I have a customer in California who is putting in solar, and currently has a late model propane furnace.  They do not want air conditioning, nor the expense of switching to a heat pump system.

Does anyone have any thoughts or experience adding a secondary heat coil to the propane furnace?  The heat load for the house is 32K BTUs.  20 watts equals 68K BTUs.

Who might manufacture an add-on electric heat coil?  The plan is that the electric heat will handle normal loads, but the propane will be there if needed for additional heat at very cold times (not often).

I would appreciate any input.


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Hi Rich, I think you meant to say 20 KW = 68K BTUs, although it is nice to dream :).

I'm no help on the conversion.


you can put strip heat in all air handlers.  just put on top of furnace and wire in.  Its just an add on that can take $2 a hr to use  so 2000 hr.  a heat pump can cost just $.3 hr with same BTU yr in building but runs longer.  most Heat Pumps will cost 1/3 the power cost   but $1600 more to install over AC

How expensive is propane vs. strip heat in his area? Neither are economical choices in our area...

The cost of adding an electric duct heater may have a very long payback time.

What kind of sun exposure do they have?   Perhaps splitting between solarPV and solarH2O, but use H20 for coil in the furnace.    Gary Reysa -- Bozeman MT is heating house mostly using solar h20.   It just takes a larger storage tank and collector...

In our mid-Atlantic region, $3.90/gallon propane in an 80+ heater is the same cost to heat as 15¢ kWh electricity.  Higher in a more efficient heater.

True that the electric strip is cheap to install, but until propane gets a bit more expensive (it is below $3 here) don't do anything.

And even though a heat pump is more expensive to install, it will heat so cheaply that it will pay back quickly - you should do the calculations based on your costs.  Then he will have air conditioning even though he doesn't want it, but at re-sale time he may be happy he has it.

About the biggest joy I get from my mini-split is the gawks. First, they gawk, listening FOR the dang sound! They can FEEL the coolth or warmth spreading across the room, but they gawk and balk and twist their head to bend the ears and shrug. Age. Rock n' roll.

Then they gawk again at the electric eye scanning the room and aiming the flow to the warm spot in the summer cooling mode or the cool spot in heat mode. They'll open the door and time how long the eye takes to aim the fins at the idiot by the barn door grinning! 

But the best gawk is the utility bills where the tiny little toe-print barely scratches the checkbook. If there's solar on the roof, one guy gets home, walks through the comfy house to the back door, opens it and watches the meter spin...BACKWARDS! "Ha printin money while I'm makin bacon!' 

Just the de-humidification feature of the mini-split is a real comfort booster here in Seattle about 20 or 30 days a year in a climate where the actual cooling is needed only a few days by the squeamish finicky persnickety sickos like me, geezers on pills like me or gorgeous young porn stars who are convinced the sun rises to splay THEIR shimmering alluring beauty, like my young room-mate:)

I applaud your customer for willing to go without air conditioning in California.  It is great that they are adding solar, but hopefully they are willing to buy some conservation carrots to go along with their solar candy.

It is difficult to answer your question regarding the feasibility of adding an electric heating coil without knowing the type of existing propane furnace, physical space limitations, tightness of the existing duct system, air flow limitations, and the size of the existing electrical service to the home.  Adding 10 kW of heat (32,000 BTU/Hr / 3,413 BTU/kWh) may require upgrading the size of the electric service from the electric utility.  Old forced air systems are often leaky and inefficient, especially if the ducts run through an unconditioned attic or crawl space. The older furnace also may not be set up to simply bolt on an electric coil section.  To prevent a fire, the electric coil section will need a flow switch to turn off the heater if no air flow is detected. 

Would it be possible to install wall mounted heaters instead?  That way they could provide heat only to the rooms that need it, while keeping the doors closed to the other rooms. 

A gallon of propane contains approx. 91,600 BTUs of heat.  Dividing this by 3,413 BTU/kWh means that there are approximately 28 kWh of raw energy in a gallon of propane.  The heater is not 100% efficient, so you have to multiply this value by the system efficiency to determine the useful energy in a gallon of propane.  An 80% efficient furnace would have 21.5 kW of useful energy per gallon.  Don't forget to include the energy used by the pilot light in estimating the overall efficiency of the propane furnace. 

If propane costs $3.50 per gallon, the equivalent cost per kWh would be:

$3.50/Gallon x 3,413 BTU/kWh / (91,600 BTU/Gallon x 80% Eff.) = $0.16 per kWh

If their electrical costs are anywhere near this high, they should keep the propane for back-up, and install a mini-split heat pump to provide a majority of their heating needs.  The mini-split is not cheap, but neither is $1,200 per year propane bill.

Few electric heaters of any type have flow switches for lack of airflow. Most use thermal limit switches mounted near the elements.

this is why most electric  strip heat do not work after 5 years.   The worst is very high Static pressure system or lack of return systems,  I just can not keep all parts working with on off and high amps.   On a 4 ton drive system I see 25 KW of strip heat there is not the air flow to keep all of them on at same time.   The  builder likes the install price and the building owner can not pay the heat bills.   

Sequencers and limit switches are the high failure items, they are marginal capacity for the high amp load. Elements rarely fail. Contactors last much longer but tend to be noisy when switching. Crappy ductwork and high static pressures will cause heating elements to switch off on high limit. Heating capacity is often oversized, many houses will heat fine with 10KW if decently insulated.

I have seen lots of heating elements melt though duct board when breaking apart.  I like to put long time sequencers when putting in back up strip heat so it takes a very long time to turn on.   You are very right about over sizing strip heat - 3-4 times over sized is norm.   8 KW  strip heat in a 400 sf is very common  I have put back 3 KW after sealing and will last a long time

Strip heaters in ductboard? That's just hack work IMHO.


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