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.

Rich

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The add-on electric heat coil has morphed into a ductless heat pump that eliminates duct loss, pressure-related convective losses associated with both the propane 80%'er at best before duct losses whack it and also the hapless unitary heat pumps that were noisy blends of old technologies and centralized mistakes of last century. Dump the 80% er, the tank and the smell. Drop the ducts, seal the penetrations. Run the whispering suit-case. With the sun.

I can't blame them for wanting to avoid the expense and penalties inherent in that old time unitary, ducted heat pump- noisy, needs back-up, expensive! But look at mini-splits for significantly higher HSPF, better than anything in the strip heat arsenal, like COP of 3.8. handling low exterior temperatures without need for backup, invertor driven to maximize efficiency and minimize racket while eliminating health, safety and environmental issues.  and running off ~ 450 to 1000 watts in Cali winter, 2 - 4 more panels of 250 watts each worth to go net zero. Why not go for the whole NINE yards?

Adding electric resistance coils in the ducts would likely have caused the problem discussed in the post;

How Do We Make Energy Efficiency Sexy, or at Least Interesting?

With your customer ending up with a higher electric bill.

Adding Solar often drives people to want to convert from gas to electric, solar installers often encourage this.

For smaller houses, and very energy efficient houses, and conservative occupants, switching to electric can be a good choice, but it increased the primary (source) energy consumption at the power plant (aprox 3x the energy at the power plant than used at the house). Of course heat pumps can lower the site energy with minimal impact on the source energy, and are almost always less expensive to run than propane.

Switching to ductless mini-splits eliminates the duct losses. Although air tight ducts in conditioned space have no loss either. The problem can be distribution, and many builders install small bath fans w&w/o ducts to help distribute the heating / cooling. Having an energy efficient home will help make distribution easier.

Have they, or are they doing any efficiency upgrades to the house to reduce energy use?

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