A solution to defrosting with an Oil furnace

[VanIsleRefrig]

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.

all help and criticism is appreciatedhttp://hvac-talk.com/vbb/images/smilies/2cents.gif

[BaldLoonie]

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.

[southshorejohn]

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.

[chuckcrj]

We have a lot of satisfied customers with that setup, do it just like Baldloonie said.

[jhef83]

Or switch to oil at 50 degrees with an outdoor stat and no need for a defrost.

[chuckcrj]

Or switch to oil at 50 degrees with an outdoor stat and no need for a defrost.And defeat the purpose of having a heat pump.

[pacnw]

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.

[stopro1]

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.

[beenthere]

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.

Bonnet sensor should be used on any oil furnace with a heat pump, but they are really needed on over sized oil furnaces.


OP, don't put the evap in the return, it will rot out the furnaces heat exchanger prematurely.

[VanIsleRefrig]

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.

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?

[beenthere]

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?

Manufacturers tell you not to put the indoor coil in the return.

Use a dual fuel thermostat with an outdoor sensor, no extra relays required then. And your customers will get what they are paying for then. Economy, and comfort.

[millertime77]

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?

[beenthere]

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?

Its more important when it switches back to heat mode. over sized oil furnace tend to cause a high head and trip the HPS on some units. As far as when its in defrost, its not a real problem, but best if not left to rise to more then 100°F.

[millertime77]

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?

[millertime77]

I would also like to add that if possible I think the indoor coil should be on the return side of furnace.

[millertime77]

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?

[beenthere]

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.

[millertime77]

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.

[mlstark]

Beenthere is not that great, he's just old and seen it all.:D

Sent from my PC36100 using Tapatalk 2

[beenthere]

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.

[beenthere]

Beenthere is not that great, he's just old and seen it all.:D

Sent from my PC36100 using Tapatalk 2

And bald. LOL

[pacnw]

Open forum, so do not want to get too detailed.

As Beenthere said, evap in the supply above the oil HX, defrost limit switch, dual fuel stat with OD sensor AND move the original fan/limit switch if normal older style mechanical type.

I have seen this several times this year, customer going on the cheap picking company and they melted the evap pan. Most likely cause, fan/limit switch in original spot and it allowing too high temp before activating( won't get anymore specific than that in this forum).

[VanIsleRefrig]

thanks for the input you guys this site is awesome