really dumb switch. Need to replace switch - but I think I should have some monitor before I do that - because I will be tempted to shut it off when it is particularly cold out (or some obnoxious smell) and I want to make sure that I have hard data before I do that.

I was thinking a few days to even out the thermal mass but a week of data to smooth that effect out.

 

[holeydonut]

Um...yes, of course.

For similar reasons, why we stripped the roof off and added as much insulation as we could before adding solar, and boy did that improve the comfort of our home.

We selectively use things like fleece, 100w spot heaters etc., to be comfortable and warm when it is cold out. Why heat any more space than you have to? And, yes, we all know how many kW the AC draws, and how many $/hr that is when it is running. (Ditto the furnace) Personally, I can't wait to replace the AC with something far more efficient, but as it keeps on running, and we use it so infrequently, it is hard to rationalize replacing it. (ROI, and all that)

Don't get me wrong, I would much prefer a home with R80 in the attic, R40+ walls and floors that could get by on passive heating and cooling, but the ROI of that in this house is never, so we make do.

@swedge what kind of automatic sun shades do you have?

All the best,

BG

My wife just set the heater/thermostat to 74F since she said everybody sets it to 74F. She says nobody heats a home to only 70F in the Winter, and I'm out of my mind having it so cold in the house of only 70F.

So yeah... dino farts are being lit on across two NG furnaces (luckily 96% efficient I guess). PG&E gas bills in the Winter are insanity.

 

[Big Earl]

My wife just set the heater/thermostat to 74F since she said everybody sets it to 74F. She says nobody heats a home to only 70F in the Winter, and I'm out of my mind having it so cold in the house of only 70F.

So yeah... dino farts are being lit on across two NG furnaces (luckily 96% efficient I guess). PG&E gas bills in the Winter are insanity.

I can’t imagine what my electric bill would be if I heated my house to 74°. It was set at 67° yesterday and I used 100 kWh for heat. But hey, I guess if “everybody else“ is doing it maybe I should, too!

 

[ohmman]

I'd be wearing my swimsuit all day if it were 74 in my house.

 

[mspohr]

I can’t imagine what my electric bill would be if I heated my house to 74°. It was set at 67° yesterday and I used 100 kWh for heat. But hey, I guess if “everybody else“ is doing it maybe I should, too!

Just keep adding solar panels until you reach parity
FALC

 

[mspohr]

I'd be wearing my swimsuit all day if it were 74 in my house.

Puts your "upright member" tag in a different context

 

[STS-134]

I can’t imagine what my electric bill would be if I heated my house to 74°. It was set at 67° yesterday and I used 100 kWh for heat. But hey, I guess if “everybody else“ is doing it maybe I should, too!

Holy crap, in ONE day?!? Most I've ever used in a MONTH is less than 1500 kWh and that includes the pool pump. My heat is set at 64°F

Just keep adding solar panels until you reach parity
FALC

If you reach parity in the winter, you'll have to keep the house at 65°F in the summer to use up all of that extra energy. And then I'd have to wonder why you need to keep the house at 74°F in the winter if you're keeping it at 65°F in the summer...

 

[mspohr]

Holy crap, in ONE day?!? Most I've ever used in a MONTH is less than 1500 kWh and that includes the pool pump. My heat is set at 64°F

If you reach parity in the winter, you'll have to keep the house at 65°F in the summer to use up all of that extra energy. And then I'd have to wonder why you need to keep the house at 74°F in the winter if you're keeping it at 65°F in the summer...

Net metering is your friend

 

[wwhitney]

I can’t imagine what my electric bill would be if I heated my house to 74°. It was set at 67° yesterday and I used 100 kWh for heat.

I take it you have cheap electric rates there in Washington State, and you have electric resistance heating. 100 kWh = 340,000 BTUs, so your average heat loss was 14,000 BTUs/hr.

Eyeballing a graph of yesterday's temperature in your location, I estimate the average outdoor temperature was 33F. So you were maintaining a 34F temperature delta. If you increased that to a 41F delta, you would have used 20% more energy, or 120 kWh.

Cheers, Wayne

 

[STS-134]

Net metering is your friend

Using the grid as a 100% efficient battery hasn't been allowed since NEM1, and certainly wouldn't be allowed for any panel additions.

 

[Big Earl]

I take it you have cheap electric rates there in Washington State, and you have electric resistance heating. 100 kWh = 340,000 BTUs, so your average heat loss was 14,000 BTUs/hr.

Eyeballing a graph of yesterday's temperature in your location, I estimate the average outdoor temperature was 33F. So you were maintaining a 34F temperature delta. If you increased that to a 41F delta, you would have used 20% more energy, or 120 kWh.

Cheers, Wayne

Thanks for the stats, Wayne! You're right on the money - fairly inexpensive rates and straight electric baseboard heat. It was very windy here yesterday with gusts to 50 MPH. Our 1974 house is pretty leaky. We've only been here for three months ... on the project list is replacing the weather stripping on the exterior doors and adding mini-splits to the large rooms. Also considering the Sanden CO2 hot water heat pump, although those are in short supply and probably very expensive.

 

[holeydonut]

I wonder if radiant/floor heating is better than minisplits (that is, assuming the issue is just heating a certain area of the home and not cooling it).

I just put down $35k to overhaul every goddamn window in this house. Biden Money isn't going to help since the IRA encourages spreading out window replacements over a goddamn decade. Barf.

House = money pit. Uggghhhhh.

 

[mspohr]

Using the grid as a 100% efficient battery hasn't been allowed since NEM1, and certainly wouldn't be allowed for any panel additions.

Sorry net metering won't work for you