Heat pump question

[supersurgilator]

My grandparents mobile home in North Carolina has a packaged 3 ton Janitrol heat pump with a 5kw supplemental heat strip. I am still confused about how these things work as this thing doesn't really heat that well. Granted it was exceptionally cold when we visited them, lows in the teens and low 20's. When the actual heat pump is running, the air is warm, but not as hot as I am used to with our gas furnace. However, every time the heat pump goes into defrost mode, the thermostat will illuminate that the heat strips have been activated, however the vents just blow cold air. The defrost cycle lasts anywhere between 5 and 15 minutes, and it seemed that it defrosted way too often (again not sure if it was due to the cold temps or not). The unit regardless runs constantly all night long, but it doesn't have any trouble maintaining the temp during the daytime. My experience has always been that heat pumps are really made for places like Florida where it really doesn't get that cold. Anybody have any experience with these to see if this seems to be proper operation or not.

 

[neptunebob]

We had a heat pump before it was replaced 3 years ago with a traditional furnace/air conditioner and at least for the Pittsburgh area, I did not think it was all that great though newer ones may work better.

Actually, ours was the heat pump with a gas furnace backup. There may be an adjustment on the unit that determines what temperature the heat pump shuts off and the backup heat comes on. With our unit, on a sunny cold day it could not make up its mind. It would run as a heat pump, cannot handle it, on comes the gas furnace, decide it could heat pump, the heat pump comes on, cannot handle it, do you get the idea? All the clicking, strange noises, and cold and hot air was annoying.

A heat pump will not have as hot air as a furnace or electric elements because it is moving heat "uphill" from where there is less heat energy an area where there is more. There is no combustion process as with a fossil fuel or use of energy directly. When the element is on, you consume 5KW of energy from the power company and get 5KW of heat energy in the house but the idea of a heat pump is that with 5KW of work from the power company you will receive 10-15 KW of heat that is moved from the outside. It just doesn't feel that way because the output temperature is not much higher than your 98 degree body temperature. Actually, this is somewhat helpful if there are seniors in the house - it does not dry their skin out the way a furnace can and there is less static electricity.

The 3 tons refers to the units cooling capacity and heat pumps are sized for summer air conditioning loads. It is possible that your heat pump is just a little too small for the winter difference. It is possible that you could use larger heat strips (if the utility will let you).

When a heat pump defrosts itself it turns into an air conditioner for a few minutes and so turns on the heat strips so as not to chill the house. There are sensors to start the defrost cycle and if it is cold and damp outside it may have to defrost more often.

This is what I know, if you have questions just ask again. Hope this helps.

 

[powerfin64]

alot of variables to this..

how old is the system? is it a matched system(IE: all the same brand and age?)Has it been serviced in the last couple of years, checked for freon level, ect? Is the air filter clean/changed recently?

The heatpump, depending on the tonage of the unit, could be too small for the SQ feet to heat/cool. A heatpump, pulls warmer air from the outside, and pumps into the home, which is going to be cooler air than electric heat. When its below freezing, it can run non-stop to attempt to keep the temp at the set level desired, and will go into defrost mode as needed. IF its going into defrost mode alot, I would switch it to the AUX heat mode, (and have it serviced) which will be more cost effective in below freezing temps outside, otherwise, all your doing is running up your powerbill MUCH faster having the heatpump trying to keep the set temp when it can't effectively.
The electric strips should come on in defrost mode, to maintain the desired heat setting, then go off after defrost mode is done.
If the heatpump is 7 years and older, I would look into a newer one, they are now MUCH more cost effective to use.

Rich

 

[DADoES]

 
The heat strips should be sized to fully counter the cooling capacity of the unit when it switches back to air conditioning during defrost. If the unit blows chilled air during defrost (check the output air temp at a register against the room temp at the return-air grill), then 5KW of heat strips is too small.

Weather conditions have a very large effect on defrost frequency, of course. Rainy, wet, snowing, sleeting, defrost will occur much more often than during a dry cold. Two kinds of defrost control are common. Timed defrost triggers a defrost cycle after a set number of hours of run time, whether it's really needed or not. Demand or sensed defrost has the control board check whether a thermistor or temp sensor in the outdoor coil indicates frost accumulation.

To be more accurate, a heat pump doesn't pull warm air from outside to inside ... it pumps heat from outside to in. Think of it as installing a window unit into the window BACKWARDS ... it'll cool the outdoors and blow the collected waste heat INTO the house.

 

[andrewinorlando]

A good quality heat pump (modern one) will work well down into the teens if it is sized correctly. It is 25 degrees here tonight, and my heat pump has the house at 70 degrees and is having no problem keeping up during this sustained cold snap. It continues to cycle on and off during most of the night.

If the unit uses a programmed defrost versus a demand defrost function (cheaper units use programmed defrost), it will defrost on a preset schedule (which can be changed on nost units) much like older frost free refrigerators do, by using a timer. A demand defrost unit will only defrost when the unit has a significant ice buildup outside. When the machine goes into defrost mode, the reversing valve switches between heating and cooling, and shuts the condensor fan off such that hot refrigerant is passing through the outdoor condensor's coils to melt the ice buildup. While it's doing that, the cold refrigerant is being passed through the evaporator coil inside, thus the neeed for back up electric resistance heat. That cold air you feel coming out of the vents during the defrost cycle means that either the electric heat strip is not working, or that it is insufficient for the capacity of the unit. A 3 ton unit could probably make due with a 10 KW strip. Most air handlers have space for up to (4) 5KW heat strips, dependent on the household wiring going to the unit.

