Re: Oversized Boiler, Is It Costing Me Money, How Much?

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Posted by Harold Kestenholz - Hydronic Network on October 29, 2001 at 11:44:52:

In Reply to: Oversized Boiler, Is It Costing Me Money, How Much? posted by Joe Howell on October 29, 2001 at 07:58:39:

If the boiler is I=B=R rated, then the piping and pickup allowance of 50% extra is already included. Be aware of the difference between IBR rating and heating capacity, the boiler output does not include a piping and pickup allowance. Adding 50% to an IBR steam rating makes a gross oversizing.

: The problem I am having is that the boiler will come on and within 10 supply steam to the radiators.

*** This is the initial steam pressurizing before air is purged from the returns.

However, it will reach the high pressure on the Pressuretrol (1-1/2 psig) and cut the fire, then the pressure will decrease and start the fire (.5 psig), and continue on like this for about 45 minute before satisfying the thermostat.

*** This is the air purging and warming cycle. You do not want the cycle to take longer. Larger air vents may shorten the cycle and permit less boiler input.

Once everything is hot the boiler will fire for approx 2 minutes until it reaches 1-1/2 psig, then off for about 1 minute until it reaches 1/2 psig and continue on like this.

*** This indicates high input and a small boiler and water content mass. The accumulated burn time of the on and off cycles over a clocked hour is valuable information. If the total time is 20 minutes out of the hour during the coldest weather, the boiler input is excessive. Over half an hour of burn time is getting closer to optimum.

: Should the boiler input (gas burner and valve) be sized to supply the amount of energy needed (in my case only 210 is needed)and no more or should it be oversized? Are these components automatically sized for the input when one buys a boiler?

*** The boiler size (rated input) should be selected for the radiation output and piping and pickup allowance. The boiler is manufactured to give the maximum efficiency at the factory designed burner input. An oversized burner in a boiler means that higher exhaust temperatures than necessary will occur, wasting energy - and may also cause flames to burn outside the boiler.

: All other things being the same (piping, etc). Is this way oversized boiler costing me on my gas bill. I am thinking that I can save a lot on gas costs if I install the proper sized boiler since the input will be way less and the efficiency will be approx 20% better. Is this correct?

You can save gas by using a more efficient boiler properly sized for the system that is part of the test paramteres.

: How can I fiqure out what my input is currently if its not written on the boiler?

*** Clock the gas meter over one minute and multiply by 60 minutes for the hourly rating. The dial on the gas meter is marked for quantity per rotation.

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