Heat Pump confusion

[TheElectricLight]

I have a Lennox Heat Pump and a Honeywell Chronotherm IV 8611 Thermostat. I understand the concept of the heat pump. I am a little confused about the Auxilary electric heat though. I live in Connecticut and it's been ranging from 2F to 20F degrees at night. I have the thermostat set at 65F. Some nights the unit runs almost constantly. I had a service call yesterday and the tech said this is normal in the winter. He checked the Auxilary heat and it does work. He explained that it is only supposed to come on if the temperature is 2 degrees or more from the target temperature. If this is the case it seems to me that the Auxilary heat would NEVER be used as the heat pump itself will activate as soon as the temperature falls 1 degree below the setting. My question is does the Auxilary heat work by temperature or should it help the heat pump reach the desired temperature if it knows it will take an hour to get back to the deired temp. If I set the temperature 2 degrees or more higher the Aux heat kicks in, but if I leave it at 65 constantly it never comes on regardless of the outdoor temp. Is this normal?? I just want to make sure it's working the way it's supposed to be! Thanks for any advice!

PeterV

[Dr. Hardcrab]

It's normal.

If it is getting down to the low teens/single digits, it WILL run constantly. Hopefully the aux heat will cycle and not run constantly too.....

[Don McClimans]

That's true, but you're wasting a lot of energy melting the ice off the outside unit during the defrost cycle.

I live in western NY, so have similar or colder temps than Connecticut in the winter. We have a similar heat-pump/electric-furnace system, with a Chronotherm III.

When it gets that cold out, I switch the thermostat to "emergecy heat", which disables using the heat pump and just uses the furnace (auxiliary heat). This is as efficient (probably more efficient) than the heat pump at these temperatures. That's because a) the heat pump inherently gets less efficient as the temperature difference becomes greater, and b) to avoid ice build-up on the outside unit, the system has to run a defrost cycle. In the defrost cycle, the system runs backwards (essentially in air conditioning mode), to heat up the outside unit and melt off any ice. Similar to defrost in a freezer. All that energy is wasted of course, as you're literally heating the outdoors.

One thing to watch out for if you stay in emergency-heat mode for a long time. Snow and ice can build up on the outside unit. When you switch back to heat pump mode, the fan may not have any place to push the air to. So you may need to go outside and brush off the unit. (If you're running the heat pump all the time, the fan being on keeps the snow and ice from building up.)

Terry

[trader4]

"I live in western NY, so have similar or colder temps than Connecticut

in the winter. We have a similar heat-pump/electric-furnace system, with a Chronotherm III. "

Just out of curiousity, why do you have a heat pump system in that climate? I know heat pump systems can be economical and practical in moderate climates, but I would think natural gas or oil would easily win out in your area. Am I missing something?

[Don McClimans]

Yes, you're missing the weirdness of municipal electric systems and the american political system. :-)

I live in a town that has a municipal electric system. This means the system is owned, operated, and maintained by the town government. Municipal electric systems have a sweatheart deal which is somehow based on the thinking that they get first access to hydro power (think Niagra Falls). I don't really understand the logic of it (if there is any logic to it), but it's been in place for a long time and there's a political fallout if they change it, so nobody changes it.

(By the way, what we call a town here is what many other places call a township. It's bigger than a village, and smaller than a county.)

The net result is that electric costs me about 3.5 cents/KWH in the spring/summer/fall, and in the coldest 3 months it costs about 4.5 cents/KWH. This makes electric cost effective. Also, I live outside the gas distribution, so the alternative is oil, or propane tanks, I suppose.

Even in the midst of winter we get a fair amount of weather where the days are above 32 deg F, so we use the heat pump itself a fair amount even in the winter. In the spring and fall it's the only thing that runs. In the summer, of course, it's the A/C system. But even when we use the electric furnace rather than the heat pump, it's pretty inexpensive to run.

Within this town, most people have electric heat pumps with electric furnaces. Sometimes you will see heat pumps with oil furnaces. A few miles away, when you get to the next town, you're using a commercial electric supplier, where rates are > 12 cents/KWH, I think -- I'm not really sure, it's been 12 years since I moved here. Anyway, in other towns, everybody uses gas or oil, and no heat pumps, just A/C.

My house, built in 1984, has an unused flue in the chimney intended for an oil furnace if the electric rates someday skyrocket.

Now, don't blame me! I did't make this crazy thing up, and I didn't move here just because of the cheap electric rates, I just bought a house here. :-)

Terry

[Stretch]

Heating Capacity of a heat pump drops as it gets colder outside. The heat load load on the house goes up at the same time. Around 35 degrees oiutside the heat pump's heat output drops below the heat load of the house. This is called the "balance point". Below this outdoor temperature the heat pump will run constantly.

Note that at -20 degrees outside the COP (Coefficient Of Performance) Of a heat pump will be about 1.5. The COP of strip heat is ALWAYS 1.0! Therefore, switching to emergency heat will increase your electric bill, as with both heat strips and heat pump running your average COP will be around 1.25 at -20 degrees outdoors. Not great, but better than heat strips alone. Also since swithcing to emergency heat drops all the heat from the heat pump mout of the system, you have just lost some of your capacity and the system may not be able to keep up.

