View Full Version : Setback with Dual Fuel HP/NG
02-23-2010, 09:05 PM
I see a lot of discussion around setback or not to setback regarding heat pumps with electric backup but not much in the way of setback for HP/NG. What are the experts thoughts on setback for this type of system and what is considered a best practice strategy? I try to heat as much as possible with the heat pump as my electric rate is pretty cheap right now. My economic balance point is less than 20 deg but that's the temp I set the lock out because the heat pump starts to struggle below that temp. So with a priority on heat pump use would you recommend a setback at all or does NG being a relatively inexpensive fuel give a better opportunity for savings using it only to recover as necessary
My heat pump (American Standard 2-Stage) has an stated HSPF of 9.2 but the AHRI says more like 8.8. The furnace is an American Standard 3-Stage 95%.
02-23-2010, 10:19 PM
If this were my system I would set it back every night around midnight or so say 3°.
Then in the am I would enable the furnace to let it recover with the Furnace. It is good for a furnace to actually use it..and the mornings are when the heat pump would struggle the most.
Once it recovered then I would let it switch back to heat pump to maintain set temp.
02-23-2010, 10:36 PM
Are you saying that because of the 3 deg differential the system will use the furnace to recover or is there something on my T-Stat to tell it to do so? I have the American Standard controller for the hybrid communicating system (ACONT900AC43UAA). Currently, I believe it starts with the heat pump and will switch to furnace if the heat pump can't keep up. Is there something I need to set differently to for the furnace to recover? My current setback is 3 deg but when I get up in the morning the heat pump is running full throttle and the front of the controller says "Recovery" on it. If it's more economical and efficient to operate with furnace for recovery then I would like to give it a try.
Thanks for the response
02-23-2010, 11:03 PM
You would have enter the installers menu on the tstat and make some adjustments to make the unit shift over. You would want to raise the temp of the dual fuel balance point. A 2° temp droop should be fine. That way with 3° temp setback at night it should be running with furnace now in mornings.
02-23-2010, 11:15 PM
Better set back tstsats were equipped with "intelligent adaptive recovery" (if enabled)that was supposed to raise the temp back up 1 dg. at at time to avoid bringing on the aux. heat. Setting the temp way back and raising way back up in the morning usually brings on the aux heat defeating the whole purpose of having a HP, IMO.
I don't usually recommend setting dual fuel HP systems back at nite to my customers.
02-23-2010, 11:22 PM
Would the furnace kick on for recovery with a 3+ deg setback even if I have the Aux Lockout function enabled? I have that set to 35 deg. Would I need to disable that and only have the heat pump lockout set in order for the furnace to operate for recovery? I wish the thermostat manual was a little more comprehensive in its explanation of the settings and how they operate. I would imagine the primary function for the Aux Lockout is to prevent expensive back up sources like heat strips from coming on unnecessarily, but I'm certainly no expert.
02-23-2010, 11:26 PM
Did you already raise aux lockout temp from previous days? Was it below 35 and furnace still didnt kick on?
Yes you are correct about aux lockout primarily being for elec aux strips.
02-23-2010, 11:44 PM
I set my thermostat to drop from 68 degrees at 11:00 PM to 63 degrees during the night. I lock out the furnace from the CS 7000 thermostat and allow the heat pump to come on at 5:00 AM to raise the temperature using the smooth set back feature which raises the temperature up to 2 hours before the programmed time. I set a differential on the thermostat such that, if the heat pump has not reached 68 degrees by 6:40 AM, the backup furnace will come on to achieve 68 degrees by 7:00 AM. It works well. Am I saving any money? I'm not sure, but I sleep better at cooler temperatures.
Above the balance point I would just hit hold; most heatpumps don't have the capacity to recovery quickly near the thermal balance point. If the HP can't do the recovery on it's own, no intelligent t-stat will be able to get more heat out of it.
02-23-2010, 11:59 PM
Thanks for the responses guys. I'll try some scenarios and see what works best for my family and my system. On a more general note...all of my equipment is multi-stage. My heat pump is 2-stage and my furnace is 3-stage. So is there any additional value in using setbacks on multi-stage hardware? I'm speaking more from an efficiency/economic standpoint. If you need it cooler to sleep then you do and you should set it to what's comfortable, but other than that is it better to let the system run at low stages vs ramping up to full throttle every morning during recovery? Especially when using larger setbacks with the intent of using more expensive (for me anyway) NG aux heat to recover with?
02-24-2010, 05:01 AM
Would need to know your economical balance pint, to know if set back is saving you money if the gas furnace is used to recover.
If your locking out the heat pump at 20. Then set back under 20 will save you money.
02-24-2010, 10:02 AM
To be honest I'm not sure how the furnace was behaving with the aux lockout option set. Last night I changed the "Dual Fuel Heat Pump Control" setting from 2 to 1. This basically sets only a HP lockout and brings the furnace on as needed regardless of temp. This morning wasn't a very good test though because it was 8 deg and it would have been running anyway.
I've calculated my economic balance to be below the 20 deg lockout temp, but the thermal balance is hit just below that temp (at 18 deg it will always upstage to furnace). I'm on a special heat pump program from my utility co. during the heating months. The electric rate is around $0.06 delivered and NG is around $0.92/therm delivered. The only COP data I have is based on the HSPF/3.3 = COP formula (at 47 deg I believe) which is 2.67. The AHRI stated HSPF is 8.8 while American Standard says 9.2. Based on that information, am I doing my math right?
Thanks for the help guys.
02-24-2010, 03:03 PM
Afraid you can get an accurate COP from the HSPF.
A heat pump that has a time and temp defrost will have a lower HSPF then an on demand defrost unit. But can still have the same COP 17°F outdoor temp(or even a higher COP on some units).
At 20°F, your COP could be 2.6, or 2.1
Your contractor should be happy to tell you the COP at 47 and 17 so you can plot a economical balance point.
Give him a call.
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