# Help Me Understand Heat Pumps



## kcarter609 (Nov 14, 2012)

Hi everyone,
I hope this is the right forum to post in. I'm renting a house currently. It's new construction, built in 1999/2000. It's pretty well insulated, and uses a heat pump as its heat source. Now, I have a big problem (or what I believe is a problem anyway) and I don't understand why this happens.

If I set my thermostat (it's an ancient looking analog one that looks like it belongs in 1980) to say 60 degrees for heat, it kicks the heat pump on until it hits 60 degrees. Then, the heat portion shuts off but it still circulates air. Well, that runs my electric bill up just as fast as if the heat was on. Why does it not shut off when it hits 60 degrees? I mean, it might shut off after a couple hours, but that doesn't help when it uses 100 kWH per day and sends my electric bill through the roof.

They just checked the heat pump 2 weeks ago when they came to fix a leaking pipe under the house, and said everything was good.

Also, why does the emergency heat kick on sometimes? I was told it's not supposed to turn on unless the temperature is 10-15 degrees below what you have it set at when it gets turned on. Sometimes it'll kick on for that, but even if it gets within like 1 or 2 degrees of the set temperature, the emergency heat won't shut off.

Lately to cut back the power usage (and it's working big time) I just leave it off during the day and when it drops to about 55 degrees, kick the heat on for maybe 10 minutes. This happens a few times a day, keeps me relatively warm, and has cut my electric usage to about 10-15 kWH per day instead of 100 kWH per day. I am not exaggerating on the usage. Meter read 89011 on 1/18/14 and as of 1/24/14 it was reading 89633. As soon as I shut off the heat pump, the usage goes to almost nothing.

I've always been told heat pumps are super efficient, but then someone said in very cold temperatures they're horrible. Now I don't understand why all of this is the way it is, and my experience with them is very limited.

If I could figure out how to make the heat pump shut off when it hit the set temperature, I'd be able to set it at 60 or so and leave it on during the day so my house isn't 45 degrees when I get home and I wouldn't have a power bill that's astronomical.

Thanks,
Kevin


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## MushCreek (Jan 7, 2008)

It might be the switch on your thermostat. Some have 'Fan Auto', for instance, which should shut the blower off when the heat pump stops.

What are your outdoor temperatures? Heat pumps generally only work down to about 35 degrees or so, although recent technology has improved on this. If it's below the point that the heat pump can make heat, there are supplemental heat strips that use resistance heating so you don't freeze to death. These use a LOT of electricity!

It's hard to diagnose from a distance, but your thermostat might be set wrong, and you might be using a lot of resistance heating if it's too cold outside. Heat pumps were touted as saving a lot of energy, which in FL they do. NJ? Not so much.


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## kcarter609 (Nov 14, 2012)

Sorry I moved and haven't updated that. Just did. I live in VA now. Outside temps have been below freezing for the past 2 weeks or so. Single digits in the mornings. Uncommon, but caused by that arctic air. The thermostat has the fan set to auto, the emergency heat setting is on normal. I'm wondering if the thermostat is so old something is just wrong with it.


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## Guest (Jan 28, 2014)

The fan continuing to run is a function of the lower limit switch.


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## kcarter609 (Nov 14, 2012)

zong said:


> The fan continuing to run is a function of the lower limit switch.


What is this? How do I adjust it/make it stop?


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## Harry Chickpea (Dec 19, 2008)

If I set my thermostat (it's an ancient looking analog one that looks like it belongs in 1980) to say 60 degrees for heat, it kicks the heat pump on until it hits 60 degrees. Then, the heat portion shuts off but it still circulates air. Well, that runs my electric bill up just as fast as if the heat was on. (NO it doesn't.) Why does it not shut off when it hits 60 degrees? I mean, it might shut off after a couple hours, but that doesn't help when it uses 100 kWH per day and sends my electric bill through the roof.

Once the setpoint temperature is reached on the tstat, the fan should shut off... generally. If you bump the tstat to a colder temp and it doesn't shut off within two minutes, you have a problem - possibly a sticking contactor.

Also, why does the emergency heat kick on sometimes? I was told it's not supposed to turn on unless the temperature is 10-15 degrees below what you have it set at when it gets turned on. Sometimes it'll kick on for that, but even if it gets within like 1 or 2 degrees of the set temperature, the emergency heat won't shut off.
 
Emergency heat is not "emergency" in the sense that it is an emergency when it comes on. It is more accurately the back up heat (often electric heat strips or a propane furnace) that comes on under certain conditions. Three or four degrees difference between the setting on the tstat and the inside air temp will kick in the backup heat. Why? Because people moan and groan about "cold" air coming out of vents when they want the place to heat up quickly. You want quick heat? We give you quick heat. People freak out when a heat pump runs constantly, thinking that whenever the fan is going there are major dollars being used. It doesn't work that way.

Buy one of the non contact infrared thermometers I mentioned in a different thread. Aim it at a duct with "cold" air coming out. You may find that it is actually 75 or 80 degree air that FEELS cold because the dry air blowing has a wind chill factor on even marginally moist skin. 

In addition to the backup heat coming on when there is a 4 degree difference, it MUST come on periodically when the heat pump goes into air conditioning mode.

