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Dual Fuel Heat Pumps

 
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Old 02-25-2009, 01:24 PM   #1
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Dual Fuel Heat Pumps


I have been checdking this forum for about a year now but need some input for HVAC. Has anyone installed a dual fuel heat pump? I have a customer that is interested with new construction. I've also seen them called hybrids also but don't know much about them except what I read in building product magazines.

THX Ahead
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Old 02-25-2009, 02:42 PM   #2
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Re: Dual Fuel Heat Pumps


We do alot of Heat pumps with gas backup. We used to use the fossil fuel kits, but now we run them through Honeywell Vision Pro 8000 stats. Makes it easier to wire. We use a 2 stage gas furnace that way we can use AUX heat option AND Emergency heat. AUX would be HP and 1st stage gas. EMER heat would be all gas. The programming on the stat (coupled with an outdoor sensor) covers everything from compressor cutoff temps to types of backup heat, gas, elec, oil...Etc. Hope this helps.

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Old 02-25-2009, 09:45 PM   #3
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Re: Dual Fuel Heat Pumps


The hybrid is a great concept. The heat pump does the work, and when necessary, the gas serves as a backup. It is great because a heat pump is cost efficient to operate during mild temperatures. But, they can't keep up with the load during real cold temperatures. Having said that, you can buy a two stage furnace which is cost efficient as well. The drawback to the hybrid system is the equipment costs. You need to buy the fancy t-stat and a heat pump does cost a bit more than a straight a/c. Where do you live? If you live where it gets real cold from October through May, you may be better served with a 90+% two stage furnace. Around here, we have only two months of real cold weather, and our electricty is cheap (.07/ kwh), so a hybrid setup is a good way to go.

With a hybrid setup, you want the coil on the return side of the furnace.
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Old 03-01-2009, 07:22 PM   #4
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Re: Dual Fuel Heat Pumps


I did the design work for the dual fuel changeover kit for when the local utilities were pushing them in texas, (thousands sold until the electric utility bought the gas utility, now they don't care) you can still get them from Retrozone.com (dfac40f??). No fancy stat, standard honeywell heat pump stat with a relay card and a precision outdoor stat that would do the changeover at 39 degrees. Basically the changeover card provided logic to make it all work ok, the trick was to have a highly accurate outdoor thermostat with a very small deadband. Some products offer this feature described as "add on" or "add on heat pump" and do it by dropping first stage heat when aux calls, but this is pretty cheesy with no outdoor temp input-you really want that as an input. Anyway, its a great concept for many areas.
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Old 03-06-2009, 05:47 AM   #5
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Re: Dual Fuel Heat Pumps


Dual fuel/hybird systems have been around for many years.
First one I did was early 80s.
They are a great way to save on heating cost.

You just have to decide, if you want it set up for max efficiency, or max comfort.
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Old 03-12-2009, 10:08 PM   #6
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Re: Dual Fuel Heat Pumps


I agree with flash and verbal. Some of my happiest customers have had hybrids installed. My heat pump runs down to 20 degrees outdoor temp.
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Old 03-12-2009, 10:41 PM   #7
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Re: Dual Fuel Heat Pumps


To those who mentioned their heat pump installs, are these air-exchange, ground loop, or water loop systems?
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Old 03-13-2009, 04:43 AM   #8
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Re: Dual Fuel Heat Pumps


Air to air, here.

But, we never put the coil on the return, since it tends to rust out the furnaces heat exchanger.
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Old 03-13-2009, 09:44 AM   #9
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Re: Dual Fuel Heat Pumps


Quote:
Originally Posted by beenthere View Post
Air to air, here.

But, we never put the coil on the return, since it tends to rust out the furnaces heat exchanger.
Glad you mentioned the coil thing. If one has a better idea on how wire a hybrid setup, I am all ears/eyes. The problem is that if you use the honeywell 8000 stat with the sensor, there is no control to keep the h.p & gas furnace from operating simultaneously. At least, if the coil is on the return, both can operate.

My friend has a gas furnace, and heat pump (he thought it was only a/c). I recommended that he buy the hoeywell stat + sensor. To make a long story short, I got both appliances to work simultaneously. The amp draw on the compressor was below the rla, but the liguid pres. going to the compressor was about 325 ( I did not check s.c.).

Do you guys think this is a breakdown waiting to happen? Or, if it aint broke...? Generally, I believe that if the motor doesn't over-heat, cool. But in this scenario, I am not sure.

Well, any thoughts / comments are appreciated
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Old 03-13-2009, 01:23 PM   #10
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Re: Dual Fuel Heat Pumps


If you have the outdoor sensor connected.
You set the HP and aux lockout temps.
Then they won't run together.
The only exception would be during defrost, which is ok.

