Heat pump - drop temperature at night or keep constant?

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ThankYouJack
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Heat pump - drop temperature at night or keep constant?

Post by ThankYouJack »

I have a heat pump with a programmable thermostat. It's effecient except when the outside temperature drops below ~40 degrees and the the auxiliary unit (electric coils) kicks in. The nights almost always drop below 40 during the middle of winter and my electric bill goes up quite a bit. However when I drop the temperature at night it pretty much guarantees the auxiliary unit will need to kick on for a while in the early morning to help get the house temperature back up.

So do you think its better to program a heat pump thermostat to drop the temperature at night or keep it constant throughout the night?
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Kosmo
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Re: Heat pump - drop temperature at night or keep constant?

Post by Kosmo »

Lower it at night but have a more gradual temp rise in the morning. If all of a sudden the thermostat gives an instantaneous temp increase of 5 degrees that will kick on the aux heat. But if it's 2 degrees higher or might not. Then 30 minutes later go another 2 degrees. Slowly work up to your desired morning temp. If it's really cold outside and the heat pump can't keep up anyways, then nothing will stop the aux heat from turning on.
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magellan
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Re: Heat pump - drop temperature at night or keep constant?

Post by magellan »

It's too bad that the control logic for these things isn't better designed to match the capabilities and economics of the system.

It would be relatively easy to build a control system that allows the user specify two temperatures, say a minimum temperature and a desired temperature.

The system would only use the heating element if the temperature is below the minimum temperature. When the temperature is between the minimum temperature and the desired temperature the system would engage the heat pump, but not the heating element.

Does any one know of a system that works this way?
Valuethinker
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Re: Heat pump - drop temperature at night or keep constant?

Post by Valuethinker »

there are controllers you can get which take into account outside temperature and the heating characteristics of your house.

Generally as per the other poster, with HPs you want big, slow moves in temperature-- ie the pump to come on hours before you need the house at a given temperature.

http://www.amazon.com/Honeywell-C7089U1 ... B000979ERY
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alec
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Re: Heat pump - drop temperature at night or keep constant?

Post by alec »

Kosmo wrote:Lower it at night but have a more gradual temp rise in the morning. If all of a sudden the thermostat gives an instantaneous temp increase of 5 degrees that will kick on the aux heat. But if it's 2 degrees higher or might not. Then 30 minutes later go another 2 degrees. Slowly work up to your desired morning temp. If it's really cold outside and the heat pump can't keep up anyways, then nothing will stop the aux heat from turning on.
+1. we keep the thermostat at 68 during the day, and 65 from 9pm - 6 am during the winter. The aux heat only comes on when the outside temp gets below about 10 degrees F.
"It is difficult to get a man to understand something, when his salary depends upon his not understanding it!" - Upton Sinclair
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walkabout
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Re: Heat pump - drop temperature at night or keep constant?

Post by walkabout »

magellan wrote:It's too bad that the control logic for these things isn't better designed to match the capabilities and economics of the system.

It would be relatively easy to build a control system that allows the user specify two temperatures, say a minimum temperature and a desired temperature.

The system would only use the heating element if the temperature is below the minimum temperature. When the temperature is between the minimum temperature and the desired temperature the system would engage the heat pump, but not the heating element.

Does any one know of a system that works this way?
Every heat pump manufacturer makes such a system and there are many after market thermostats that provide that capability. Typically these systems are used to control a heat pump with gas heat backup. They provide exactly the kind of control you describe. I'm not sure if any of these systems allow you to apply the same kind of control to the backup electric heat since the whole point of the heat pump is to optimize electric use by using the backup kept only when necessary.

To the OP... If you are using a heat pump compatible programmable thermostat, it should ramp up your temperature in the morning (from your setback temp to your desired temp) such that the use of the backup heat is minimized. The backup heat will engage if there is a sudden large jump in temperature or if the heat pump is unable to keep up (or reach) with the desired temperature. 40 degrees is not that cold for a modern heat pump. A modern heat pump should be able to easily generate heat at that temp.

Are you sure that your heat pump is operating efficiently? Does it need a service?

Good luck!
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ThankYouJack
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Re: Heat pump - drop temperature at night or keep constant?

Post by ThankYouJack »

wageoghe wrote: Are you sure that your heat pump is operating efficiently? Does it need a service?

Good luck!
It probably does need servicing. The thing is, I haven't had good luck with the HVAC companies. It has never defrosted well, and the HVAC guys never seemed to fix it.
The unit is about 8 years old and I've never been that happy with it.

