> >
>

# What does pressure gauge show and what's optimum pressure?

#1
05-08-12, 08:06 AM
Member
Join Date: Dec 2010
Location: usa
Posts: 44
What does pressure gauge show and what's optimum pressure?

Hi,
I've got Williamson gas-fired steam boiler GSA-250. I have two questions.
By searching this forum and the web I saw most people recommend in private homes pressure settings 0.5psi cut-in with 1psi differential. However, I could not find any official recoomendation neither in boiler manual, or anywere. I wonder if anyone has any reference to a building code or things like that on where did the above numbers come from and how high could they go?

My second questions is what does the pressure gauge show? With the above settings in my pressure control, burners actually turn on at 2psi and turn off at 3.5psi according to the gauge. It is Honewell PA404A and it is installed exactly as shown in the Fig.15 page 14 of the manual ( http://www.williamson-thermoflo.com/...0-738_0210.pdf ).
Thanks,
lzr

#2
05-08-12, 11:59 AM
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 16,321
Obviously the pressure gauge shows the pressure inside the boiler that is higher than the atmospheric pressure outside the boiler. For almost all residential steam heating systems you don't need but the barest amount of pressure, generally about 1/2 psi. Using these low pressures it is difficult to get a decent pressure gauge without spending a ton of money so the accuracy of the gauge is questionable right out of the box. This is especially true since the maximum pressure the boiler can operate is 15 psi and the gauge generally reads to 30 psi. Even with a pressure as high as 1.5 psi in the boiler you are only at 5% of the maximum gauge reading and those gauges are rarely more accurate than +/- 2% of full scale. That means that the allowable error in the gauge (0.6 psi) is about the same as the desired operating pressure so any reading between zero and 1.2 psi could in fact be 0.5 psi.

This is even more pronounced when it comes to the set points of the pressure switch which has far greater allowable error. You could try lowering the setpoint on your control to see if the pressure runs at a lower indication and still provides adequate heat.

This is one more reason why I detest residential steam heat. The control is just plain lousy and it is all but impossible to get better control without spending a boatload of money.

#3
05-08-12, 03:01 PM
Member
Join Date: Oct 2006
Location: USA
Posts: 19,710
Repeat after me:

Never trust a pressure gauge. Never trust a pressure gauge. Never trust a pressure gauge. Never trust a pressure gauge. Never trust a pressure gauge. Never trust a pressure gauge. Never trust a pressure gauge. Never trust a pressure gauge. Never trust a pressure gauge.

Repeat ad infinitum until you believe it.

#4
05-08-12, 04:21 PM
Member
Join Date: Dec 2010
Location: usa
Posts: 44
Furd and NyTrooper,
Thanks for the replies. My old boilder [before it started leaking] had the same pressure settings, but its gauge was showing up to 6.5 psi, while the new one shows up to 3.5 psi. So, I don't know if I now indeed have a lower pressure (inacuracy of pressure control?), or it's the gauge. The boilder reaches the pressure limit very fast and radiators don't seem to get hot enough before it shuts down, although it's hard to say since it's warm outside and it works only briefly.

The installer suggested to raise the pressure setting, that's why I wondered where does the number 0.5psi which everybody suggests come from. Would (say) 1.5 psi cut-in, 2.5psi cut off be a risk?
lzr

#5
05-08-12, 05:36 PM
Member
Join Date: Oct 2006
Location: USA
Posts: 19,710
The installer suggested to raise the pressure setting, that's why I wondered where does the number 0.5psi which everybody suggests come from. Would (say) 1.5 psi cut-in, 2.5psi cut off be a risk?
The installer, to use a well-known term, is a 'knucklehead'.

Raising the pressure on a steam boiler is almost NEVER the 'cure'.

Proper steam vents on the main line, and at the rads is the proper way to balance the system so it heats properly and evenly.

Have we suggested Dan Holohan's book "We've Got Steam Heat! ..." and if we did, did you buy and read it cover to cover? If not, you should! You will know more than your installer when you are done, and it's not all techie mumbo-jumbo. It's FUN to read! Written explicitly for the homeowner.

Heating Help

This is the best \$25 you could spend on your heating system.

#6
05-08-12, 05:40 PM
Member
Join Date: Oct 2006
Location: USA
Posts: 19,710
Where does the number come from?

The bottom line is that the proper pressure in your boiler, assuming that it is correctly installed and all the correct venting is in place and fully functional, is the LOWEST PRESSURE you can run.

Every little bit you increase the pressure costs you proportionally more in fuel. It's not a linear function either. That 'phase change' from boiling water to steam costs a LOT more BTUs than one would imagine.

Don't raise the pressure. Focus on the venting. Buy the book.

#7
05-08-12, 06:30 PM
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 16,321
Quite honestly, I don't know why the residential steam heat community doesn't have a fit over the use of pressure controls. Ideally, I would use a a digital temperature control with an RTD (very accurate thermometer) sensing device. Set it for 213 degrees F. and be done with it. Okay, I'd allow adjustment up to maybe 215 degrees in .01 degree steps for specific systems.

Otherwise, heed what Trooper wrote, I taught him everything I know.

Oh, I almost forgot. If your boiler comes up to pressure that quickly it is being overfired and is likely massively oversized for the heat load.

#8
05-09-12, 08:29 AM
Member
Join Date: Jul 2009
Location: Toronto
Posts: 1,043
oversized steam boilers, really people would do that ..

LOL

#9
05-09-12, 06:06 PM
Member
Join Date: Oct 2006
Location: USA
Posts: 19,710
I taught him everything I know.
And I'm deeply grateful for that! Thank You!

The sad fact is that in a year or so, yer gonna have to teach me all over again! How quickly we forget stuff.

#10
05-09-12, 07:49 PM
Member
Join Date: Dec 2010
Location: usa
Posts: 44
Have we suggested Dan Holohan's book "We've Got Steam Heat! ..." and if we did, did you buy and read it cover to cover? If not, you should!
No, no, yes.

@Furd
Quite honestly, I don't know why the residential steam heat community doesn't have a fit over the use of pressure controls.
Since they do use pressure controls, I wonder why do they use a device with 10psi scale when you need to set below first visible mark?

#11
05-09-12, 11:38 PM
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 16,321
Several reasons. One is that designing and building a pressure-activated control that operates at very low pressures yet is robust enough to handle 15 psi is very difficult and rather expensive. The reason why it has to withstand higher pressures than will ever be used is that residential steam boilers have a maximum pressure rating of 15 psi and that is the setting of the safety valve. This even is true if electronic controls are used.

However, the pressure of saturated steam (the kind inside a boiler) has an absolute relationship with its temperature and that means that pressure can be controlled by controlling the temperature. Utilizing an electronic device called a resistance temperature detector (RTD) it is possible to measure temperature with both a high degree of accuracy and repeatability. Couple the RTD to an electronic control and you can get a very accurate (and repeatable) temperature controller and even calibrate it to read out in pressure if desired. Such a control could be made for two-three hundred dollars but the pressure switch, bad as it is for this application, is probably less than one hundred dollars. Since cost is everything the higher quality control is sacrificed for the control that just (barely) sort of works more or less.