Heat pumps

[Country Geek]

The advantage of a propane backup to a heat pump is that you can run the propane furnace off a consumer-grade generator if the power goes out. That's an important plus if you live in a rural area where ice-storm induced power outages can last a long time.

I had a propane backup furnace installed as part of my geothermal system this past fall. The installer thought I didn't need it and was wasting my money but I wanted it for the ability to easily run off a generator (my house is wired for a generator and the furnace is on the generator panel). He was right in that the system hasn't needed to use the propane backup. Even with temps down into the teens the geothermal heat pump has no problem holding 70 degrees inside with no backup. But they are calling for zero degrees later this week so that will be the real test.

 

[LD1]

We bought our 1970 built 2500 sq ft 2 story 4 bedroom home in September 2013. We put a new Heat Pump in with propane backup when we moved in. Our Honeywell thermostat has a lockout setting for the heat pump which is programmable. I have it set for 30 degrees I believe and I have an outdoor temp sensor wired directly to the thermostat. It is a Heil brand. I'm not overly impressed with the system. Seems to do weird things. Like the heat pump comes on, then shuts off, then the propane comes on for a while, then turns off only after about a minute and the blower continues to run. It only does this when it's around the lockout setting. When it's colder than the lockout then the propane works fine. Just acts real weird right around the lockout temperature. If I had to do it all over again, I would probably just go all electric.

Right around lockout temp is finicky. A few things I can think of....

IF the unit is in sunlight, it may warm up the unit (wherever the temp sensor is) just above that lockout. Once it kicks on and air moves, it cools back down to ambient temp and locks out.

Another things is once it is running, lets say its 30.5 degrees outside, the unit is blowing 15 or 20 degree air outside. If that is circulating to getting any bit of that super cold air where the temp sensor is....lockout.

And I am not sure how your system is setup, but alot of propane furnaces continue to run the blower after flame is out. On a timer I believe. You burnt propane and heated up everything including the ductwork. The blower continues to run to extract all that heat and distribute it in the house, instead of letting it slowly cool off in the furnace and ductwork. Especially improtant if you have ductwork in an unheated crawl.

 

[kcflhrc]

Right around lockout temp is finicky. A few things I can think of....

IF the unit is in sunlight, it may warm up the unit (wherever the temp sensor is) just above that lockout. Once it kicks on and air moves, it cools back down to ambient temp and locks out.

Another things is once it is running, lets say its 30.5 degrees outside, the unit is blowing 15 or 20 degree air outside. If that is circulating to getting any bit of that super cold air where the temp sensor is....lockout.

And I am not sure how your system is setup, but alot of propane furnaces continue to run the blower after flame is out. On a timer I believe. You burnt propane and heated up everything including the ductwork. The blower continues to run to extract all that heat and distribute it in the house, instead of letting it slowly cool off in the furnace and ductwork. Especially improtant if you have ductwork in an unheated crawl.

Very good info, thank you. The temp sensor is fairly close to the outside unit. The unit doesn't get a lot of sun, it's on the north side of my house so in the winter not much sun there. Full basement so no crawl space and all duct work is in the walls, none in the attic.

 

[JRobyn]

In TN, we have a pair of fairly new (2yr old) Rheem variable speed blower, two-stage compressor, high efficiency heat pumps with the Rheem "communicating" thermostats. The thermostats have a variable/programmable "Balance Point" which I at first found to be confusing terminology. It is the lockout temperature below which the compressors are locked out, based on outdoor temperature. Rheem considers the "balance point" to be where the heat pumps can just keep up with the house heat load, running continuously. The thermostats also run the emergency heat strips whenever it deems it appropriate, like when I set a thermostat setpoint that is 2 deg or more away from the room temp, or of course, when the compressors are locked out. I didn't think I was getting any appreciable amount of heating from the compressors when it got down below 32deg. They seemed to run nearly continuously then, and I don't like the compressors running continuously, so I set mine at 32deg. I may now adjust them down a bit based on the excellent discussion in this thread!

You can only get to this setting of the thermostat using the "Installer Setup" menus, so to find it on yours, you should try locating the full install instructions for your thermostats on-line.

Good luck getting yours sorted.

