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Old 11-13-2008, 08:04 AM   #1
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Dual Fuel LP/Heat Pump Questions


Hi all. I recently had a friend (licensed hvac installer for 20 years) install me a 3.5 ton add-on heat pump for my 80k btu LP furnance. This replaced a 10yo 3 ton 10seer A/C unit. We also installed a White-Rodgers 1F97-1277 thermostat that has some nice features for controlling the heat pump (algorithms for programmed setbacks, energy management for peak cooling, etc.)

Im in Ohio and it occasionally gets very cold, but average January low is 20F, average high is 38F. Ive historically used 1,000-1,200 gallons of LP for heating and the last fillup was $2.79/gallon. Our electric rate is around $.09/kwh.

Now on to my concern: I have the option to cancel the HP at a certain temp. I currently have not programmed in this cut-off temp so it will kick on the LP if it cant keep up. I have been playing with the settings/program a little and noticed that when I jump the thermostat up 3, it kicked on the LP immediately (fine with that). What I found strange was it also ran the HP during this time. That doesnt seem efficient to me because its going to be producing a much lower temp than the gas. I would think it should only call for the backup heat at that point. Ive also read some stuff online that says you shouldnt/cant run both systems simultaneously on a dual-fuel. My friend didnt seem to think that was a problem and said his runs that way down to around 20F. We had a freak cold snap the other day where it dropped to 22F overnight and the HP kept up (ran pretty much non-stop) and only kicked on the LP when I raised the temp up a degree just to see what would happen. Surprised it was able to keep up down to that low of a temp, but again concerned that at times it still might be trying to run both systems. Im just looking for some extra opinions. Ive had this guy do systems in several of my commercial buildings and two homes and trust him, but still like to do my research and make my own decisions. He is of the opinion that with $2-3.00 LP its worth milking every ounce out of the HP. Im more of a math guy so I like to make sure it actually adds up in $$. Also, is there any harm in both systems running at same time? Thanks for any help.

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Old 11-13-2008, 10:13 AM   #2
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Dual Fuel LP/Heat Pump Questions


The closer the temperature gets to freezing the efficiency of a heat pump drops dramatically. Having it on during those times is really a waste of money without an additional heat source.

Twenty two degrees already in your area??? So much for global warming.

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Old 11-13-2008, 10:55 AM   #3
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Dual Fuel LP/Heat Pump Questions


if it were mine i wouldn't want both running together there should be a switch in the fossil fuel kit which kicks out the h/p when the gas takes over I'm assuming you have more than just the tstat to make the decision of which unit is running. years ago fossil fuel kits were these ungodly large boxes filled with relays and transformers (i never did understand why it had to be so complicated) now i think it little more than an outdoor sensor that makes one circuit at a chosen temperature and breaks the other circuit but if the refrig. pressures get too high from the refrig. (a coil) being heated from the propane the hi pressure switch will trip on the h/p so if that happens you know pressures are to high and, no they cant run together

and agreed with marvin usually 35-30 is the design temp that the h/p starts losing ability unless you keep your tstat at 60 at night

Last edited by kennzz05; 11-13-2008 at 10:57 AM.
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Old 11-13-2008, 11:49 AM   #4
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Dual Fuel LP/Heat Pump Questions


Your HVAC guy may be right to run it as low a temp as possible to get the greatest economic benefit. For some people, having cooler air blow from the registers and the draftiness causes comfort issues, so they lock out the HP at a higher temp. So there is the economic balance point and comfort balance point.

What is the SEER/HSPF rating of that unit, efficiency of your furnace and does that KWH cost include taxes? Knowing these I can show you some calculations you'll appreciate as a math guy. You could even do a spreadsheet.

As far as running them in tandem, it can be wired exclusively or together. Together you are obviously getting more BTUs than just the furnace can provide. If your indoor coil comes after the furnace, there is some controversy it may not be the best for the compressor to dump the furnace BTUs onto the coil. It think it maybe okay to do.

Last edited by dac122; 11-13-2008 at 11:58 AM.
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Old 11-13-2008, 01:19 PM   #5
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Dual Fuel LP/Heat Pump Questions


Thanks, yeah I was gonna do a spreadsheet once I got all the facts together and see if I could find the sweet spot. What I have now is a 92% 80k LP furnace with 3.5 ton, 13 Seer, 7.9 HSPF. The electric is about $.095 which includes taxes/etc. I've only got $1500 (after electric company credit) in the heatpump so my gut was it would be a fairly quick ROI with LP prices where they've been. My prior A/C was a 10 seer but only a 3 ton unit. Plus I can probably get a couple hundred buck out of it in the spring.

So far the house actually feels warmer running the HP. It's a very old home and while I've done all the windows and such, there are still some inefficiencies in insulation, particularly the walls. I've been holding out until I can put on an addition, insulate, and re-side all at the same time. But I think the fact that there's warm air running all the time is better than the blast of air. Plus our old t-stat was had 2 drop so the house would cool off and then heat up.
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Old 11-13-2008, 03:51 PM   #6
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Dual Fuel LP/Heat Pump Questions


Using Goodman's 13 SEER 3.5 HP COP data we can compare the cost of 1 million btu's of heat.

