Oversized Boiler

pr8891

Is there an way to calculate the extra cost of running a boiler that is about 30% oversized? The system appears to be running fairly efficiently but trying to decide if it would be worth our while to change the boiler and what impact that would have on our heating bill.

ChrisJ

How

was it determined to be 30% oversized?  Is this based on the amount of radiation or heat loss?

Jamie Hall

There is some cost...

associated with running an oversized boiler but unless it is really seriously genuinely short-cycling -- that is,, shutting off on pressure several times before the thermostat is satisfied (and not on recovering from a setback, just normally running) and if that can't be helped by down-firing the boiler (if it is presently running in the middle of its firing range, it should be possible to down fire that far in most cases) the cost isn't going to be much -- perhaps 5 percent of the annual fuel cost, if that.



And it's going to take a long time to pay for a new boiler at that rate...

Long Beach Ed

Downfire?

Many manufacturers specify that their boilers can be down-fired up to 20%.   This may be one tool you can use if the thing is cycling on pressure. 

ChrisJ

what is

considered short cycling on a steam boiler?  I've yet to be around a steam system working correctly so I have no idea what a normal cycle is. 



My previous boiler was significantly undersized and could never build any pressure even when it was in the 40s outside.

Mark N

Short Cycling

The boiler is short cycling when on a call for heat from the thermostat the boiler is cycled on and off multiple times by the pressuretrol or the vaporstat before satisfying the thermostat. An oversized boiler will do this because it makes more steam than the system can condense. I have a boiler that is rated for 271sqft EDR. My system has 260sqft of EDR. From observing my system over the past 3 years it always run between 20 to 25 minutes on a call for heat. Never have observed it cycle on pressure. I have a low pressure gauge and my boiler runs at .5oz of pressure even with outside temps as low as zero outside.

Jamie Hall

Just to add to Mark's comment

which is quite correct -- the boiler in the building I super runs about the same -- even a correctly sized boiler will begin to cycle on the vapourstat or pressuretrol after it has been running for half to three quarters of an hour, as might happen when recovering from a big (say 5 degree) setback.  Then, depending on the relationship between the size of the boiler, the radiation, the length and size of steam mains, the quality of insulation -- and possibly the phase of the moon, for all I know -- the boiler will typically run for a minute or two or three, then shut off for about a minute, and repeat.  This is perfectly normal.



Emphasis: this cycling towards the end of a really long run is perfectly normal1



Now if it's doing that within say five to ten minutes of startup, when things are just beginning to get warm (not all radiators blazing hot all the way across!) you do have a size problem...



Do investigate down firing first.  It's cheapest and almost always improves overall efficiency, it the technician doing it knows what he (or she!) is doing.