- Jay

 

[LD1]

In TN, we have a pair of fairly new (2yr old) Rheem variable speed blower, two-stage compressor, high efficiency heat pumps with the Rheem "communicating" thermostats. The thermostats have a variable/programmable "Balance Point" which I at first found to be confusing terminology. It is the lockout temperature below which the compressors are locked out, based on outdoor temperature. Rheem considers the "balance point" to be where the heat pumps can just keep up with the house heat load, running continuously. The thermostats also run the emergency heat strips whenever it deems it appropriate, like when I set a thermostat setpoint that is 2 deg or more away from the room temp, or of course, when the compressors are locked out. I didn't think I was getting any appreciable amount of heating from the compressors when it got down below 32deg. They seemed to run nearly continuously then, and I don't like the compressors running continuously, so I set mine at 32deg. I may now adjust them down a bit based on the excellent discussion in this thread!

You can only get to this setting of the thermostat using the "Installer Setup" menus, so to find it on yours, you should try locating the full install instructions for your thermostats on-line.

Good luck getting yours sorted.

- Jay

Like I have mentioned before, without doing BTU calculations and determining how many watts your system is using, Setting a lockout point is nothing more than a guess. And neither you nor I will know if it is actually helping or not.

And if you have propane for AUX heat, the lockout number at which is most efficient is ever changing. Because electric rates go up, and propane fluctuates. If all electric, you set it once and you are done unless you change some part of the system.

 

[techman]

The balance point is the temperature where the heat demand equals the heat pump output. On a conventional HP, it is the temperature where the aux heat would be enabled, not where the compressor is locked out. In dual fuel systems, it is common to cut out the compressor at that temp, due to the problem with defrosting. On units with electric heat, you still get usable heat, at higher efficiency that electric strips, just not enough of it. If you lock out your compressor at that temp, you are losing money.

paul

 

[JRobyn]

Yup. Agreed and agreed!

- Jay

 

[buickanddeere]

The advantage of a propane backup to a heat pump is that you can run the propane furnace off a consumer-grade generator if the power goes out. That's an important plus if you live in a rural area where ice-storm induced power outages can last a long time.

I had a propane backup furnace installed as part of my geothermal system this past fall. The installer thought I didn't need it and was wasting my money but I wanted it for the ability to easily run off a generator (my house is wired for a generator and the furnace is on the generator panel). He was right in that the system hasn't needed to use the propane backup. Even with temps down into the teens the geothermal heat pump has no problem holding 70 degrees inside with no backup. But they are calling for zero degrees later this week so that will be the real test.

Ground source geothermal is much different than an air source heat pump.
Air source the available heat from outside air drops rapidly and the energy required to defrost the outside air coil increases when temps drop below freezing.
Obtaining usable heat from an air source as outside temperatures drop to or below 0F maybe possible. Even acceptable if you reside in an area that suffers only one or two brief cold snaps per winter. Operation of any heat pump from November to March with outside air temps near or below 0F is using the heat pump outside of it's intended design curves.

 

[buickanddeere]

Be aware that the issue with heat pumps is NOT efficiency, it is capacity. A modern heat pump will still output more energy in heat than it consumes in electricity even in the single digits. As it gets colder out, the energy draw for the heat pump drops as well. The problem is that the amount of heat that it is generating also drops with temperature. This is due to the fact that the colder the air, the less heat that can be drawn from it. So in very cold temps, the electric heat is added as a SUPPLEMENT to the heat pump. So if you require 30,000 BTUs and the heat pump is only generating 15,000 at the given outdoor temp, the electric heat makes up the difference. So when the backup is on, the heat pump will still run and is still generating its portion of the heat at efficiency greater that straight electric heat. A a low temperature the heat pump will reach a point where is will generate less heat energy than it consumes. This point is generally zero or below for a modern heat pump. So it is normal for it to run, and usually run 100% of the time at cold temps. It is still giving you more than the electricity it is using.

paul

I wonder how many folk believe the HP is warming the house in below freezing temps and are unaware the
Resistance heating is adding another 10-20F to the plenium temperature ?

 

[kcflhrc]

Yep, dual fuel heat pump with a lockout is a completely different animal than an air handler with electric strips. Can you change over a dual fuel system with propane backup to an all electric with heat strips easily? Or is it a completely different inside unit?