I do have a heat pump specific programmable thermostat -- I usually set it for 63 at night and 69 during the day.

How much should I expect to pay if I have it serviced? I wonder how much it would save on my electric bill. Last month, I think the bill was $255 so not that much for a 2,500 sqft house but it would be nice to lower it a bit. During months where I don't use heat or AC my electric bill is around $70
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walkabout
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Re: Heat pump - drop temperature at night or keep constant?

Post by walkabout »

We have two heat pumps and probably pay around $200-250 to have both serviced. One key to efficient operation is the right amount of refrigerant. If your refrigerant is low, your unit won't be able to make enough hot air via the heat pump mechanism and will have to fall back to the electric heat. Another key is a clean air filter.

I would try not setting your temp down so low at night. Try 65 and see how that goes.
Ninegrams
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Re: Heat pump - drop temperature at night or keep constant?

Post by Ninegrams »

From what I've read it's more efficient NOT to let the heat run unneeded all night. My particular heat pump will slowly raise the temperature by default, however it has a mode that I can select that will raise the temp at a more rapid rate.
cpumechanic
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Re: Heat pump - drop temperature at night or keep constant?

Post by cpumechanic »

Hello

I got a Nest thermostat as a Christmas Gift. Installed on my heatpump in eastern PA with Aux heat.
After install it clearly pointed out that Aux heat was the default in the AM after a setback from 70 to 65 at night.

Unit is very well engineered (designed by ex-Apple engineers)... and was smart enough to point out (with a picture) when one of the thermostat control wire got disconnected, when I re-seated the wires (pushed them back into the wall).

I corrected the problem, and was impressed with a thermostat that "talks back" and shows an image of the exact problem that needs to be corrected.

Google says Aux heat is default on if/when the called for Temp is > 2F higher than current building temp. House will heat up pretty quickly, but no heat pump advantage, 1:1 Kilowatts> Heat.

Nest has a minimize Aux heat feature that I turned on, and so now.. it may take 2+ hours to go from 65 to 70 in the morning but I prefer that lower cost option compared to the Aux heat. Length of time needed to reach the new higher temp is a function of your outside temp, and the efficiency of your heatpump.

Hope this helps

CPU
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Ketawa
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Re: Heat pump - drop temperature at night or keep constant?

Post by Ketawa »

I have an ecobee Wi-Fi thermostat and a heat pump for my condo. The thermostat takes into account the outside temperature to estimate how early it needs to start heating before the desired temperature goes up. It's not a perfect feature. It regularly underestimates the amount of time needed. If it hasn't reached the desired temperature after 30 minutes of heating, I have it set to turn on the auxiliary electric heat. This morning, the aux heat came on for about 30 minutes.

I think it's better to have the temperature set back at night. Here are some actual numbers from last night.

Set on 71 for 8:00 before the overnight setback:
60% heat pump on
1% aux heat also on

Overnight for 7:15 total it was set back from 71 to 65, then reheated the condo:
45% heat pump on
7% aux heat also on

Set on 71 during the day for 3:55:
60% heat pump on
4% aux heat also on

I could probably take a closer look with my heat pump's performance on average over a longer period of time. Maybe I can find a way to trick the thermostat into starting the heat pump earlier so it uses the auxiliary heat less often.
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Toons
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Re: Heat pump - drop temperature at night or keep constant?

Post by Toons »

Heat Pump -70 during the day 65@night . :happy
"One does not accumulate but eliminate. It is not daily increase but daily decrease. The height of cultivation always runs to simplicity" –Bruce Lee
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just frank
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Re: Heat pump - drop temperature at night or keep constant?

Post by just frank »

I got a nest stat just to get better hp/aux control, which it advertised as the best on the market. After the class action lawsuits, they have scrubbed that claim from their website. After 18 mos playing with it myself, I concluded that it does no better than a cheap programmable stat at minimizing aux usage (its algorithms are quite primitive and not adjustable for 'swing' or staging control), and got an ecobee instead. The ecobee runs a lot less aux and makes the house much more comfortable due to the smaller swing.

To the OP, in your case, it is prob cheaper to not setback at all. If your bedrooms are too warm that way, close down the air register partially. When you replace the unit in the future, think about upsizing the unit to get a lower operating cost.
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Re: Heat pump - drop temperature at night or keep constant?

