Cut In on New Steam Boiler??

MikeyG

Hello Steam Heat Experts-  I just had a new steam boiler installed in my property and see a setting for "Cut In"  Please advise what is the proper value for safety and best fuel usage since I believe the heating contractor set it too high.  Thanks for any insight on this.

Mike

nicholas bonham-carter

Pressuretrol settings

This is a good reason to have a good low-pressure gauge, such as a 0-3 psi gauge, marked in ounces. Modern pressuretrols are notorious for their inaccuracy. Use the gauge to verify the setting for the lowest pressure you can attain, with a maximum of 1.5 psi. With a cut-in set as low as can be.

If the main vents are functioning correctly, and the boiler has been sized properly, steam will get to all radiators at the same time with a few ounces of pressure. If the boiler cycles a lot, then the main vents are probably not working, or an excessively large boiler has been installed.--NBC

Steamhead

Those pressure controls

usually come from the factory set at the top of the scale. The installers should have cranked it down- since they didn't, we know they weren't paying attention.



Crank it down!

MikeyG

what number do you recommend?

when you say "crank it down" do you mean to put it at the lowest setting on the scale?  or some other value?

Jamie Hall

Residential steam

such as yours should not operate at more than 1 1/2 to 2 psi -- which is a real stretch for a pressuretrol.



However, with a great deal of care, it can be done.  There are two types of pressuretrols: subtrractive and additive.  In the subtractive variety, you set the cutout pressure on the main scale and the cutin pressure is set by the differential, which is subtracted from the main scale cutout pressure.  If that is the flavour you have (it will say on the control) set the main cutout pressure to 1 1/2 psi, and the differential to 1 psi.  If you can... You will need to fire up the boiler and wait for the pressure to rise (which, if you don't have king valves to shut off the system from the boiler, may take a while -- it should!) and check to make sure that the boiler does cut off and cut back in.  Set that low pressuretrols can be just a bit flaky.



If the pressuretrol is additive, the principle is exactly the same, except that you set  the cutin pressure on the main scale and the cutout is determined by that plus the differential.  In that case, set the cutin at 1/2 psi or as low as it will go, and the differential at 1 psi.  Test it as above.



A better approach is to buy a vapourstat for the main control, setting it at 12 to 16 OUNCES per square inch cutout, and keeping the pressuretrol as a safety backup.



As NBC noted, getting a low pressure gauge in on the act isn't a bad idea -- then you  can see what's really happening.

Jamie Hall

Low end...

I think a number of us have, but I'll reply... the place I take care of is largish (about 7,000 square feet usable, on three floors).  The boiler is in the middle of the basement.  The longest run to any radiator is around 80 feet.  The system is controlled by a vapourstat, with a cutin currently of around 3 ounces and a cutout around 9 ounces -- which it never reaches, except on recovering from a setback situation (and I don't use setbacks).



It takes around 9 minutes for heat to reach the farthest radiators from the time the thermostat calls for heat -- that includes the time it takes the boiler to raise steam, which is about 5 minutes.  The pressure at that point at the boiler is on the order of 2 ounces, more or less.  It will rise -- but only very very slowly (the boiler is an excellent match to the system) to perhaps as much as 5 ounces by the time all the radiators are hot across. 



So, bottom line is no.  No problem with getting steam to the farthest radiators on close to no pressure at all.



Now.  If your venting isn't adequate,  all bets are off.  You can raise enough pressure to move a locomotive and, without adequate venting, you will still not get steam to the poorly vented radiators.

ChrisJ

Where to set it to?

For the most part, the lower the better as long as you get enough heat to all parts of the building.



My system is currently set to trip at 4 ounces and wait for 10 minutes but this cannot be done with the typical pressuretrol. My Pressuretrol which is a backup is set to trip at 1.5PSI via my 3PSI gauge.



Most of the time my system sees very little pressure, in fact my 3.0 PSI Wika gauge never even moves. I'm planning on replacing it with a more useful gauge this fall.