It sounds like you have ISU 200 set to 0 instead of 1.
If thats set to 0, then the stat doesn't know that its controlling a dual fuel system.
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Old 03-13-2009, 06:12 PM   #11
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Re: Dual Fuel Heat Pumps


I am certain that I set the system for gas furnace with a h.p. I set the lock out compressor lock out @ 35 degrees, but I would prefer to set it to about 15-20 degrees. I don't remember the auxlockout portion. Will that lock out the gas furnace at a determined outdoor temp?

You know, I after posting my question, I called the Goodman people. He said that they now have an "all fuel board" to prevent them from running simultaneously. Well, better late than never. Thanks for the reply
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Old 03-13-2009, 09:54 PM   #12
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Re: Dual Fuel Heat Pumps


Thats just add on dual fuel control.

Did you tell the thermostat that its the dual fuel control.
Or did you set it up that there is a external dual fuel control.

If you set it up that it thinks there is an external dual fuel control. Then it will allow both to run at the same time, because it thinks that there is a external control preventing that.

That would be ISU 210, 0=thermostat controls dual fuel, 1=external fossil fuel control.
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Old 03-14-2009, 09:58 AM   #13
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Re: Dual Fuel Heat Pumps


Scary thought, but I am confident that the system was setup ok, BUT.... If you have time, could you type out how you configure the t-stat? Then I could check it out at the friend in question's house.


I remember talking with the Honeywell tech people, and they mentioned that the t-stat was not designed so that h.p would be locked out when the furnace operates. The Goodman guy said that one needs to go with the t9000 (all bells + whistles) stat or use the all fuel board. What the heck? Things keep getting more spendy and after all is said and done, it may be best to just use a two stage 90+ furnace and straight a/c. Especially with the new stimulus tax rebate program.

At this time, I think it's just more practical to put the coil on the return side. I understand the rusting h.e. issue, but I think it would take a long time before rust will penetrate through the h.e.. Now, if one lives in an area that is quite humid, and the a/c operates about six months of the year, that's another story. Actually, they might as well just use strip heat as the heat source...keep things simple.
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Old 03-14-2009, 07:05 PM   #14
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Re: Dual Fuel Heat Pumps


First, if the Goodman guy told your a TH8320, or TH8321 can't lock out the furnace or heat pump.

But, if you want to control both stages of a 2 stage gas furnace also, you do need the IAQ(YTH9421).

He doesn't know his butt from a you know what. As far as what which thermostat can and can't do what.

Second. The heat exchanger can rust out in less then 5 years in a climate area that only has a 2 or 3 month cooling season. If you put the coil in the return.
And voids heat exchanger warranty.


Skipping the time and scheduling set up.



0170=7
0180=0
0200=1
0210=0
0240=3
0250=3
0270=3
0340=2(outdoor sensor)
0350=35(depends on the homes heat loss, and the customer preference)

If you set 0350 to 0(no heat pump lock out) the heat pump will be used until the indoor temp drops, then it will switch to the gas furnace.

I seldom use the 8000series anymore.
The IAQ doesn't cost that much more. And it does more then any of the 8000series can.
Along with no clicking at the stat. Since the EIM is mounted on or near the furnace. The customer no longer has to put up with the clicking.

Plus, if using a furnace with a VS blower, it will slow the blower if the humidity is high.
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Old 03-14-2009, 11:13 PM   #15
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Re: Dual Fuel Heat Pumps


Thanks Beenthere. I will look into how I configured my friend's system. If necessary, I will make it right with my other clients as well. This has been an issue that I have tried to clarify with my supplier, and other hvac guys, and I came to the conclusion that it would be best to... well you know what I would say. But, as I said before, if there is a better way, I am all ears/eyes. Thanks again.

I don't need to be right, just need to do it right. Have a good week-end, and look forward to seeing your "insight" in the future.
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Old 12-29-2010, 09:27 AM   #16
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Re: Dual Fuel Heat Pumps


can anybody please decribe what happens if both hp and furnace runs together at the sametime? what kind of damages to their units?
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Old 12-30-2010, 05:29 AM   #17
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Re: Dual Fuel Heat Pumps


Quote:
Originally Posted by seanzzang View Post
can anybody please decribe what happens if both hp and furnace runs together at the sametime? what kind of damages to their units?
If its during defrost, no harm, and it is ok to allow and set up for it to operate that way.

If both run together in heat mode. If the heat pump doesn't have high pressure safety switches in it. The compressor will be over worked. And will generally wear out quickly. The furnace won't be harmed.
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Old 01-11-2011, 07:54 PM   #18
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Re: Dual Fuel Heat Pumps


You can not use the honeywell 8000 on a duel fuel system due to the tsat does not have the brains to figure out if the heat pump can not keep up, or has a issue, and is above lock out temp, it will not turn on gas back up. you have to use the honey IQ series tsat. I found that out the hard way. The only way you could use the 8000 is if the coil is below the heatexhanger or was in the return of the duct.

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