Analyzing and improving heat pump energy usage

[brkaus]

Fan on will also keep the ducts in the attic at house temperature. If the attic is uninsulated this will cause more heat loss in the ducts. Ducts are typically around r-6.

Sure, a system that is off will also loose heat in the ducts, but they will eventually approach ambient attic temp and stop loosing more heat.

 

[Dave EV]

Now I wish I'd kept the Canadian research on fiberglass insulation and how cold attic temperatures affect its R value when I researched how to build an energy efficient house back in 1989.
I do remember a 50% R value drop in very cold temperatures, in the minus range just don't remember how minus for that 50% loss.
It had figures for above zero temperatures as well.
Their conclusion was convective losses and recommended bats in plastic bags and layered.
This is what I did by using 6" bats, in bags, covered with a sheet of Tyvek then another 6" bats and another layer of Tyvek.

Good tip on using bagged insulation to prevent convective losses with attic insulation!

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Thing to remember is the "Fan on" is helpful at extreme's sufficient to force high stage to recover. For my home if I'm below say 20-30f (depends on wind) the home perimeter gets cold enough that an "auto" call will require more than 2 (or maybe it's 5) minutes to satisfy the whole house. Whatever that value is, if the home isn't satisfied within that time the unit kicks on high stage. All HP's (I'm not air source, I'm ground-loop), run more efficently at lower stages and for longer periods. Testing suggests I'm better off running "Fan On" at such times. Hard to be certain, but every multi-day test I run shows it pretty conclusively (I use emporia, can see most of my home loads).

Wow, only 2-5 minutes before it kicks into stage 2? That's barely enough time for the compressor to reach steady state! Most will wait at least 20 minutes, sometimes longer and monitor the thermostat to see what direction the temperature is going.

My t-stat will also call for high-stage if difference is over 2 degree's, and has some learning capability.

Most of the time, 2 degrees is too low, it would be nice to be able to configure that. It should look at the inside, outside temps and ideally map out the rate of temperature change in both stages with a variety of outside temps to determine which stage should be used...

 

[dafish]

Ground source. ambient is irrelevant.

20 minutes to stage up? wow. i would not be happy..

the zone controller is pretty smart, dynamically balances opening of zones to arrive at set point at a coordinated time. The stat is integrated w/same. the zone controller has stage up logic, as does the unit. I think, but am not certain, that the 2 or 5 minute stage up call is integral to the unit. But maybe not. I did all this 20 years ago, so…

 

[nanchux]

Cathedral ceiling is a killer. The heat difference between the ceiling and the floor can be 5 degrees and lead to discomfort. It also takes the heat pump a long time to heat that much volume. Is the fan set on high? It will help to circulate air and heat the area more uniformly.

 

[zƬesla]

Cathedral ceiling is a killer. The heat difference between the ceiling and the floor can be 5 degrees and lead to discomfort. It also takes the heat pump a long time to heat that much volume. Is the fan set on high? It will help to circulate air and heat the area more uniformly.

I'm currently having this issue. I see a 9 degree difference between the temp reported by the mini-split head (high up) and baseboard thermostat on wall. Recently replaced the outdoor unit and trying to figure out a happy medium. So far, I can either have a really hot upstairs or really cold downstairs. There is even a fan above the upstairs landing that I always have on to circulate the hot air.

 

[BGbreeder]

Cathedral ceiling is a killer. The heat difference between the ceiling and the floor can be 5 degrees and lead to discomfort. It also takes the heat pump a long time to heat that much volume. Is the fan set on high? It will help to circulate air and heat the area more uniformly.

We are fortunate to have the air returns centrally located and high, to get that warm air back down to the level of the occupants.

I'm currently having this issue. I see a 9 degree difference between the temp reported by the mini-split head (high up) and baseboard thermostat on wall. Recently replaced the outdoor unit and trying to figure out a happy medium. So far, I can either have a really hot upstairs or really cold downstairs. There is even a fan above the upstairs landing that I always have on to circulate the hot air.

Improving air circulation is the only way that I know of for a single heat source, but if upstairs and downstairs are heated separately, you might consider a door, or Japanese Noren style door curtain to keep the warm air from readily flowing into the upstairs. Sometimes, a little bit goes a long way.

All the best,

BG

 

[zƬesla]

Improving air circulation is the only way that I know of for a single heat source, but if upstairs and downstairs are heated separately, you might consider a door, or Japanese Noren style door curtain to keep the warm air from readily flowing into the upstairs. Sometimes, a little bit goes a long way.

Cathedral ceiling with open stairs/landing/hallway. Door is not an option. These are unducted mini-splits too. I'm hoping it's a matter of finding the sweet spot between the multiple heads settings as we didn't have as much of an issue with the old outdoor unit. Was hoping the ceiling fan atop the landing would handle the circulation but it doesn't seem to.

