Electric heat retrofit?

[jfreze]

Last year I added a heat pump to an existing oil-burning furnace and am quite happy with it. Now I'd like to go a step further and add electric heat as the primary backup, using oil only for emergencies and extremely cold weather. The first question is what wattage is appropriate? The existing furnace can put out 138,000 btu/hr. The house has about 2400 finished square feet. Obviously matching the oil furnace is both overkill and impractical. But can I get away with a quarter of the output (about 10 kWatts) or do I need more like a half (20 kWatts)?

The second question is about the heating elements. The duct exiting the furnace is now taken up by the heat pump coils. So I will need to put a heater in each of the two main distribution ducts, one being 16" x 8" and the other 20" x 8". Can the replacement heating elements you sell (e.g. 3500-410P/A) be mounted horizontally in these ducts and still function properly? Or am I better off trying to custom-string elements to fit the ducts? Or are there other stock elements that are better-sized for this application?

Thanks in advance...

[Robert]

This is not something that can be answered without being there.

You will need to get an HVAC Contractor out who can see what

best way is to accomplish your goal.




Take care and best wishes,
Robert

[jfreze]

FYI, I had the same company out who did the heat pump and they confirmed the element placement but never got back to me with a quote. It seems to be too small of a job to be of interest. Your answer may be applicable to the first question, but the second is a straightforward technical issue: can these heating elements function normally if they span about half of the duct and air flows along their long axes?

[Robert]

Hi,


My answer is applicable for both questions. Without being there, I cannot give a quality answer.


Would I advise it, no I wouldn't with limited info I have.

Have I ever done it or been asked that in last 29 years, no I haven't.


You would be restricting the airflow through the duct for one thing and not sure heat pick up would be sufficient that way.


Most things can be accomplished, but this would entail an unordinary modification and without being there, would be wrong to try and say for certain.


Call them back, you paid for the heat pump install, so they should be willing to assist with this.

Basicly, you're wanting 3 heating sources to work with each other, so it will not be easy if it is possible at all.


The heat pump will work down to 35-40*. Trying to fit electric in between that and the oil furnace may or may not be feasible, but will definately be a in person determination.


This is in the Dual Fuel catagory, but needs onsite inspection for correct advice on it.


Thanks for inquiring,
Robert

[jfreze]

I appreciate your taking the time to reply.

As I implied, the heat pump installer is the one who said he would put the heating elements in the two main ducts, and this seems to be the only reasonable solution.

You would be restricting the airflow through the duct for one thing and not sure heat pick up would be sufficient that way.

I asked about airflow and he said the impact would be minimal. I share your concerns about heat pickup if the element only spans half the duct width.

Basic[al]ly, you're wanting 3 heating sources to work with each other, so it will not be easy if it is possible at all.

In fact it is easy. Even the thermostat I already have is equipped to deal with this situation. Instead of leaving Auxiliary and Emergency heat lines tied together, Auxiliary would control the electric heat and Emergency controls the oil furnace. In this scenario one can program the thermostat to enable both heat pump and electric heat simultaneously in a given outdoor temperature range, which isn't possible with a fossil fuel backup.

Trying to fit electric in between that and the oil furnace may or may not be feasible...

There is only one correct ordering here: first and most-upstream is the oil furnace, next the heat pump coils, and finally the electric heating elements. The oil furnace must precede the heat pump because of the danger of condensation in AC mode. Electric heat follows the heat pump coils to allow efficient simultaneous operation in mid-range temperatures.

Have I ever done it or been asked that in last 29 years, no I haven't.

Given the price of fuel oil I think this should be coming up more often. Electric heat is now considerably less expensive, especially here where electricity is only about 7 cents per kWH, and at most a penny-a-year increase for a while. I also own a mobile home and am going to add electric heat there as well, but at least there's no heat pump to deal with.

[Robert]

Hi,

The order I was referring to was order in which each is used, not the layout of them.

Thermostat hookup would be easy enough, it's the layout allowing proper operation that may prove difficult.


Have done similar, just not this particular set up.


The mis-communication is why I cannot give advice from here.


I'm sure in person, it would be different and feel it should be ok.

Would appreciate an update when all is done, first time for everything and you're right, things are changing.


Take care and best wishes,
Robert