Pressure settings for PRV and expansion tank


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Old 02-08-09, 08:38 AM
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Pressure settings for PRV and expansion tank

I have a 32 year old oil fired baseboard heating system in my house. I suspected that the old pressure gauge was giving me incorrect readings so I installed a water pressure test gauge on the boiler drain and confirmed that the old pressure gauge was reading about 5 psi higher than it should.

PRV - Watts S1156F
Extrol Tank - Model #30
Vertical Height from boiler drain to 2nd floor - 15 feet

So even though the old pressure gauge was reading 12 psi, the true pressure was really less than that (7 psi?). Also, I determined the expansion tank air pressure was probably less than 5 psi.

I wanted to correct the pressures for the PRV and expansion tank so I can keep the stress on this old boiler and expansion tank low.

After cooling down the system and reducing the pressure to zero by bleeding water from the 2nd floor vent, I raised the PRV setting so the pressure reading at the boiler drain was 10 psi (15 feet x .43 plus 3 or 4). Since the expansion tank is located about 3-1/2 feet higher than the boiler drain pressure gauge, I adjusted the air pressure in the expansion tank (while still connected when the system was at zero) to 9 psi.

Generally speaking, I think the system is working better now. Starting at the cold pressure of 10 psi, it now "tops out" at 18 psi when the temp gauge "says" 180 degrees.

Is my reasoning correct on the pressure settings that I am using? Should I change anything? Should I drop the expansion tank pressure a little more?

Thanks,

Gary
 
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Old 02-08-09, 10:06 AM
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Gary, 18 psi seems a little high for the cut-out of 180*. Could be you need a larger expansion tank. I'm sure there's a sizing tool on the 'net somewhere (maybe even here).
Tom Beer 4U2
 
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Old 02-08-09, 02:08 PM
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I wouldn't worry about 18 PSI... I wouldn't even start getting concerned until it was at like 23 PSI hot... but I would go ahead and use the minimum pressure of 12 PSI in the tank and system... because there are other reasons for the minimum... you don't want the circulating pump cavitating from too little pressure on the suction side...

Should I drop the expansion tank pressure a little more?
No, that will probably actually make it worse.
 
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Old 02-08-09, 09:30 PM
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Before I do this, I want to make sure I've got my numbers right.

First, after dropping the system pressure to zero, I will change the air pressure of the expansion tank to 12 psi. Then I will adjust the PRV valve so the system pressure is also 12 psi. at the expansion tank. The pressure gauge on the boiler drain is located 3-1/2 feet below the expansion tank. Does this mean that the pressure reading at the boiler drain should read 13-1/2 psi for the system pressure at the expansion tank to be 12 psi?

Thanks'

Gary
 
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Old 02-09-09, 02:18 PM
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Hi Gary, don't worry about the couple of feet difference there... just set 'em both at 12 PSI and you'll be just fine. It doesn't have to be that close... as long as there's enough room in the tank to take up your expansion, that's all that matters.
 
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Old 02-09-09, 03:08 PM
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OK This forum is a great help. I think relying on my faulty pressure gauge has been causing me some problems for awhile. I read a lot of postings on this forum before it dawned on me that a bad pressure gauge was sending me on a "wild goose chase".

I will adjust the pressures and post back.

Thanks,

Gary
 
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Old 02-09-09, 03:58 PM
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Have I ever said, "I HATE GAUGES!" ?

Now ya know why!
 
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Old 02-22-09, 01:44 PM
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Update:

Per suggestions, I readjusted the system pressure and expansion tank pressure to 12 psi. After heating up, the pressure was topping out around 22-23 psi so I was a little concerned, but I thought maybe some air might have gotten into the system, so I monitored things closely hoping that it would purge out.

About a week later we had a power outage because of bad weather, so the boiler cooled down. When power came back on I happened to be in the basement so I watched the gauge as it heated up. When things heated up to about 140 degrees, all of a sudden there was a little spritz of water from the pressure relief valve. The pressure had not even climbed to 20 psi yet, and was well below the rating of 30 psi., so I was a little surprised. But other than that one incident, the heater seemed to be working OK.

I mentioned this to an HVAC mechanic that I knew and, without hesitation, he told me to change the pressure relief valve. So I cooled things down and changed the pressure relief valve, and while I was at it I checked the pressure on my expansion tank, and noticed that it had dropped from 12 psi to 10 psi in one week. So I installed a new expansion tank of same size (#30).

So now with the new pressure relief valve, new expansion tank, cold temperature system and expansion tank pressures set at 12 psi each, the pressure reading on the new gauge tops out at 20 psi when the temperature reads 180 degrees, and drops down to 17 psi when the boiler kicks in at around 150 degrees.

My aquastat is a Honeywell Type L8124A, C L8151A. I have it set as follows:

Low Limit - 140 degrees
High Limit - 170 degrees
Differential - 10 degrees

With the High Limit set at 170 degrees, the temperature gauge reads 180 degrees after the lag. I will wait until I take a shower this summer before I adjust the Low Limit.

Do the readings and settings that I have seem to be reasonable? Thanks for all the help.

Gary
 
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Old 02-22-09, 05:12 PM
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It don't get much better than that Gary! Good job!

Only one thing left to do... get out yer black Sharpie and write the installation date on the tank and relief valve.

Looks like yer good to go for another bunch of years... but do check the pressure in the expansion tank at LEAST every two years, better every year... replace the relief valve like every 5 years or so...
 
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Old 02-22-09, 06:51 PM
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Originally Posted by NJ Trooper View Post

Only one thing left to do... get out yer black Sharpie and write the installation date on the tank and relief valve.
Good idea. This boiler may just last longer than my mind!

Thanks for the help.

Gary
 
 

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