 

[aesculus]

we didn't have as much of an issue with the old outdoor unit

Was your old unit ducted? If so perhaps you can create your own air handler (just a fan?) that moves air from the top to the bottom during heating season using the old duct work.

 

[BGbreeder]

Cathedral ceiling with open stairs/landing/hallway. Door is not an option. These are unducted mini-splits too. I'm hoping it's a matter of finding the sweet spot between the multiple heads settings as we didn't have as much of an issue with the old outdoor unit. Was hoping the ceiling fan atop the landing would handle the circulation but it doesn't seem to.

Is your ceiling fan pulling air up, or trying to push air down? Pulling up would be better for equalizing the gradient. Can you go to a larger ceiling fan? Have you tried a "Vonrado" type fan blowing up? It doesn't sound like there a bunch of options.

All the best,

BG

 

[nanchux]

Cathedral ceiling with open stairs/landing/hallway. Door is not an option.

Do you have separate compressors to handle the cathedral ceiling area and the upstairs area? Do you run only the compressor in the cathedral ceiling area during some time of the day? Since the heat is escaping that compressor is going to be inadequate for that volume of area. You need to turn on both compressors. You will still end up with hot upstairs but you will have warmer cathedral ceiling area.

 

[dafish]

Huh. I have an 18" family room and a pretty large second floor with no real heating or cooling problems. Here in the midwest I think we insulate pretty well, and most homeowners here learn quickly that a second floor home needs great ducting plus zoning or a 2nd floor dedicated system. Now this is a wild assed guess, but I have to wonder: Is there any chance your home attics are getting really hot and you're picking up a lot of heat from the attic? Have things like radiant heat barriers, better attic airflows, and increased ceiling insulation been looked at?

Again a WAG, but my unskilled/WAG leans towards "man, something else is up. I'd want to fix that, not throw more cooling $$'s at it".

 

[zƬesla]

In answer to above posts:

Old unit was also unducted. There was an issue with the old outdoor unit where it would occasionally stop working. Best guess was the high pressure switch but no guarantee it would resolve the issue. The unit was 10 years old and a new one would not be unreasonably more than just replacing the switch after rebates.

We’re in northern NH where we’re still seeing freezing temps nightly. Also a supposed advantage of the new “hyper heat” unit vs the old is that it can handle lowers temps.

The fan is in counter-rotate mode (pushing heat down) during winter and running at all times.

There are 3 heads attached to the outdoor unit. 18k btu for living room, 12k and 6k for two other rooms. The old unit would keep temps more even but that may be that we’re in transition season right now with 20s at night and 40s-60 during the day. We usually keep the 6k unit off, but after leaving it on las night found the living room to maintain temp a little better (67F vs 64).

Attic over open area of house had insulation upgraded years ago. We’ve participated in the energy company’s weather tightening program a couple of times now. Although there is plenty I don’t like about the program (mostly contractor related), major improvements have been made to the house envelope. We have not tackled the cathedral ceilings though. A possibility for when the roof is next replaced (hopefully a ways off).

 

[DrSmile]

This month (July) was very hot in NJ, like everywhere else... This year is the first year that I have rewired the old attic fan that was already present with a slower motor and a thermostat set to 110 degrees. It doesn't pull air through the house, only through the attic space when the roof heats up. The fan typically starts running at 10-11am and runs until 8-9pm at night. It uses about 600 watts but the A/C savings are much, much greater. Last year during a much cooler July I imported 500kWh, this year I exported about 50kWh. So my suggestion is that if you have an attic space, consider venting it with a thermostat wired fan.

 

[Dave EV]

This month (July) was very hot in NJ, like everywhere else... This year is the first year that I have rewired the old attic fan that was already present with a slower motor and a thermostat set to 110 degrees. It doesn't pull air through the house, only through the attic space when the roof heats up. The fan typically starts running at 10-11am and runs until 8-9pm at night. It uses about 600 watts but the A/C savings are much, much greater. Last year during a much cooler July I imported 500kWh, this year I exported about 50kWh. So my suggestion is that if you have an attic space, consider venting it with a thermostat wired fan.

Do you have enough attic insulation? With sufficient insulation you shouldn't have that much heat seeping into the house from the attic...

 

[DrSmile]

Do you have enough attic insulation? With sufficient insulation you shouldn't have that much heat seeping into the house from the attic...

First thing I did is add a ton of insulation, including covering all the access points, but it didn't make a significant difference. It is a split level 50s era ranch so heating and cooling it is not very easy. The fan was a way cheaper solution than the insulation!

 

[charlesj]

First thing I did is add a ton of insulation, including covering all the access points,...

Was this weight in the engineering calcs?