Can solar panels help reduce heating costs?

I'd like to connect solar collectors to my forced hot water heating system and reduce the amount of fuel I have to buy. Is this type of arrangement possible?

It's certainly an appealing idea. You pay for the solar collectors once and enjoy free energy for the life of the system. And although what you're suggesting can be done, it may not provide nearly as much heat as you'd like.

Both freestanding cast iron radiators and fin-tube baseboard radiators are designed to work with water temperatures of 165 degrees or more. A solar thermal system isn't likely to get water that hot, so your system won't be putting out the heat the system was originally built to deliver. In other words, you may be chilly.

A radiant-floor heating system is much better suited to solar input because it runs with much lower water temperatures. Even in the dead of winter, the house might not need water temperatures any higher than 100 degrees, even less.

But there's a catch. Solar panels are collecting energy for only part of the day. During the winter, when the demands on the heating system are maxed out, sunlight is limited. To work around this problem, a solar space heating system includes a storage tank. Antifreeze circulates through the collectors and a heat exchanger inside the tank, keeping water for the heating system warm.

When the heating system calls for heat, it taps into the storage tank first. Although that helps, it's usually not enough to provide all the heat you need, especially in areas that get really cold weather. What the solar system can't provide must come from a backup source--and that usually means a boiler that burns a conventional fuel.

Of course it's theoretically possible to install enough solar collectors to handle the whole heating load. Not only would that cost an arm and a leg, the bigger issue is what you'd do with all that hot water during the summer.

The other wild card is how well your house has been built, and how much insulation it has. If you live in an old place with lots of air leaks and minimal amounts of insulation, a reasonably sized solar space heating system might meet 10% or less of your heating load. That's a very low return on your investment. You'd be much better off spending money on new windows, adding insulation and sealing air leaks.

The numbers get much more attractive in a tight, well insulated house. You could see a 50% contribution from your solar collectors. The savings in reduced fuel use might offset the cost of a loan for retrofitting an existing house, or the higher mortgage payment in new construction.

In a super-insulated house with walls rated at, say, R-40, and the roof at R-60 or higher (plus some very careful air sealing), you could probably do much, much better than that. But in houses built to those standards, heating loads are so low that almost any heat source is very economical.

Solar space heating is a great idea. It's just not very well suited to most old houses.