5M BTU Boiler Pressure

Charles7540

Last year we installed all new main vents, and decreased our cutoff pressure from 5-6PSI down to 3PSI. We had asked for modulation at 1PSI and cutoff at 2PSI, but the boiler company protested that it was too large of a change. This year I'm asking again. Having read "Greening Steam" I think there is not any benefit to the extra pressure from 2-3psi, and we burn fuel to get there. The comment from our boiler company is: "Thats very close tolerance to set for two spring action pressure controls. We will give it a shot." This makes me concerned that this isn't what they normally do, and so I'm out on a limb here. What do you think? Is modulation at 1PSI and cutoff at 2PSI wrong OR very difficult to achieve on a boiler of this size?

Jamie Hall

Not at all difficult, and no reason to go over 2 psi for normal heating applications. The potential problem is that the usual Honeywell/Resideo prussuretrol calibration is none too accurate, so it really helps to have a low pressure gauge (in addition to the code required 30 psi gauge) so you can see what pressure the pressuretrol is really operating at.

EBEBRATT-Ed

It is very difficult to get both modulation and limit controls set to those low pressures and prevent short cycling

What they should do is use the modulation control to keep the burner modulating at a pressure as low as possible with just enough pressure to heat the building. That may be 1, 1 1/2 or 2psi as low as you can make it to get heat.

Then set the operating control above that. Start with 1 psi above the point where the burner modulates. You may get short cycling which also wastes fuel and is hard on the equipment "starts & stops" beat equipment.

You probably going to have to make the operating control adjustments yourself as you going to have to live with the boiler to get it right.

To low on the operating control will short cycle you to death, too high will build too much steam.

Your not going to get perfection in most cases this is a compromise. Most of the building were originally coal fired and most steam boiler are oversized.

JUGHNE

Do you actually have "modulation" controls on the boiler?

That implies lowering the firing rate of the burner.

Pictures of the controls would be good, actually the entire boiler, piping and all.

retiredguy

I may get a lot of pushback from the steam guys on this sight but depending on the type of steam system you have and the type of radiation installed, lowering the steam pressure from 5-6 to 2-3 serves no real purpose except to make it hard for the boiler to utilize a modulating burner. Before everybody goes bonkers, if you have standing cast iron radiators as your heat source then a very low pressure may work. However, if the heating system is univents and heat exchangers then a higher pressure is warranted. That said, sitting at my desk and telling you what the steam pressure set point should be is not possible. Some steam systems require a higher steam pressure.

bburd

My high school in Massachusetts was a large building with #5 oil fired steam heat. Controls were pneumatic and steam pressure was maintained throughout the heating season.

The older buildings built between 1930 and 1959 operated between 3-5 psi. I don't believe the burners modulated. They had cast-iron radiation, fin tube draft barriers and univents.

The newer buildings built in 1965 had a separate steam plant and operated between 5-10 psi. These had univents and AHUs with steam coils only, no radiation. I don't know if the higher pressure was due to smaller pipes or the coil design.

This was all explained to me by the full-time stationary engineer who operated and maintained the system, a meticulous ex-Navy man. The boiler rooms were spotless.

Bottom line: Although most steam space heating needs 2 psi or less, different buildings may have different designs and require different pressures.

realliveplumber

What does the 5M translate to?

JUGHNE

I would guess 5,000,000 BTUH input?

That's why the pictures would be interesting.

realliveplumber

I get puckered up starting a 250,000 btu unit.

(M in Roman is 5000.)

Can even imagine one million let alone 5

Jamie Hall

The comment by @retiredguy is quite correct. Some types of installation do need the higher steam pressures, so it would be helpful to know what the boiler is powering. I was assuming, perhaps incorrectly, "normal" residential type equipment -- which doesn't always mean residential (consider our proverbial go-to installation, the Empire State Building!) However, one should be aware that some fittings used on residential type equipment, such as many types of main vents and thermostatic traps work poorly -- if they even survive -- at higher pressures.

EBEBRATT-Ed

Largest burner & boiler I worked on was 18,000,000btu. I think the boiler was 400hp

But most were smaller14,000,000 or less.

But residential steam is different than a hospital or an industrial plant those were not designed for 2psi.

I agree with @retiredguy a 5 million btu burner is going to be very difficult (I would say impossible) to make it work at 2psi.

Many burner manufacturers used to claim that there burner turndown (diff between high and low fire) could be 5 or 8 or 10-1. That was all pure nonsense. Most large burner will turn down 3-1 and that is that, maybe a little bit better on some fuels.

JUGHNE

Pictures will tell a lot.

Steamhead

@Charles7540 , bottom line is, use the lowest pressure that will heat the building.

retiredguy

My normal limit for start-up and service were boilers firing nat gas or fuel oil up to 21,000,000 BTU's or approximately 500 HP with the occasional 30,000,000 BTU's or 750 HP and up to 300 PSI of steam pressure. When I started these boilers new or seasonal, I rarely let anyone in the boiler room with me unless they were well versed service techs. So when it comes to residential units I usually let the experienced guys on this site do the explaining and I get educated reading their responses.