Minimum Pressure for Hydronic Boiler?

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LDPosse
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Post by LDPosse » Sun. Dec. 21, 2014 10:48 pm

At my 2 unit rental property, I have each side of the house on its own oil fired boiler.

I installed both of these units in October 2006. It seems, like clockwork, that the T&P valves on both boilers have failed this year. The boilers are Burnham LEDV-2 units, and have been solid performers. Both boilers seem to be working fine, but they are running with virtually no pressure in the system, maybe 1-2 psi. If I try to turn the supply water on, they start to leak water out of the T&P valve. I used to let them run with the supply water on all the time, but I decided to change that a few years ago, just in case of a system failure, I don't want a basement full of water.

Will running with the system pressure this low lead to any problems, such as premature failure of the boiler or radiators?

At this point in the heating season, I don't know how to replace these valves, without causing a major disruption to the heating system, and causing inconvenince to my tenants, by them not having heat for half a day and by needing to go into their homes to bleed all of the rads/baseboards.

Any input/suggestions/advice appreciated!


 
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Post by lzaharis » Mon. Dec. 22, 2014 12:36 am

The preset inlet water pressure regulators
for feed water are set at 12 pounds.

Do you have two back flow preventer's installed for both boilers???

So what you are saying it you cannot physically shut the
hot outlet and cold return lines from both apartments
in the basement; not good as you could or should be able
to isolate both apartment units from the boilers to solve problems
in the basement.

if you have low pressure that may also indicate a low
water level in both boilers.(not good)

If you had a pair of spirotherm air vents you may not
need to bleed the baseboard at all .

If your pressure relief valves are leaking and you shut off
the feed water supply lines you will have low pressure as you
have a low water supply, not good like I said.

If your not comfortable doing it have plumber do it and put in
two pairs of isolation valves and a pair of spirovents.

 
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Post by Rob R. » Mon. Dec. 22, 2014 5:15 am

Before you get too far, check the expansion tanks. A water logged diaphragm tank is the usual cause for a leaking relief valve. They might as well sell a kit with an expansion tank, float vent, and relief valve - they all seem to fail at the same time.

If you can isolate the boilers from the rest of the heating system, you should be able to replace whatever is needed without too much excitement...but if you aren't sure where to start, I suggest calling a plumber and getting the job done.

 
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Post by lsayre » Mon. Dec. 22, 2014 6:13 am

I believe that cold system fill pressure depends upon the height of the house. A Single story house can get by on as little as 8.5 PSI, but a two story house needs 12 PSI. See the link I provided below, where the pressure requirements are displayed on a chart that forum member and Engineer 'Yanche' provided.

I believe that Yanche's chart provides a cushion of 5 PSI, and it multiplies the height by 0.433. Therefore to duplicate his chart you multiply the system height by 0.433 and add 5 PSI to determine the proper fill pressure.

For 8 ft (single story): Fill PSI = 0.433 x 8 +5 = 8.464 PSI
For 16 feet (two story): Fill PSI = 0.433 x 16 + 5 = 11.928 PSI (and this is why expansion tanks come standard preset at 12 PSI)
For 24 feet (three story): Fill PSI = 0.433 x 24 +5 = 15.392 PSI

Single Story House : System and Exp. Tank Static Pressure?
Last edited by lsayre on Mon. Dec. 22, 2014 7:34 am, edited 1 time in total.

 
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lsayre
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Post by lsayre » Mon. Dec. 22, 2014 6:56 am

OK. After some quick research I believe the Yanche chart actually divides height by 2.31 (since 2.31 feet of vertical water column has a static pressure of 1 PSI) and then it adds 5.

Thus the real formula:

For 8 ft (single story): Fill PSI = 8/2.31 +5 = 8.463 PSI
For 16 feet (two story): Fill PSI = 16/2.31 + 5 = 11.926 PSI (and this is why expansion tanks come standard preset at 12 PSI)
For 24 feet (three story): Fill PSI = 24/2.31 +5 = 15.390 PSI

But since 1/2.31 = 0.4329 my first guess was essentially spot on, and both sets of equations are equivalent.

I also learned that the 5 PSI cushion is there to avoid circulator cavitation. The minimum cushion to meet NPSH (net positive suction head) requirements for water at up to 230 degrees is about 4 PSI, so if you truly want to live on the edge you can use 4 instead of 5 in the above equations.

IMPORTANT: If you decide to decrease your cold fill pressure for a single story house (or increase it for a three story house) you must alter the pressure of your expansion tank to closely match the pressure of your cold fill. Increasing it for a three story house is a must. Decreasing it for a single story house is not a must.

LDPosse, if you are only seeing 1 or 2 PSI, you risk cavitation and the rapid destruction of your circulator(s) impeller(s) and/or impeller housing(s) and seals. You also potentially risk boiling the water within your boilers.

