It gets frosty at night. The clock is ticking and we need to make decisions on the heating system. Let’s start with the easy ones.
Very early on we dismissed the geothermal option. The economics don’t work out, nor is it the right match for our building envelope and energy goals.
Instead, we would like to go with a solar hot water system. The question is, how can we make it work?
The basement received a new concrete floor, which made it easy to integrate radiant floor heat, which in turn is compatible with the 100 to 120 degree Fahrenheit of a solar hot water system. One problem solved.
The 1st and 2nd floor would need some kind of radiators. Our mechanical engineer recommended small, baseboard style, steel radiators. I had never heard of steel radiators but learned that they can operate at lower supply temperatures, compared to the cast iron kind, and are thus also compatible with a solar hot water system.
Another problem solved, although at an expense.
Steel radiators can be used at higher water temperatures, i.e. 180 degree Fahrenheit. When I spoke with a sales representative inquiring about pricing, he assumed I had a big project on my hands based on the number of radiators. He was confused when I mentioned that we only talk about three units.
It comes down to the correlation between supply temperature and Btu output. Here are examples taken from the product specifications for Runtal radiators, model type RF-2:
- 215 degree – 870 Btu/hr/ft Ratings @ 65°F EAT (entering air temperature)
- 180 degree – 620 Btu/hr/ft Ratings @ 65°F EAT
- 140 degree – 350 Btu/hr/ft Ratings @ 65°F EAT
A supply temperature of 215 or 180 degree Fahrenheit would require significantly fewer radiators to meet a certain Btu/hr demand, compared to the 140 degrees Fahrenheit, or in our case 120 degrees. Thus the extra expense and this problem I can’t solve.
We need a high efficiency boiler as a backup to the solar hot water, for those cloudy days in November and December. This is a given.
While developing a schedule for the installation, we realized that the same boiler can also be our primary energy source for now, while we wait to get ready for the solar hot water system on the roof. Once installed, the furnace can be relegated to its backup function only. In the mean time, we can move into the garden unit and work our way up. That solves the scheduling problem.
To sum it up: Start with a high efficiency boiler to run the domestic hot water and space heating. Use radiant floor heat in the basement and steel radiators on the 1st and 2nd floor. Install a solar hot water system once the roof is ready. Once installed, relegate the boiler to backup use only.
And, maybe one day, we can replace the boiler with a high efficiency biomass furnace.