The defrost board in the HP will trigger heat during defrost. I'd put in a limit above the heat exchanger so the burner can't put too much heat in the coil on a long defrost. We had dual fuel oil heat for years in our office.
Where I live there are a lot of houses with oil furnaces, so granted some of our customers don't see the necessity in putting in an electric or gas air handler when they already have a 'perfectly' fine oil furnace*cough*. So we tend to do alot of heat pump add ons to these oil furnaces and throw a coil in the return or supply obviously depending on what is more accessible with the existing ductwork. moral of the story is new electric furnaces turn on the auxillary to keep sending warm air through the ductwork while the heat pump is in defrost. but with these old oil furnaces when defrost comes on of course cold air comes out of the registers and the oil furnace does not come on to supplement the heatpump while it is doing its defrost. of course it doesnt cool there house down due to how long defrost runs but why would someone want cold air coming out of the registers when its below 32 ambient.
it tends to be a big complaint in the winter time and I hate having to say 'sorry defrost is an essential part to a heatpump being used in winter and there is not much we can do about that'
Does anybody know of any relays or controls that we could use that would turn the oil furnace on to offset the inside coil being used as an evaporator for those few minutes.
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The defrost board in the HP will trigger heat during defrost. I'd put in a limit above the heat exchanger so the burner can't put too much heat in the coil on a long defrost. We had dual fuel oil heat for years in our office.
I would think any GP relay to TT of primary will turn the burner on, the real problem would be cycling the blower if board controls blower. If older style fan/limit, would be easier, and could even be added onto unit easy enough.
We have a lot of satisfied customers with that setup, do it just like Baldloonie said.
Or switch to oil at 50 degrees with an outdoor stat and no need for a defrost.
we do this set up all the time and only one customer has requested we disable the oil in defrost.
we use a HW 8000 series stat, outdoor sensor, proper stat programming and a plenum switch in the evap coil plate- basically like Baldloonie said.
blower control is STILL on from call for heat pump in this set up.
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from an excerpt by Paul Jacob in Sun City, AZ
furnace bonnet temp (before evap) should be limited to 88F max. This will prevent overloading the indoor coil, which is now in cooling mode, with too much heat. it also should keep discharge air no higher than 68-70F which is desirable during defrost, so you dont satisfy the heating call while in mid-defrost.
That's an excellent idea for a heatpump with a coil in the return I will definately pass that on to my boss and we can pick away at it.
But when there is a coil in the supply we use an isolation relay so that the furnace and heatpump will never run at the same time so it will not overheat the coil. Wondering if there is something that we could use in cooperationwith the isolation relay?
I always wondered why people worry so much about how much heat is being blown over the evap coil during defrost in dual fuel applications, I dont see why it matters what the air temp is going over an evaporator in ac mode?
so when it finishes defrost and switches back to condenser inside the left over heat in heat exchanger could cause a high pressure trip. Am I understanding that correctly?
I would also like to add that if possible I think the indoor coil should be on the return side of furnace.
Read back down through the thread and saw where been there said not to put coil in the return, what is the reasoning behind that?
if the heat exchanger is too hot when it comes out of defrost, it can trip the HPS and lock out the heat pump.
An evap coil in the return will cause condensation in the oil furnaces heat exchanger in the summer, and rot it out from the inside out. You also lose some of your heat pumps heat capacity to heating the chimney.
Thanks been there. As an inexperienced tech I appreciate the fact that you take the time to answer my questions. I feel blessed to be able to come here and ask questions and get answers from guys who are probably some of the best inn the trade, definitely a fortunate thing for younger technicians.
Beenthere is not that great, he's just old and seen it all.
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if you get to work on any gas/electric RTUs. look at how rusted out the heat exchangers get with the evap coil before the gas heat exchanger. It can be real eye opener as to how quickly a HE can rust out from condensate forming in the HE from the A/C.