Pressure setting and fuel/gas consumption

Med618

Is there any way to quantify the amount of fuel oil or gas which is burned at different pressure settings?



In other words, how much more fuel oil will a boiler at 5 PSI burn compared to one set at 2 PSI assuming the heat cycle is the same.



And if it can't be quantified in gallons or therms, can it be quantified as a percentage(ie, it will use 20% less fuel)?



Or is this not really an answerable question?

BobC

How much

The amount of fuel depends on the specific system. A boiler has to run longer to produce more pressure and the steam temperature only goes up marginally so your wasting the fuel used for the excess pressure. If the boiler runs for 2 minutes more to make that extra pressure EVERY TIME it runs the fuel adds up - say a boiler runs 6 times a day then it's costing you 12 minutes a day or 84 minutes of run time a week.



Another factor is a lot of air valves will not function above 3PSI so a system will heat up initially but the air valve will not open back up until the pressure drops below 3PSI so you will burn a lot of fuel and not get any heat into the house. Some air valves can be permanently damaged by high pressure.



Most steam systems run just fine a 1 PSI or less, some less than 4 oz, you just have to decide if it's worth it to spend more to replace a pressuretrol (1.5 to psi) with a vaporstat (0 to 16 OZ).



Bob

Big-Al_2

Depends

5 PSI steam is about 15 degrees hotter than 0 PSI steam.  That basically means that the flue gasses going up your chimney will be about 15 degrees warmer too.  That 15 degrees is heat that could have gone into making more lower pressure steam instead of floating away.



However, even though my Pressuretrol is set at it's lowest reliable setting, 2 PSI, the boiler almost never reaches that pressure before the thermostat is satisfied.  Most of my heating is done at close to zero pressure.  If I set the Vaporstat higher than 2, it probably wouldn't change my gas bill much at all . . . because the boiler would very rarely run long enough to make any appreciable pressure anyway.



The same thing holds true for me installing a low-pressure Vaporstat.  My boiler spends so little time above 1/2 PSI that the amount of heat wasted by the few minutes a day that the boiler might operate at a little higher pressure would not mean much. That's for residential.  If I had a multi-unit apartment building where the boiler was steaming most of the time, then lower pressure would probably be worth it.

MrDvorak

Some guessing

According to my naive calculations and this table: http://www.engineeringtoolbox.com/saturated-steam-properties-d_457.html, rising the pressure from 1. 5 PSI to 3.0 PSI requires additional 8.5% steam mass to be generated ~> about 9% more heat is needed to reach 3.0 PSI (excluding small temperature differences). Going from 3.0 PSI to 4.5 PSI, extra ~8% heat is required accordingly. But the extra energy does not get lost; it just is not used as efficiently as if you stayed at 1.5 PSI all the time IMHO (bigger pressure and heat losses in the system, blocked vents, larger temperature swings caused by imperfect thermostats, etc.).









Or I have it completely wrong.

Edit: I had a gross logical error with a negligible effect in my original post, mixing up latent heat and specific heat. I am removing it now.