ReplyVaries with the lenght of defrost cycle, and the BTU of the furnace.
Its possible if you have room for the strip heaters. And can find someone thats good with low voltage wiring.
Regular Guest
Is it possible to have a heat pump with a heat strip plus furnace??
Here is what I'm thinking; If I could have a heat pump w/heat strip, so in defrost mode the strips would come on down to a temp of 32*, where the heat pump and strip is locked out so the furnace would take over.
Is this possible ??
It seems almost a waste to have a furnace kick on for just a few minutes during defrost. How much heat does a furnace even create during defrost?
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God Speed, Jim. We'll miss you - 2-26-23
Varies with the lenght of defrost cycle, and the BTU of the furnace.
Its possible if you have room for the strip heaters. And can find someone thats good with low voltage wiring.
Professional Member*
I've done it
Membership Chair/ARP Committee / Professional Member*
No way I'm firing a 93% gas furnace for every defrost.
I going to try to not use any heat during defrost as I plan the chnage-over for a fairly mild ambient. But if that is uncomfortable - I have ten KW of electric heat ready to go.
PHM
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When faced with the choice between changing one's mind, and proving that there is no need to do so, most tend to get busy on the proof.
Professional Member*
Dude different folks different strokes.Besides down here you don't have that cold of a winter.For some reason that is what the customer wanted so that is what he got.I really didn't see the point but I get paid by the hr.Originally Posted by Poodle Head Mikey
God Speed, Jim. We'll miss you - 2-26-23
What do ou considr a mild change over ambient..
Membership Chair/ARP Committee / Professional Member*
With a 90+ furnace cycling on - it's probably the better part of five minutes before it is producing any noticable amount of heat. And by then the defrost is likely over anyway.
The electric heat elements are almost instant-on.
For 'mild ambient change-over' I have in mind maybe thirty or thirty-five degrees. But like so many things in my Life there will, no doubt; be some tweaking involved.
Of course; I do live ever in the hope that beenthere will come to my design-rescue, tell me what to do for the best, and so spare me the tedious agony of overcoming my own fumbling attempts at empirical sorting. <g>
PHM
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When faced with the choice between changing one's mind, and proving that there is no need to do so, most tend to get busy on the proof.
God Speed, Jim. We'll miss you - 2-26-23
Don't know if I ever seen a 90% furnace take 5 minutes to put out heat.
2 to 3 minutes though.
Best set up. A unit with On demand defrost. Cuts down on those needless defrost cycles.
You must have a tall basement to get a heat kit in your plenum and be far enough above your coil.
Membership Chair/ARP Committee / Professional Member*
And let's say that the defrost time on the HP will be maybe five to ten? So do we really want the furnace starting and then stopping in five or ten minutes? I don't.
Aren't all heat pumps using 'on-demand' defrost now?
My cellar isn't all that tall - but it is sixty feet long.
PHM
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Don't know if I ever seen a 90% furnace take 5 minutes to put out heat.
2 to 3 minutes though.
Best set up. A unit with On demand defrost. Cuts down on those needless defrost cycles.
You must have a tall basement to get a heat kit in your plenum and be far enough above your coil.
PHM
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When faced with the choice between changing one's mind, and proving that there is no need to do so, most tend to get busy on the proof.
God Speed, Jim. We'll miss you - 2-26-23
Lot of HPs still use time and temp defrost conttrols.
6 one way, half a dozen the other.
May not get to full efficincy in 5 minutes. But the furnace putting heat into the evap coil, also shortens teh defrost time, by heating up teh OD coil quicker.
Electric strip after the coil, 100% efficiency right aways, but adds no heat to the ID coil, so the OD coil takes longer to defrost.
Which method is better, will depend on your rates, winter OD ambient conditions.
Membership Chair/ARP Committee / Professional Member*
Wouldn't that give me the best of both concepts?
PHM
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PHM
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When faced with the choice between changing one's mind, and proving that there is no need to do so, most tend to get busy on the proof.
God Speed, Jim. We'll miss you - 2-26-23
Yes it would.
How practical it is to do it. I guess would be dependant on the install set up.
But should for all practical purposes give you shorter defrost cycles, and tempered air temps during those defrost cycles.
Let us know how it works in the real world. Cause it does look good on paper.
Banned
all that dust burning off the heaters at the beggining of the season getting impacted in the evaporator
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God Speed, Jim. We'll miss you - 2-26-23
Most of that dust will go through the dry coil.
If it was wet, then more of it would stay on the coil.
Regular Guest
Ok, learning time for a homeowner. At what OD temp is a HP defrost necessary??
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God Speed, Jim. We'll miss you - 2-26-23
Depends on outdoor humidity. And varies with the HP a bit.
Generally under 40°F. but once in a while it can happen above 40.
Professional Member
most homes in Bucks county have a plenum on top of the furnace which feeds a tee and goes in both directions to feed the conditioned space. You could easily install strip heat in each side of the tee, and interlock those so that on a call for defrost, these are enabled, and controlled off of discharge air temp. These could be staged for better control, or even done with an SCR if you want to get real fancy. Depends on which part of bucks you live in and how big your lawn is I suppose.....
Membership Chair/ARP Committee / Professional Member*
One problem with the strips before the evap coil is that then I can't use it for reheat.
PHM
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When faced with the choice between changing one's mind, and proving that there is no need to do so, most tend to get busy on the proof.
God Speed, Jim. We'll miss you - 2-26-23
No, but pre heat can still increase humidity control.
Banned
i seem to remeber a lot of your posts about reheat (i assume for dehumidification) or to cut down on system cycling
strip heat makes for some very expensive reheat
you ought to look into heat pipe technology
it cost zero to operate heat pipe reheat!
i used to be involved with the installation of heat pipes in miami fl.
look at this link for kicks, its very interesting, of course initial installation is a bit pricey
http://www.heatpipe.com/
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