This question is about my own home (I work in energy policy and have only a passing knowledge so far of actually working on home equipment). But I know this is a very knowledgeable group and I couldn't find a definitely take via google. I know this is out of the usual discussion topics, but maybe a good case study? :)

Here goes: I noticed a week or so ago that my programmable thermostat read "aux mode". A little investigation revealed that whenever the heat has been coming on, it has been blowing cool air for a minute or two, then switching to auxiliary (which I know is resistive heat). I didn't notice because it was still making the apartment comfortable. I believe this explains why my last few bills have been high, and I'm embarrassed not to have noticed earlier. So, my basic question is what could be wrong? I would assume the compressor is bad and call an HVAC person, but there is one potentially complicating factor: last fall, I signed up for Pepco (my utility) demand response, and they affixed something to my external unit. So - is it possible that is incorrectly not allowing my heat pump to run in normal mode? Or, is it the heat pump itself that is to blame? Is my first step to call Pepco to see if they are messing up my system, or get in an HVAC professional? Even better, is there anything I could check myself? Appreciate any suggestions!

Some other facts:

The fan in the outside unit does turn on when the heat pump tries to run

The unit is only ~3 years old, the programmable thermostat 2 years.

My unit is an english basement, ~800 sq feet that is usually quite cheap to heat and cool (~$50 a month electric utility in most circumstances, recently more like $90)

We have smart meters, so I can see in the last month my peak has been (in the morning and evening when the heat comes on ~4kW for an hour at each peak). My daily average when letting the unit run in the last few months has been ~22kWh. In the October-November timeframe it was 10-15.

I have csv files of representative periods if that would help!


Austin Brown

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It seems like the device installed by the utility would allow them to prevent resistance heat from coming on, assuming it's a load management device. That way they can manage demand during short periods when they have too much. You should call them and ask what the intended function is, and what the parameters are.

It should be fairly easy to find out if the compressor is working. When the system is running, go to the outdoor unit, or to the air handler, pull back the pipe insulation, and put your fingers on the larger of the two copper lines. If it's hot, the compressor is working, although this is not a way to check if it's working right. Do this when the system has turned itself on, don't bump the t-stat to get it running. You may have to wait a bit for the lines to get warm.

Another thing to do is to read the load on your meter when the heat is running--that should tell you whether the heat strips are on or not. If your meter is like mine, there is a way to time it and determine the load. Heat strips would pull several thousand watts, maybe even 10-15 kw  or more, so the meter would be zipping right along. An easier way to do this is the measure at the electrical panel, but you need a clamp meter to do so. 

Anyway, you could do some detective work and get a picture of what's going on in various circumstances. I always try to figure out what the system does when (a) it turns on by itself, and (b) it turns on because I bumped the t-stat +1 degree. What I find is that a lot of systems go into strip heat right away if I touch the stat, even if the outdoor temp is relatively warm.

What you may ultimately need is a better t-stat, one that has an outdoor temperature sensor and heat-strip lockout function. You probably need an HVAC tech to handle that. 

Depending on your skills, it may be wise to have an HVAC tech check out the whole thing.

All Heat Pump Stats that I use will turn on strip heat W2 when in defrost mode, If not you will get a lot of late night calls.  Defrost is taking heat from inside to melt the ice on outside coil, with out using strip heat the air temp off the coil gets below skin temp and feels cold - then I get a "I am cold call".

If you get a tier 1 Heat Pump the controls will not let the heat pump run defrost till its needed or ice on coil(28').  I have taken a switch or breaker to keep the strip from coming on, just turn on in dead of winter not in spring or fall  You can spend $50-$100 a mo just on strip heat.  If the defrost happens just in day light you will have short time with strip,  most defrost happens in night.   Some controls just use time to turn on strip that's the low end, high ends will use time and temp.  Old controls use  presser also but bugs get into the hoses.  

The COP just drops with strip heat. the new low temp Heat pump will use gas by pass to warm up the coil so strip will not be used

Yes, I think a call to Pepco would be a smart first step, since your unit is relatively new and has worked properly for two seasons prior to the installation of their device. If nothing else, they should be able to identify what the problem is, in the course of ensuring that the problem isn't theirs. Beyond that, the David's suggestion of feeling the refrigerant lines to see if the compressor is functioning is a good one.

We often don't install any strip or backup heat at all with our heat pumps, and we've never had a complaint (3,000 HDD climate and design temp about 30F). The issue is in air distribution, and if you don't design correctly for a mixed air system, then the user satisfaction rate will be low. We have logged data of our units running in AC mode (defrosting outdoor coil) for 10 minutes with outdoor temps in the 20's and the people are unaware because we only introduce higher velocity supply air within the unoccupied zones of the room (above 6' and around the 1' perimeter of the room). The occupied zone contains only comfortable air moving at <25 fpm.



The heat pump / back up relationship is an easy one.  If the house won't heat with the heat pump, the back up comes on.  On a cold day (in your 5,000 degree day climate), the heat pump can't keep up and the temperature slowly drops to about 1-2 degrees below the thermostat setting, which then triggers the back up.  You can also force the back up on by asking for 1-2 degrees higher than the room temperature.

The "auxiliary" and the "emergency" lights both tell you the back up is on.  The difference is that the "emergency" switch cuts off the heat pump so it won't damage itself in an "emergency".

If the large pipe at the air handler does not get hot (hot enough that you can't hold your hand on it is best), it looks like one of two things is happening.  FIrst your heat pump is broken (tripped breaker??) and  the symptom is the cold air that blows for a minute while the thermostat figures out that the heat pump isn't keeping up before it adds the back up.

Second is the new smart thermostat has somehow locked out the heat pump, forcing the back up to run.  This thermostat is not sophisticated enough to prevent the back up from running.  It just allows the utility to turn off your entire system in times of need.  These will typically be in the hottest part of the summer when demand is highest, rarely in winter - that is why they give you a lower price in the winter because they have excess that they built for summer and they want to encourage you to use electricity to heat.

Did the house air condition properly last summer?  On your bills, can you tell what month the problem started?  Does it correspond with the installation of the thermostat?  My bet is with a broken heat pump.  

Let us know what happens

Ed Minch

Thanks all for the helpful responses. I followed the suggested diagnostics and found that there might be a little warm but not very hot to the touch. The fan on the outside unit does run, and the compressor sounds like it's running (though maybe a bit rough).

It has been running in aux mode even when I set the setpoint to be 1 degree over the measured indoor temp. It hasn't been below freezing here in general, and previous winters when it has been significantly colder for periods this never happened. I have had the thermostat for ~2 years and it has worked fine last summer and previous winters.

Pepco's demand response folks have been really helpful when I called to check, and will be out to make sure. They think, however, that the DR unit is unlikely to be to blame. They said that if it were on and preventing the unit from running it cuts the electricity to the unit so the fan wouldn't run, which makes sense to me. They are going to stop by and check, but I'm assuming I need to get in an HVAC professional to check the compressor function.

So, thanks to all your input my confidence is pretty high that it's a real problem with the compressor. On the better t-stat, I have been thinking I might try out the Nest since I think it's a creative business model. It has me wondering if this would be a good time to do it, although that I do feed comfortable installing myself since the wiring seems clear. Have yours or your customer experiences with it generally been good? Worth the expense? (For me, part of the goal is wanting to walk the walk with newer EE tech).


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