A temperature differential is built into the thermostat such that if the temperature of the house drops, usually by more than 1 or 2 degrees from the temp setting while the heat pump is running, the thermostat will engage the backup electric resistance heat to try to bring the temperature of the house up to the preset temp. This usually happens when it's just too cold out for the heat pump to be able to produce sufficient heat. If the temp inside the house is dropping and the house isn't heating correctly, the backup heat strip(s) may not be working. If you manually raise the thermostat to a higher temp, you should see the backup resistance heat go on to help the heat pump warm the house more quickly. Most folks with a heat pump will raise the temp only one degree at a time to prevent the electric backup heat from engaging, as it is far more expensive to run the electric heat than it is the heat pump.

Heat pumps generally have lower outlet temps than say a gas or oil fired heating system, but a properly sized modern system can heat a home when the outside temps drop into the teens with no problem, though it will take a little longer.

To test the system in your grandparent's house, if their thermostat has a setting that says "Emergency Heat" or something similar, slide the switch to that setting. It should shut the heat pump off, and turn on the electric resistance heat. If the unit blows cold air, then you'll know the heat strips are not working and most likely need servicing. If it blows warm air, then you'll know that a single 5 KW heat strip is not sufficient for that system, and a second 5 KW strip should be added.

 

[DADoES]

 
A typical electric portable space heater is 1.5KW (1,500 watts). So 5KW of heat strip is roughly equivalent to 3-1/3 portable space heaters.

My system also is having no trouble keeping up over the last 3 days with outdoor temps of 20°F and below, although it is a bit oversized (5 tons for 2,550 sq ft). The run-cycles get longer and frequent ... but so they should. One night I had the setpoint at 67°F and raised it to 69°F with an outdoor temp of 21°F (and/or less) and it recovered the difference in about 40 minutes, without using the auxiliary.

 

[SactoTeddyBear]

Heat Pump Heating Problem:

I had a Heat Pump in one of my houses in the 1980's and I replaced it with a "Gas Pack" Roof Gas Heat/AC Unit. When the company removed the Heat Pump Heating Unit, they showed me that the Strip Heaters were never hooked-up, no wonder I froze my Back Seat off, when the unit would go into the Defrost Cycle.

I would suggest to you Bruce "Supersurgilator" that you check the Strip Heaters, or have someone else check them out, if you don't live very close to where your Grandparents live. That might be the only actual problem with the System and certainly would save them quite a lot of Money, over a Service Call, or Replacing the Unit.

Good Luck, keep us "Posted" about what was wrong and the remedy.

Peace and Kind Regards, Steve
SactoTeddyBear0503...

 

[toggleswitch2]

A good rule-of thumb (applicable even the the coldest regions of the USA) is 10 watts of heating (electric coil /resistance) per square foot of room/home space. For rooms with a taller than average ceiling (normally 8 sometimes 7 feet) or strange configuratins (catherdal ceilings, slope ceilings etc.) use 1 watt per cubic foot.

1 watt = 3.4 +/- BTU, therefore 1kw = 3,400 BTU

A typical USA heater of 1,500 watts therefore throws 5,100 BTUs of heat.

Another rule-of thumb is to a allow 20 BTU per square foot for cooling purposes. Another method of estimation is to allow 3k to 4k per room.

 

[DADoES]

Yes, 1,500 watts = 5,100 BTU.

So 5KW = ~16,830 BTU ... which isn't enough to counter a 3 ton heat pump (A/C) during defrost. 3 tons = ~36,000 BTU.

The heat strips running during defrost do not directly provide the heat for defrosting (they're inside the house, the frosted coil is outside) ... they're "downwind" of the (cooling) coil inside the air handler, and serve simply to reheat the cooled air.

Note that ground-source heat pumps don't need to defrost, but still typically have auxiliary heat strips for emergency backup in case of compressor or circulating pump failure.

 

[supersurgilator]

This unit is 15 years old and is a packaged all in one. There is no indoor air handler, everything is housed in the unit outside. They were having a problem with it after they first moved in a few years ago about it provided NO heat whenever the temp got to 40 degrees outside. After having a service call, they did find that the heat strips weren't hooked up like someone else mentioned, however they are hooked up now, but I suspect that they are simply undersized. COuld part of the problem during defrost be the fact that since it is a packaged unit, the heat strips are having a hard time providing heat when it turns back to ac mode?

 

[DADoES]

 
The heat strip capacity is too small. Doesn't matter if the air handler is inside or outside as part of the "package" ... the concepts and operation are the same.

 

[toggleswitch2]

~My experience has always been that heat pumps are really made for places like Florida where it really doesn't get that cold.

Heatpumps are said to wor in any US climate, but yes, warmer climates are best.

The biggest problem is that their use is so limited (rightfully so)in colder places that heating contractors don't properly install them in that they dont understand the practical aspects and the theory behind them.

It is said that the least efficient gas furnace is more efficient than the best heat pump in cold climates. Efficiency is one of those words that is techincal in meaning and very rarely used propeorly in thougth and speech. I'm guessing this truly meand that less cost is involed in heating as well as fewer energy resources with gas.