Heat

DRE_79

I live in northern VA. We just had a new unit installed in 2018. We have a heat pump and  in the winter when the temperature falls below 32 our unit goes into AUX heat mode stage 2. However the temp could be set to 68 but the temp shows less like 64 and the air coming from vents seems cool. 

We’ve had two different HVAC companies come out and they both said nothing is wrong with the system. The last guy said we needed an extra return in the basement which we did but still the same issue. It seems there is an issue with the emergency heat because when temps outside are above 32 the system works fine. Does anyone know the issue?

DRE_79

JUGHNE

So below 32 degrees the heat pump locks out...does not run?

If so then you rely on the electric elements in the air handler/furnace,
you may have 2 elements...maybe 10 KW of heat. Sometimes 3 for 15 KW.

Perhaps only one element is engaging giving you lukewarm air.

A simple check with a clamp on meter would tell.

There are simple controls for staging elements.

Perhaps when it was checked, all the elements were engaged but some dropped out after a short time. (either the tstat did this or the staging controls in the air handler-or only one element is working at all)

Your tstat may be too "smart" and thinks it is saving energy....which it is....at your discomfort.

Has this always been this way since day one or is it something recent?

Jamie Hall

An even more basic question... is 10 KW of heat -- about 34,000 BTUh -- even remotely enough to keep the place warm?

HVACNUT

How is the Nest wired?
What's on W1, W2, aux, (E) emergency?
O or B is energized or de energized depending on manufacturer, and Y is your heat pump. W1 should energize at the offset temperature and run with the heat pump, not instead of it. Depending on wiring and manufacturer, W2 will energize during defrost, W1 and W2 for emergency heat.

pecmsg

Loose the nest. 

Run the Heat pump all the time. 

Minimum COP with a heat pump is 1.1 - 1.25 to 1. Straight electric is 1 to 1

EBEBRATT-Ed

I would ditch the nest as well. If the heat pump is in defrost below 32 it will blow cooler air (it runs ac to defrost) as @JUGHNE mentioned maybe the electric heat is not set up properly or one element is bad

JUGHNE

Jamie's question of if 10 KW producing about 34,000 btu being enough heat for the house. If the house is comfy down to 32 with only the HP, (we assume no back up elements on). Then my best guess is that 10 kw will heat the house.

IIWM, I would run the HP down to maybe 20-25 degrees before going over to the elements. It would depend upon how many defrosts are required. Temp and humidity determine this. The Nest should cycle some heaters on if needed should the HP fall short of adequate heating.

And I would take 1:1 resistive heaters over HP at 1to1:1 COP.
The wear on a compressor and doing defrost cycles IMO, are not worth the small energy savings. Plus higher temp from elements brings added comfort, for this is the business of indoor comfort, if I recall.

I suppose I am of the crotchety point in life where I want comfort and can afford a little more expense to be there.

As a kid I was happy to not have a glass of water at my bedside freeze, I wished we had more than one dog, and I kept my clothes in bed with me.

As for my "green" report card when I was a Freshman, 40 years ago, I would have gotten high marks......now being a Senior I probably am failing in that respect.