Terry,

The outdoor coil freezes because the outdoor coil gets colder than the outdoor air in order to extract heat from it. If the outdoor coil temperature is below the dew point of the outdoor air, frost will form on the outdoor coil. Leaving the system in emergency heat will shut the compressor and outdoor fan off, so the outdoor coil will be at the same temperature as the outdoor air. NO frost will form on the outdoor coil unless it is from horizontally falling snow.

When the heat pump goes into defrost, the strip heat comes on inside to temper the air, the reversing valve switches to cool mode so the outdoor coil will get hot enough to melt the ice and the outdoor fan will shut off so most of the heat in the outdoor coil goes to melting the ice (latent heat of fusion at 144 BTUs per pound) and very little goes directly into the outside air.

The fan running on the outdoor unit CAUSES the frost to form in the first place, not prevent it form forming. If the outdoor fan was not running, VERY LITTLE heat would be extracted from the outdoor air.

Stretch

[RP]

In the south the thermal balance point typically runs much lower than this. The unit is sized for cooling -- backup strip heat is rated as necessary to provide sufficient heating capacity (with heat pump disabled). As a result the heat pump is oversized by northern standards. It isn't unusual for a heat pump in the south to keep up with the load at 15ºF or even lower without backup. Your argument is good when taken as an example, but not as a general rule. I'm just clarifing for any readers who might fail to take into account your implied context.

For an average heat pump. But I wonder about that number :)

hvacrmedic

[Don McClimans]

Only if there is no back-up heating unit. In real systems, below this outdoor temperature the heat pump and resistance heating will both turn on, but when you reach the setpoint temperature, the heat pump will turn off.

Does this calculation of the COP include the defrost cycles? Or are you saying (below) that the cost of the defrost cycle is negligble even at -20 def F?

I believe units installed in northern climates are sized so the back-up furnace can handle the entire heat load.

Yes, I thought this is what I was saying. If you switch to emergency heat, no ice forms on the outside coil.

That heat that goes into melting frost ends up in the outside air.

In practice, what I see in very cold temps is that the defrost cycle is *very* frequent. It's hard to believe that I'm actually getting any benefit from the heat pump.

Yes. What I was trying to say is that when you switch to emergency heat, the heat pump turns off and so the outside fan stops. If you stay like that for a couple of weeks during a cold spell, and there is lots of snow or freezing rain, etc., you may end up with a crust on top of the unit that you have to manually remove. The fan alone (when you turn the unit back to heat-pump mode) cannot blow away the snow and ice. So that's a disavantage of switching to emergency heating mode. If you leave it in dual mode, you may be getting zero or negative savings out of the heat pump, but at least the blowing outside fan keeps the snow and ice from building up on the outside unit.

Terry

[Peter V]

Thanks for all the replies, I am still learning a lot about these heat pumps. One thing I am still stuck on, I know my Auxilary Heater is operational only because If I manually raise the temperature more than

2 degrees, I can see the indicator and I feel the hotter air. However, in normal usual operation, I never see it come on at all, no matter how cold it is outside and how long the heat pump is on. From what I can tell it only works off the heat pump. The serviceman told me the only time that the auxilary heat will ever come on is if the room temp falls 2 degrees below the set point. Is this correct?

Thanks again,

PeterV

[CBHVAC]

Depends on your thermostat, your heat pump, if you have an outdoor thermostat...

[Stretch]

Sorry RP.

My 35 degrees is based on most of the load calculations I do a nd the high and low temperature ratings of the heat pumps I use. Sometimes the balance point is as low as 30 degrees outdoors, but I have never seen a balance point of 20 degrees. But I am basing that on the climate in Myrtle Beach, SC. Apparently in your part of the south the balance point is different. I suppose the ocean has a moderating effect on the outdoor temperatures around here. Every climate has it's own balance point. Best to do your own calculations.

The heat pumps that I checked for that COP of 1.5 @ -20 degrees were rated 12 to 13 SEER and 7.1 to 8.5 HSPF. 10 SEER units would be lower in HSPF and heating COP as well.

Stretch

[Stretch]

"One thing to watch out for if you stay in emergency-heat mode for a long time. Snow and ice can build up on the outside unit. ";

I took that to mean you thought the frost would build on the outdoor unit with the thermostat was set to emergency heat. Seems you ment something else.

The HSPF is the number you look for as far as heat output including defrost cycles. You want the region III HSPF. In my area, we use the region IV HSPF due to our higher average temperatures. If the HSPF was

3.4, that would be the same efficiency as heat strips. HSPF is BTUs per watt. Most heat pumps, even the builder grades will produce a HSPF of 6.8 or higher in my area. In your area it will be a bit lower.

The COPs do not include a deduct for the cost of defrost, but according to the manufacturers I have talked to, you are better off letting the heat pump run, even down to -20 degrees outdoors.

Around here we set defrost to between 60 and 90 minutes per cycle. Most defrost systems will terminate after 10 to 11 minutes even if the OD coil is not completely clear.

Of course, if you are only paying 3.5 cents per kwh, who cares about efficiency. If electricity is free or very cheap, it is hard to justify high efficiency. we pay 6 to 8.5 cents per KWH around here, so that changes the equation. I have talked to people paying around 17 cents per kwh! OUCH!!

Stretch

[Don McClimans]

That's how my Chronotherm III works, I would guess your IV is the same.

Terry