WHAT? Air conditioning mode in midwinter? And that is NORMAL? YES!!! During low outside temps, when the outside coil is gathering heat from the air, the temperature on the coil can get below the dew point and below freezing. If there is any moisture in the air, it will collect on the coil and eventually freeze it over with a sheet of ice, making it not only inefficient, but dangerous to the compressor. To prevent that, every few minutes the heat pump reverses the refrigerant flow and heats the outside coil to melt any ice and dry off any moisture. That is normal, but it means the system is blowing cold air for about five minutes or less. The backup heat is strong enough to override this.

Lately to cut back the power usage (and it's working big time) I just leave it off during the day and when it drops to about 55 degrees, kick the heat on for maybe 10 minutes. This happens a few times a day, keeps me relatively warm, and has cut my electric usage to about 10-15 kWH per day instead of 100 kWH per day. I am not exaggerating on the usage. Meter read 89011 on 1/18/14 and as of 1/24/14 it was reading 89633. As soon as I shut off the heat pump, the usage goes to almost nothing.
 
You don't mention how many ton the unit is or how big the house is. You also don't mention what normal usage is historically.

If your electric bill doesn't give power usage for the same month last year, call the power company and find what actual usage was a year ago.

Last month we used about 2100 KWH and I suspect this month will be much more with a 4 ton unit and inside temps of 72 during the day, two space heaters going at night, plus regular loads (hot water, lights, electronics). You are in Virginia, where it is much colder in general. 100 KWH per day midwinter doesn't surprise me. Cold climates are costly. You could change to a propane or natural gas backup system, but unless your power costs are greater than about 15 cents per KWH you will end up not saving any money.


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## Guest (Jan 29, 2014)

If you look up a diagram of the model of heat pump you have, there will be several "relay" or "limit switch" shown. If you don't have some basic understanding of those sort of things, you'll get lost real fast though. I know that the last 20 years or so, with so much computerization, I don't understand them myself any more.


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## ct01r (Jan 21, 2014)

Mr. Chickpea (may I call you Harry?), I love the detailed explanatons that you give! They are simple and easy to understand. I have a question about kcarter's fan, though. On our heat pump, our fan continues to run after the temperature is met in order to pull as much heat as possible out of the "system" and discharg it into the house. (Ours is geothermal, maybe that's the difference.) Is it possible that his system is designed to continue to blow warmer than room temperature air after the themostat kicks off until all of the heat in the heat pump is exhausted? Curt


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## Harry Chickpea (Dec 19, 2008)

You can call me anything as long as you don't call me late for dinner. In answer to your question, yes. It is (also) a safety feature when the fan keeps running after heat strips are powered off, and an easy way to do that is to measure the temperature of the downstream air. Anything above a set temp (could be 80 degrees, 90 degrees, 100 degrees, don't know offhand) will scavenge that last bit of heat and keep the air handler from getting too hot for no purpose.


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## ct01r (Jan 21, 2014)

Thanks! Curt


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## Evons hubby (Oct 3, 2005)

Heat pumps (like aluminum siding) tend to be very efficient at transferring dollars from a homeowners pocket to the pockets of the salesman.


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## Clod Kicker (May 7, 2012)

" the heat pump reverses the refrigerant flow and heats the outside coil to melt any ice and dry off any moisture. That is normal, but it means the system is blowing cold air for about five minutes or less. The backup heat is strong enough to override this."
Sorry, that's wrong.
The indoor fan motor is not supposed to be powered during defrost. Neither is the outdoor fan. When defrost is complete sometimes one sees a surge of steam blown off the outdoor coil when the fan restarts. Looks awesome. The indoor fan should restart also, when the indoor coil heats up again, usually to a set point on a low limit fan control.


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## Clod Kicker (May 7, 2012)

Upon further research, I stand corrected....
"When the defrost cycle energizes in the heat pump sequence of operation the reversing valve switches mode to the air conditioning mode switching  the whole heat pump system into an air conditioner in the cooling mode. This is necessary to defrost the condenser coils but it is counteracted through the defrost control by the defrost control. The defrost control energizes the back-up heat* so the heat pump continues to provide heat. This timed cycle in the heat pump sequence of operation ends after a certain amount of time as determined by the manufacturer of the heat pump and everything switches back to normal heat pump mode."

* Backup and emergency heat are the same.

Credit to this site, and a link to further elucidation => http://highperformancehvac.com/heat-pump-sequence-operation/


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## Harry Chickpea (Dec 19, 2008)

LOL! OK. You just learned a lesson I learned a few years back - check before posting if you have any doubts at all. Welcome to the club.

FWIW, a couple of days ago my heat pump gave off an *AWESOME* plume of cold steam / fog when it went into that mode. I was just coming back from getting the mail and was suitably impressed with it doing its job. (I thanked it.)


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## Harry Chickpea (Dec 19, 2008)

Yvonne's hubby said:


> Heat pumps (like aluminum siding) tend to be very efficient at transferring dollars from a homeowners pocket to the pockets of the salesman.


You don't like heat pumps. I get it. It is simpler to say that you don't like heat pumps. (I think I may have a few salesmen buried out back, but I don't remember where.)


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## Clod Kicker (May 7, 2012)

That's worth a chortle........or two :clap:


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## fullmetal (Nov 2, 2013)

air source heat pumps are efficient in the southern states but anywhere north of say the 38th latitude or so ground source is better dollar for dollar.


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## ct01r (Jan 21, 2014)

fullmetal said:


> air source heat pumps are efficient in the southern states but anywhere north of say the 38th latitude or so ground source is better dollar for dollar.


We have a ground source and love it! The only time it can't keep up is when it's down in the single digits for several days. We use the wood stove to supplement it then. The A/C in the summer is also super efficient. Curt


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