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Old 12-04-11, 08:45 AM   #1
mincus
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Default New to heat pumps -- questions

Hello all,

We bought a 25 year old two story about a year ago now. It has two air source heat pumps, one for upstairs (about 800 sq ft) and one for the main level (about 1200 sq ft).

Overall, I'm pretty happy with them. It's an all electric house, which I was worried about buying in the first place. I thought my bills would be through the roof. However, one year in, my highest bill was around $130. And that was during one of the coldest months we've had in St. Louis in quite a while. Lowest, in spring and fall was around $55. So, I can safely assume heating is costing us around $80 for the coldest months. Considering our old house (900 sq ft) cost about $100 to heat with natural gas, I'm pretty happy.

However, after a year in, I'm starting to try to look for some savings. I've already insulated fairly well (but could always do more).

I'm thinking of a simple change, but I don't know what kind of effect it could have. I have been thinking lately about the times I'm running the heat pump. We are comfortable with a temp of 66 when in the house. However, it seems hard to try to keep that temp overnight upstairs during the very cold nights. Last year, I would let upstairs drop to around 60 during the day, then kick it up around 6:00 so it would be comfortable when we went to sleep. On cold nights, the heat pump ran often ran all night.

Would I be better to run the upstairs heat pump from about 2:00 to 5:00 (usually the warmest time of the day) up to about 72? Then, just turn it down to 66? I've tried this a few times this year so far. Being that it's not too cold yet, it works well. The heat pump hasn't turned on overnight at all. These were with overnight lows of mid 30s. However, it will be a different story when the temps get colder.

The main floor heat pump I'm not as concerned with. The temperature doesn't fluctuate as much because it mostly only has the walls to lose heat to (instead of the roof like upstairs). Plus, downstairs I can turn the heat pump down at night and up in the morning.

I guess basically it comes down to: does the increased efficiency during the day override the increased heat loss due to a higher temperature differential between inside and outside? I know this has to do with amount of insulation, outside temp, and probably numerous other factors, but are there any general ideas I can go with here?

One other question. I had an HVAC guy out to fix something else. I asked him when to turn on emergency heating. He told me at 32. This didn't seem right to me. My experience is that the heat pump can keep up down to around 17-20. Is it too hard on my heat pump to run it at those temps, or as long as it is keeping up, am I alright?

Any help is appreciated.

Thanks!

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Old 12-04-11, 10:12 AM   #2
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A lot of people believe it's better not to turn down the temp too much at night.
It many cases, it takes too long to re-heat the house.
People who go to work early, turn the heat back down, just when it starts feeling warmer.

I guess 'efficiency' depends a lot on the amount of thermal mass inside the house,
and how quickly that mass transfers it's heat, and the rate of heat loss
to the outdoors.
Do your heat pumps have the capacity to warm the house up quickly?

Plus, the type of windows used is a very big factor in our house.
Even good windows cost a lot of BTUs, if you have a lot of (window)square feet.
But good South facing windows help out a lot during the daytime.


My wife and I are retired, so we don't want to get up in the AM and have to
wait an hour or two for house to warm up. So, we just set ours back 1 click.
From 70 to 68. (or 21C to 20C). We get up to a warm house.

Since you live in a relatively mild climate, I wonder how often you need
to turn on your back-up heat.?.
If it goes down to 20F between 4 & 6 AM, only a few times a year,
why not use the back-up.

Do you have to manually start the back-up, or can it be set to come on
automatically?

What does the heat pump spec say your low temp range is?
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Old 12-04-11, 11:19 AM   #3
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Quote:
Originally Posted by mincus View Post
However, after a year in, I'm starting to try to look for some savings. I've already insulated fairly well (but could always do more).
It's pretty safe to say that most of the folks here on EcoRenovator agree that it is better to conserve energy than to try to generate more.

You might see if there is a free or cheap energy assessment program in your area. The people on those teams are usually well educated in the art of recognizing where energy losses might be and helping you to find the leaks and losses and then to prioritize the order of energy-saving measures, with biggest-bang-for-the-buck at the top of the list.

If such a program is not available to you, paying attention to stopping air leaks is the first order of business. There is a thread here on EcoRenovator, wherein one of our members uses all of the existing extractor fans simultaneously to create a negative pressure in the house and then looks for leaks using his hand moistened with water to feel the cold leaks.

You didn't give us much information on your insulation improvements, but it's probably not enough. Use the "search" function on this page to see what others have done. There are a lot of informed, motivated individuals here, and you could learn a lot from their combined experience.

Quote:
Originally Posted by mincus View Post
Would I be better to run the upstairs heat pump from about 2:00 to 5:00 (usually the warmest time of the day) up to about 72? Then, just turn it down to 66?
I think that this is really a great idea, because the warmer the outside temp, the more efficient your heat pump will operate. It's too bad you dont' have some kind of large controllable thermal mass to store that energy in.

Sleeping temperature is such a personal thing and what works for one person may not be at all acceptable to someone else... But my strategy is based on the idea that I have found that I sleep much better when I am warm and the air and room I am sleeping in, is cold. So, I have two down comforters on my bed and I usually open the window at night (I have even run a window fan in the dead of winter). I also have an electric blanket on the bed which I only use for making the bed nice & toasty. I turn it off before I get in. Pretty inexpensive luxury.

Quote:
Originally Posted by mincus View Post
...does the increased efficiency during the day override the increased heat loss due to a higher temperature differential between inside and outside?
You have put such a fine point on your question that enough specific detail would have to be supplied to do a heat-loss analysis... it's just too complex to guess.