Post by Ron »

alec wrote:<snip...>The aux heat only comes on when the outside temp gets below about 10 degrees F.
Almost the same for us, where the backup kicks in when the outside temp get's to the mid-teens. Even then, the aux heat is only on for a few minutes to "kickstart" the heating. We keep our heat at 70 degrees, regardless of the time of day. Since we're retired, one of us is in our home almost 24x7 and up sometimes during what would be considered "normal" sleeping hours since we're no longer on set schedules.

I wonder if the OP's unit is working correctly since s/he says the aux comes on at the 40-degree temperature; or it may be that it's not a high efficiency unit that can't draw warmth from the outside air at a lower ambient temperature.

BTW, we pay $20.73/month for our HVAC contract, which includes twice a year operations check (summer/winter) along with a discount on parts and priority service if we actually do have problems.

- Ron
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just frank
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Re: Heat pump - drop temperature at night or keep constant?

Post by just frank »

HPs in the US are quite primitive compared to those in East Asia, where the latest tech in HPs was pioneered over the last 20 years. And the ones that are more than 20 years old are dinosaurs that often struggle below 40°F.
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Ketawa
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Re: Heat pump - drop temperature at night or keep constant?

Post by Ketawa »

Ron wrote:
alec wrote:<snip...>The aux heat only comes on when the outside temp gets below about 10 degrees F.
Almost the same for us, where the backup kicks in when the outside temp get's to the mid-teens. Even then, the aux heat is only on for a few minutes to "kickstart" the heating.

I wonder if the OP's unit is working correctly since s/he says the aux comes on at the 40-degree temperature; or it may be that it's not a high efficiency unit that can't draw warmth from the outside air at a lower ambient temperature.

BTW, we pay $20.73/month for our HVAC contract, which includes twice a year operations check (summer/winter) along with a discount on parts and priority service if we actually do have problems.

- Ron
There's a lot of variables that can influence the balance point at which a heat pump can no longer keep up with the home's heat loss. For example, an AC system should generally be sized to run as frequently as possible to remove humidity. There should theoretically be a couple days per year where it is unable to completely keep up with the cooling demand. This means that it was sized correctly. This will inevitably affect the heating ability for a heat pump.

My heat pump's technical information manual says that its COP is still above 1.0 at -10 deg F. However, there's no way it can keep up with my condo's heating demands when it is that cold. Maybe if it was a massive unit, but that would be an overpay and too large for my cooling needs.
Paleblue
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Re: Heat pump - drop temperature at night or keep constant?

Post by Paleblue »

I am at a loss when it comes to minimizing my auxiliary heat usage. During the day it is set at 65 degrees. In the evening it is slowly raised to 68°, at night it is at 68 degrees. DW likes it warm at night. It takes two hours for it to climb from 65 to 68 during winter. What is frustrating is that when I raise it from 65 to 66, the auxiliary kicks on. When I raise it another degree i.e. from 66 to 67 auxiliary kicks on. It is around 35 degrees outside. On the nights when it is a bit cool, let's say under 20, the auxiliary will turn on. I have a old unit but I am not sure if putting in a new unit will decrease my overall cost that much. Cost of new system is roughly six grand. Last electric bill was for $320 covering mid dec to mid January. The HVAC guy told me it will take many years to recoup to cost of a new system; I was advised to keep it going unless something breaks. Anyone had a new unit installed? If so, what what's the cost difference in the utility bill per month during winter heating and summer a/c season once your new system was installed?
drawpoker
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Re: Heat pump - drop temperature at night or keep constant?

Post by drawpoker »

I used to struggle with the same question. Then, our electric supplier went to the "smart meter" model last year, and now, we can log-on to our exact kilowatt usage and track it not only by the day, but by each hour.

Looking at these graphs convinced me to turn down the thermostat to 58 when I go to bed. That way the heat pump never comes on at all during the night, unless it gets colder than around 29-30 degrees. Then, it will come on and run very briefly, using less than 1.5 kw usually. When I get up, yes, I turn up the thermostat to force the heat strips to kick in. But usually this is only needed for 10-15 minutes (I have a small, 1,200 sq. ft. house)

Of course, this would only work for people who like to sleep in a very cool room, 60-62 degrees.

I must be doing something right, as the other advantage to the smart meter and obtaining hourly graphings over the internet - elec. company tells me how my usage compares with the other houses on my street (we are all the same, a townhouse development). For last month, my bill was $132. Elec. co. informed me that average user's bill on my block was $245.

Besides the preferences for sleeping cool or toasty warm, heat pump owners may make individual choices taking into consideration how high the local elec. rates. are. Here, we are getting ripped by Delmarva Power (PEPCO) with obscenely high rates.
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