 
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Post by blrman07 » Mon. Dec. 22, 2014 9:15 am

Uhhh guys before we start going bonkers with all the calculations the OP can remedy this fairly easy. Install a combo PRV and pressure reducer in the makeup water line going to the boilers. Watts makes a good one but I don't recommend any single manufacturer. Let the price and tech sheets be your guide.

http://www.watts.com/pages/_products_details.asp?pid=3424

 
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Post by waldo lemieux » Mon. Dec. 22, 2014 10:21 am

LD,

You can change out those relief valves easily and with no interruption in service. Turn off power and water supply. Tape and dope new tp valves ,place bucket under old valve and remove the old valve. The lines will vapor lock and only a few blurbs will come out before you start to catch thread with new valve. Very little if any will come out in the five seconds the valve is out. Easy peasy

Happy holidays
Waldo


 
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Post by blrman07 » Mon. Dec. 22, 2014 10:28 am

waldo lemieux wrote:LD,

You can change out those relief valves easily and with no interruption in service. Turn off power and water supply. Tape and dope new tp valves ,place bucket under old valve and remove the old valve. The lines will vapor lock and only a few blurbs will come out before you start to catch thread with new valve. Very little if any will come out in the five seconds the valve is out. Easy peasy

Happy holidays
Waldo
Been there, did that, told em to keep the T shirt. just make sure you shut it down for a little cooling off period or that water that blurbs out gets real hot real quick on exposed flesh. Experience sure helps to keep the burning sensation down.

 
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Post by Rob R. » Tue. Dec. 23, 2014 5:40 am

Relief valves usually don't start leaking without a reason. I'll say it again, check the expansion tank(s). A failed or underinflated tank can cause the relief valve to "weep", and often they get some grit under the seat. Sometimes you can get lucky and open them a few times and flush out the dirt and solve the leak. Obviously this must be done in controlled manner so you don't get burned.
Will running with the system pressure this low lead to any problems, such as premature failure of the boiler or radiators?
If you leave the make-up water valved off with a leak, you run the risk of ZERO pressure, and a tenant calling you for lack of heat in one or more zones. There is also the risk of circulator cavitation, etc. If you leave the make-up water turned on with a leak, you are constantly adding fresh water to the system which will encourage corrosion and mineral buildup. In short, you need to get this fixed.

Air pressure in the expansion tank should be adjusted to match the cold-fill pressure of the system, which is normally 12 psi for a two story home.

 
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Post by blrman07 » Tue. Dec. 23, 2014 6:50 am

If the relief valve is good a system can function with zero expansion tank capacity if the pressure is only 2 psi. If the pressure was 25 psi and the boiler started firing the heated water would expand and cause the pressure to go up to SV setting and lift it.

The system pressure is reported to be only 2 psi and the valve is weeping. A proper safety relief valve has a lifting setting of 30 psi. He has other things to check before he ever gets to the expansion tank.

1. Verify the system pressure is only 2 psi. Change pressure gauge if necessary to verify this.
2. If it is only 2 psi change the pressure relief valve with proper size and capacity valve.
3. Open the water to the boiler and watch the pressure to see where it settles at.
4. If the system pressure overfills then change the automatic water makeup regulator.
5. THEN check the expansion tank and ensure that it is not water logged. Regulate the air in the tank to the proper amount or change the tank.

 
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Post by Rob R. » Tue. Dec. 23, 2014 8:24 am

My point wasn't that a failed expansion tank is making it leak now, but rather it may be what cause the valve to start leaking in the first place. Regardless, all of the things listed above should be checked. Good point to verify the pressure...a gauge with a hose bib fitting is a simple way to double check via a boiler drain.

 
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Post by Lightning » Tue. Dec. 23, 2014 12:13 pm

Rob R. wrote:There is also the risk of circulator cavitation,
What is that?? :?

 
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Post by lsayre » Tue. Dec. 23, 2014 12:44 pm

Cavitation is when the reduced pressure zone at the suction side of the pump drops the fluid below its vapor pressure and effectively boils the fluid being pumped. Most pump damage actually happens when the vaporized liquid (now a gas) passes through the impeller and slams into the high pressure material on the other side, being immediately collapsed and recompressed into a liquid. When the gas bubbles collapse they create hammering shock waves.
Last edited by lsayre on Tue. Dec. 23, 2014 2:59 pm, edited 1 time in total.

 
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Post by Lightning » Tue. Dec. 23, 2014 1:42 pm

Neato... Thanks for the explanation..
I thought it had something to do with the pump imploding.. :lol:

 
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Post by coaledsweat » Tue. Dec. 23, 2014 3:28 pm

Lightning wrote:Neato... Thanks for the explanation..
I thought it had something to do with the pump imploding.. :lol:
You would think it did after seeing the damage it does!


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