Gas furnace with propane at $2.79 per gallon, 92% efficient furnace:
(1,000,000 / 92,600 btu per gallon) x 2.79 / .92
= $32.75

Heat pump with electricity at 9.5 cents per KWH, C.O.P. = 3 at 35F:
(1,000,000 / 3414 btu per KWH) x .095 / 3
= $9.28

Above 35F the COP numbers get better, below 40 your defrost can increase the HP's cost. Goodman's full set of COP numbers are below. I would contact your HP's manufacturer and see if you can get exact COP numbers for your unit.

Temp COP
65 4
60 3.86
55 3.7
50 3.53
47 3.41
45 3.33
40 3.15
35 2.97
30 2.72
25 2.57
20 2.42
17 2.31
15 2.25
10 2.06
5 1.87
0 1.67
-5 1.46
-10 1.23
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Old 11-19-2008, 08:00 AM   #7
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Dual Fuel LP/Heat Pump Questions


Thanks for the info. I got an email from Payne with a 28 page spec sheet. Haven't had time to digest it yet....but I did get the outside temp sensor hooked up this weekend and programmed into the t-stat. Once I programmed it, the HP no longer runs when it calls for gas heat. I've got it set to shut down 60 seconds after the gas comes online. At this point I've set the termination temp at 22F. So basically the HP will run as much as possible as long as the temp is above 22. It keeps up pretty good really. It was in the lower 20's last night and in the six hours I was home the gas kicked on twice. Might have been during defrost, I dunno.
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Old 11-19-2008, 08:53 AM   #8
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Dual Fuel LP/Heat Pump Questions


If you need some help in deciphering the specs and performance for your HP, just post a page or two. If you're satisfying the tstat at 22 I would bump it down further. Yes, those furnace events might have been defrost, but without knowing how your HP works its hard to tell. Note that you get less changes of defrost at those lower outdoor temps as outdoor humidity tends to be very low. Its usually in the 30ish-40ish range, or so, when humidity is higher that defrost is needed. The unit may however do a defrost anyway whether needed or not depending again on how it works.

If it was me with those kind of electric rates I'd run that unit as low as Carrier/Payne says its safe and the COP numbers support a savings. Cheap BTUs that don't fully satisfy tstat is still a savings.

Last edited by dac122; 11-19-2008 at 08:55 AM.
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Old 11-19-2008, 06:08 PM   #9
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Dual Fuel LP/Heat Pump Questions


Quote:
Originally Posted by dac122 View Post
If you need some help in deciphering the specs and performance for your HP, just post a page or two. If you're satisfying the tstat at 22 I would bump it down further. Yes, those furnace events might have been defrost, but without knowing how your HP works its hard to tell. Note that you get less changes of defrost at those lower outdoor temps as outdoor humidity tends to be very low. Its usually in the 30ish-40ish range, or so, when humidity is higher that defrost is needed. The unit may however do a defrost anyway whether needed or not depending again on how it works.

If it was me with those kind of electric rates I'd run that unit as low as Carrier/Payne says its safe and the COP numbers support a savings. Cheap BTUs that don't fully satisfy tstat is still a savings.
defrost is determined by the temperature of the outdoor coil and it will go into defrost regardless of any ice (or not) based soley on the temp of that sensor i want to say it initiates at 28 and terminates at 35
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Old 11-19-2008, 09:53 PM   #10
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Dual Fuel LP/Heat Pump Questions


Quote:
Originally Posted by dac122 View Post
As far as running them in tandem, it can be wired exclusively or together. Together you are obviously getting more BTUs than just the furnace can provide. If your indoor coil comes after the furnace, there is some controversy it may not be the best for the compressor to dump the furnace BTUs onto the coil. It think it maybe okay to do.
With both of them running at teh same time. He really isn't geting any more heat.
Just wasting electric.
And over working the compressor. Runnig a high head pressure, and raising the evap(outdoor coil) pressure, that it can't absorb geat from the outside.
It will have an early death.

Heat pumps in the heating mode are not rated to have higher then an 80F entering air temp.
Which you have a much higher temp with the furnace running.
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Old 11-19-2008, 09:55 PM   #11
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Dual Fuel LP/Heat Pump Questions


Quote:
Originally Posted by beenthere View Post
With both of them running at teh same time. He really isn't geting any more heat.
Just wasting electric.
And over working the compressor. Runnig a high head pressure, and raising the evap(outdoor coil) pressure, that it can't absorb geat from the outside.
It will have an early death.

Heat pumps in the heating mode are not rated to have higher then an 80F entering air temp.
Which you have a much higher temp with the furnace running.

ill second that
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Old 11-20-2008, 08:46 AM   #12
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Dual Fuel LP/Heat Pump Questions


Thanks guys. Once I hooked up the outside sensor and programmed it in, it's not running both systems at the same time. I think I misunderstood my friend as he said he's not running both on his, but he's running the HP unrestricted. So he doesn't have a temp set for it to terminate the HP. It will run the gas only when the HP can't keep up. I've currently set mine at 22 although it seems to still be heating the house down to those temps but at times runs the gas.

I may lower the cut-off temp as from my initial run on the numbers, there's not really a point that LP makes more financial sense as long as I'm getting heat from the HP. Comfort wise, I normally have run the fan 24/7 during cold months anyway to help with circulation so the fact that there's warmer air than normal coming out is actually a plus. This White-rodgers tstat is also much nicer that what we had and seems to really do a good job of keeping the temp right where it's set. The old one would drop 2 before kicking on gas so that fluctuation would make it feel cold at times.

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