Quote:
Originally Posted by mincus View Post
My experience is that the heat pump can keep up down to around 17-20. Is it too hard on my heat pump to run it at those temps, or as long as it is keeping up, am I alright?
There's been a lot of improvement in heat pumps recently, and most of those improvements have been applied to mini-splits. That HVAC guy may only have experience with whole-house heat pumps, which are lagging behind the technology curve. So, you are right.

However, it is possible that it could get too cold outside for your heat pump to keep up. Any commercial heat pump I know of will shut itself down when this point is reached.

Air source heat pumps are great, but the huge irony is that when it gets really, really cold and you absolutely need that extra heat, they have less heat to offer. This is when and axillary heat source comes in to use.

I don't know if having a cold morning kitchen is a problem for you, but if you could easily close it off from the rest of the house-space, and run a small direct-vent gas heater for the morning only, would be a great solution. Perhaps better than keeping your whole house warm all night with your ASHP.

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Last edited by AC_Hacker; 12-04-11 at 11:21 AM..
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Old 12-04-11, 12:45 PM   #4
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Just a little info on my whole house ASHP. My backup heat is entirely controlled by my thermostat which is set to auto. Emergency backup heat kicks on at about -5C or if I turn the thermostat up "a lot" like 3 degrees in one shot. The thermostat will then kick on the backup automatically to rapidly increase the house temperature.

The thermostat also seems to be smart enough to start ramping the temperature up early so if I say I want it 19 in the house at 6 am the heatpump fires up around 5:30 to get it there.

Having said all of that I leave the thermostat at 18 and let the sun during the day and the woodstove at night heat the house most of the time. I just set the furnace fan to circulate to keep that air moving and the bedrooms warm.

I have recently been thinking about letting the heatpump handle heating on warmer (5-10C) days and save on wood. I'm just not sure how much more efficient that actually is with my older HP.
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Old 12-04-11, 08:35 PM   #5
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I believe the windows are original to the house. They're not in too bad of shape. I've repaired them here and there to reduce drafts.

Last year, I used the backup heat quite a few times. It does get reasonably cold here. For about two months, lows are consistently in the teens and 20s. I haven't found info on the low temp range. Didn't know that existed. I'll look for it. The auxillary heat can't be done automatically on the thermostat.

My windows were fairly drafty. They weren't sealing well for various reasons and years of lack of upkeep from the previous owners. I got most of that fixed.

I've also been thinking about thermal mass. That definitely helps us on the main lavel. We have lots of tile and a huge granite countertop. I think that's part of the reason the temps don't cool off down here overnight too much.

I've been researching ideas to increase thermal mass. Some say to put sand/rocks in your walls. I won't be doing that anytime soon though. But, there's got to be something else I could do. Just not sure what. That would maintain my temperature overnight a bit better. I think larger thermal mass definitely helps in ASHP houses.
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Old 12-04-11, 10:39 PM   #6
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I'm getting the impression those may be older single pane windows.?.
If they are, you might have found a way to cut your heat losses in half.

If they are double pane insulated windows, then you can still improve them
by making and installing interior plastic sheet 'storm' windows.
See this area.. Conservation - EcoRenovator


As to adding 'thermal mass', it's not something that most people start
off thinking about. First, there's the low-hanging-fruit. The easy stuff.

I looked at your City Data and saw your average lows during Dec & Jan
are not that low. 30F IIRC. Which makes it a nice place to live.

Looking at the temperature plot, I got the impression that you guys
hardly ever see a few weeks when it stays below 20F 24-7..

Find those low temperature specs, or buy a TED power monitor
and find out how much power your ASHP is using when it's really cold out.
You should be able to see how well it works in cold weather, by watching
the power use, and comparing it how well the systems are heating.

My ASHPs just coast along, turning off and on during mild weather.
Tonight, it's 45F outdoors, and both units are using low power intermittently.

When it goes down under 10F, I'll be able to tell when my systems
are running at full power, non-stop. That's when I need to start thinking
about turning down the thermostat for a few hours, or bringing out
a couple ceramic heaters..
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Old 12-05-11, 07:42 PM   #7
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No, they're double paned. They actually still do a pretty good job. Not worth replacing yet in my opinion.

I'm surprised to see that data you found. Although last year was an especially cold and snowy year here in St. Louis. I'm a teacher, and we had 13 snow days last year! A typical year gives us 5 or so. It was the coldest year I remember in quite some time.

Seeing as though my only year with heat pumps was uber cold, perhaps I shouldn't worry about it too much yet.
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Old 12-06-11, 10:50 AM   #8
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We had a large amount of snow last winter. It was a PITB keeping it away from the air intake of my main Sanyo..



We had a lot of roof collapses in this area.. Kinda had us worried..
I have now installed melt wires in roof areas that never needed them before.
Just in case!
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Old 12-06-11, 10:53 PM   #9
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Quote:
Originally Posted by Xringer View Post
We had a lot of roof collapses in this area.. Kinda had us worried..
I have now installed melt wires in roof areas that never needed them before.
Just in case!
I get to borrow my neighbors roof rake when there is snow buildup near the eaves.
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Old 12-06-11, 10:55 PM   #10
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Quote:
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I've been researching ideas to increase thermal mass. Some say to put sand/rocks in your walls. I won't be doing that anytime soon though. But, there's got to be something else I could do. Just not sure what. That would maintain my temperature overnight a bit better. I think larger thermal mass definitely helps in ASHP houses.
How about putting rainwater into empty milk jugs and store them upstairs.

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