Anybody?

There has been a lot of mixed success with Nest thermostats. One thing that may be happening is the Nest the trying to learn your living patterns. It may be may be making poor choices. You should turn that function off.

Do you have an external temperature sensor hooked up to the Nest? If you do, then you want to lock out the auxiliary heat when the outside temperature is above the 35-40 degree range. You will have to experiment what is the right outdoor temperature. If you set it too low, the heat pump may not be able to provide enough heat to keep the house at 68 degrees. Setting it too high will waste energy.

Your night time set back may be causing a problem. I don't own a Nest, so I can't tell you what it is trying to do during the set back recovery. It may be turning on the auxiliary heat in order to reduce the recovery time. If the auxiliary heat was locked out then this would not happen.

Thanks Mike,

Yes, I learned quickly to turn that function off. We get up and come home and go to bed essentially the same time every day (boring people, I know). So setting a manual schedule was easy.

I do nto have an external temp sensor with the Nest. My Trane thermostat connected tone, but the Nest doesn't. It uses the internet to get the outside temperature from Weather Underground. It is usually within a degree of the thermometers I have found my house, so I don't think that is a big issue.

We always had our Trane thermostat set to drop back to 60F at night (9PM -6AM). I have aux heat set to lock out at 35F.

It sounds like you have covered the basic things. Can you confirm the Nest is actually locking out the auxiliary heat?

Above 35? Yes. My concern is that it is running the compressor and the aux heat, even when it gets down to 10-15F. I can set the compressor to NOT run below a certain temperature if I choose. Does running the compressor use enough energy that it would be beneficial to disable it below a certain temp? I can't imagine it is accomplishing much at 10F anyway…

The COP (coefficient of performance) drops as the temperature goes down. This means it is providing less heat. The heat strips should be sized so that it is able to heat the house by itself at low temperatures. If that is the case, then I suggest the compressor be locked out in the 15-20F degree range.

That is what I have been looking for… the right temp to lock that out on the low side. Thank you!

I think your eight degree setback is too much for a heat pump. Since the outside temperature is generally higher at 9 PM than 6 PM, you are passing on relatively cheap BTUs from the heat pump at 9 PM and asking it to make it up with more expensive BTU's at 6 AM. In that time, not only the air temperature, but the walls, floor, furnishings, etc can lose their heat and must be brought up. I have a heat pump and never set back, but would not recommend more that a 3 degree setback.

A good heat pump can produce almost 2/3 of its rated BTUs at 17F, still better than resistance heat. I never turn off the compressor.

What time does set back end? How long does it take to go from 60 to 68 degrees when the temperature is in the 20s?

Set backs on heat pump are not a good idea if you have the type of thermostat that turns on the auxiliary heat whenever the room temp and the setting are greater than 2 degrees. If you can control when the auxiliary heat is on, then I don't see the problem with having a set back assuming you can recover within a reasonable amount of time.

Mike,

In the 20s, it will recover in 30 minutes. At 10 degrees, it may take 90.

Besides just saving energy, I don't like sleeping in a room that is anywhere close to 68 degrees. But I don't like aux heat running for 90 minutes either…

I wish the Nest would replicate the algorithm that Trane uses. It got the house warm quick enough, and did not break me in the process. I love the remote capabilities of he Nest, but it has cost me a fortune.

Dove,

I have read that never turning off the compressor doesn't really use much energy. Is that your experience?

Nice,

I don't have any with/without type of data, but this is where I draw my conclusions. Last winter, even when the temp went into single digits (Northern VA), I rarely saw the Aux heat go on, and then mostly for defrost cycles (I can see my compressor out the window). I don't have a fancy thermostat with all kinds of statistics, but it has two bright lights which light up when the Aux heat is on. We have an old heat pump. Admittedly, the heat pump's ability to supply heat without Aux depends on a number of factors - indoor setting (ours is 67F), the relative size of the pump compared to the heat loss load (insulation, air leaks, windows, etc) and the characteristics of the particular heat pump in cold outside air temp. Ours is a three ton for about 1750 Sq Ft single story, cathedral ceilings, 1983 original insulation.

One of my main reasons for not wanting the heat pump to go off is that I believe that it puts a strain on the heat pump to start in the cold after sitting for hours (once again, I have no firm data, just a belief). That is also why I don't like nighttime setbacks. We had one stretch last winter when the compressor ran continuously for more than three days, night and day, yet my bill was reasonable.

When you say you did not have extreme weather, are you sure that it wasn't colder in your area than past years? Our 2015 February mean temp was 7 degrees cooler than last year. How much worse was your electric bill?

I went to weather underground to check.

January 2014 had 32 heating degree days, 2015 had 27.

February 2014 had 26, 2015 had 32.

March 2014 had 17, 2015 had 14.

So February was colder, but January and March were warmer. My bill for all three months was over 30% higher than the same months in 2014.

Wow, with that kind of increase, I would suspect there is something wrong with the heat pump, not just a thermostat setting. Has the pump been inspected?

Yes, we discovered (and fixed) a leaky return air duct in the attic. Wanting to make sure no insulation had gotten sucked in, I had it cleaned the middle of January.

The unit was five years installed December 31, 2009.

When was the last time the heat pump was serviced? You may be low on refrigerant and not realize it. The heat pump is not going to operate as efficiently with a low refrigerant level.

Does the NEST use a 'smart recovery', ie uses heat pump only on recovery from a setback or does it does it fire up auxiliary heat to get the temp up as soon as possible?

It uses the auxiliary heat if needed.

The question is, when does it think it is needed? Like all the time?

The gripe I've heard about the Nest and heat pumps is that is uses auxiliary a lot. Where some thermostats use heat pump only during recovery from a setback, the Nest sees a big temperature change and immediately turns on the auxiliary heat to meet the new demand as soon as possible. This causes people's electric bills to get 'rather large'.

I believe that is my issue. I think I am just going to go back to the Trane thermostat. It always worked flawlessly.