When it comes to efficiency, size does matter.
by Ron Rothenberg, 4 Buyers Real Estate
The heating systems in most homes I see are much too big for the job they have to do.
If you have a boiler that’s too large, it will cost you more throughout its shortened life.
You’ll throw more up-front money at your heating system AND it will have a shorter life because it will short-cycle more than it should. Short-cycling is when a boiler starts, uses all its extra capacity to catch up with heat demand and then stops. The too-big boiler is wasting its life starting and stopping rather than running efficiently. Running a smaller boiler for longer periods is actually more efficient.
Old boilers were oversized, and so many new boilers are oversized. Installers use rules-of-thumb to size boilers,and they tend to overestimate. They are afraid of undersizing a boiler and getting that “my house is too cold” phone call next winter.
Of course, most won’t do the actual heat-loss calculations to ensure that you get the right size, it’s easier just to overestimate. The homeowner will probably never notice.
Hot-air furnaces tend to be even more oversized and that will make them inefficient and also noisier than the right-sized furnace.
The correct way to size any heating plant is to use the Manual J Load calculation. Getting an installer to do this generally requires at least an act of Congress, but it’s just an arithmetical calculation that closely approximates your homes heating loss on one of the coldest days of your winter, based on your floor space, ceiling height, number of windows, number of outside walls in a room, insulation and your climate, etc.
The calculations are actually simpler than they sound, and you can get help from your computer – try a web based calculator at http://www.mrhvac.com/free-hvac-stuff/free-heat-gain-and-loss-calculator/ or a program you can buy for $49 at HVAC-Calc Software.
Whether you do it or the installer does it, you shouldn’t get a new boiler without having someone do a load calculation.
Don’t forget that some day you may be using your heating plant to heat your domestic hot water, too, so be sure to figure that in.
My house came with a 135,000 BTU/hour boiler – that was first sized 80 years ago when the house was built, when there was no insulation in the walls, no thermal windows, very little insulation in the attic, and oil cost very little.
When I went to replace it, various plumbers wanted to put in boilers from 125,000 BTUH – 170,000BTUH which seemed too big, considering that the 135,000 BTUH boiler had been doing the job for so long, even before most of our energy improvements.
I eventually got a 105,000BTUH boiler – that’s probably a bit more capacity then we need now, but I plan to add an indirect water heater to that system soon, and have the boiler heat our domestic hot water, too.
Size does matter, and I’m always surprised at how little homeowners and installers thought about right-sizing their heating plant.