most efficient way to run the furnace

[Blumengarten]

Hi fellas,

Now that winter is almost over and the question is nearly moot, I hope you could answer a disagreement! We have hot-water radiators, and my husband turns the thermostat WAY down (like 55!) to save energy. I maintain it's more efficient to keep the temperature constant ... I've observed that when I'm boiling water on the stove, if I add cold water slowly, the water will keep boiling, but if I add it quickly, it will stop boiling and take a long time to return to boiling. I think the same principle would apply to the furnace; the thermostat regulates how often the pump will come on, not the temperature of water in the boiler; if we turn the thermostat way down, the water returning to the boiler will be very cold and the furnace will have to work harder to bring the water up to the correct temperature. Of course this goes against what we've been taught all our lives, to turn the thermostat down at night or when we're leaving, but I think that advice is for people who live with forced-air furnaces. I can't help but think it's better to keep the house set at a constant temperature, and only turn it down at night because I prefer sleeping in a cooler room.

Thanks!
Liz

 

[Dana]

First, if it's hot water radiators, in most of the US it's called a boiler, not a furnace. (Seems like most of the people I've encountered who don't make that distinction are from PA, it may be regional dialect thing.)

Lowering the house temperature lowers the heat loss out of the house, since the temperature difference through the exterior surfaces is lower, which is a net energy savings. The bigger the difference between the indoors & outdoors, the more heat is getting out of the house.

If the boiler is not a modulating-condensing boiler it's highest efficiency mode is when doing long steady burns, so the recovery from setback will be it's highest efficiency mode of operation. When it's cycling on/off maintaining temperature there is a small amount of energy wasted during each ignition cycle, but the jacket losses of the boiler are higher since the average temperature of the boiler is higher than it is when off for long periods during setback. A high mass boiler that is doing more than 5 burns/ hour or burns less than 5 minutes will be slipping quite a bit below it's steady state combustion efficiency. If it's burning constantly for an hour or more it's average efficiency will be extremely close to it's steady-state combustion efficiency.

Only modulating condensing boilers are more efficient when maintaining a constant room temperature, and then only if it's controls are tweaked for long burns at the lowest possible water temperature needed to keep the house up to temperature.