Too Much Pressure Increase When Hot

I have a gas fired Weil Mclain boiler. Pressure is about 15 psi cold. Pressure gets to over 30 psi when hot (between 160 and 180 degrees). I have replaced the expansion tank (Extrol No. 60) and have turned off the water cutoff valve ahead of the Watts auto-feed regulator, so I know no new water is coming in. The problem persists. What else should I be looking at?

[Please see my post below with more details and pics.]

 

Did you check the air in the new expansion tank before you put it on????

 

Yes. It was reading 12 out of the box.

 

Some more details: when the system is cold (has not run for several hours, temp less than 100 deg) pressure is about 12 psi (which I think is probably too low for a 3 story house). When the system is hot (it takes it a few hours to get to 180 deg), pressure goes as high as 32.5 psi. (Obviously, I need a new pressure relief valve.) Pressure goes back down as system cools.

But what is causing the pressure increase in the first place. Anyone have any ideas? I have a separate domestic hot water system (hot water heater, not a tankless coil), so I don't think I have water leaking from that system into the heating system. Could I?

I have replaced the expansion tank and closed the cut-off valve on the water feed.

Would a bad automatic vent cause this (see the third picture - 1262)?

Here are some pics that might help:

http://i168.photobucket.com/albums/u183/bokononfoma/DSCF1260.jpg
http://i168.photobucket.com/albums/u183/bokononfoma/DSCF1261.jpg
http://i168.photobucket.com/albums/u183/bokononfoma/DSCF1262.jpg
http://i168.photobucket.com/albums/u183/bokononfoma/DSCF1263.jpg

The fourth is the system head on.
The first is the system head on but zoomed in on the boiler (doesn't show the pipes closer to the ceiling).
The second is the left hand side, showing the expansion tank (which appears to have an automatic vent attached to a tee on the piping leading to it) and the pressure relief valve (on a separate line of pipe from the ET).
The third is the right hand side, showing the top of the circulating pump (red), and the water feed line and valve.

Thanks in advance for any advice.

 

This is a replacement boiler in an older system, right? You stated a three story house. The old boiler had a non-diaphram tank, didn't it?

You need more expansion tank capacity. The easiest way to do this is to pipe in a second tank in the same line as the existing tank. An alternate location would be to tee into the pipe between the make-up water PRV and its connection to the return line.

Expansion tanks must be sized to the amount of water in a system and the greatest temperature difference in the system. You have the boiler connected to significantly larger piping going to/from the heating system. That larger pipe contains more water.

When you install the second expansion tank I suggest that you also install a ball valve in the pipe between the expansion tanks and the boiler to allow changing the tanks without draining the system. Lock (use baling wire or a ty-wrap) the ball valve open when not changing tanks.

You also have the circulator pump installed on the boiler return, it is best if it is on the boiler supply, or at least have the expansion tank connected on the suction side of the circulator.

 

Thank you Furd. The house is recently new to me, so I don't know what replacements and other work have been done before. It is 1920's construction, and the boiler is much newer than that, however, so it is reasonable to assume this is a replacement boiler.

I don't know what expansion tank was original to the system. I replaced the expansion tank with the same model that was already in place (Extrol No. 60) thinking the old tank might have gone bad. The old tank was not water logged but was registering a low pressure (less than 8 psi). The new one came at 12 psi out of the box and I pumped it up to about 15.

Yes, the house is three stories (although the 3rd floor is really a finished attic with just 2 radiators). My adventures all began attempting to diagnose why I wasn't getting heat to the 3rd floor. Bleeding the radiators up there solved that problem, but then I noticed the wild pressure swings on the boiler between hot and cold.

Do you think I could get away with just putting in a larger expansion tank (maybe Extrol No. 90) rather than adding a second? If the diameter is bigger, however, I won't have room, as the current expansion tank is pretty much right up against my water heater.

Thanks also for the tips on the circulator pump.

 

The tank needs to have about 1-2 PSI _LESS_ air pre-charge than the normal system pressure when cold. If the pressure is too high on the air side, and you adjust the system pressure to match, you might not have enough tank capacity when it's hot.

Recheck to make sure you've got just a little less pressure in the tank than in the system.

 

I'm going to disagree with NJ. If the air pre-charge is lower than the system cold pressure then the tank will already have some water in it when the system water starts to heat and expand. This will DECREASE the usable size of the tank.

Quite honestly, the initial system pressure and the initial tank pressure are not as important as one might be led to believe. What IS important is that the usable VOLUME of the tank is greater than the amount the water will expand from the lowest (usually ambient) temperature to the highest achievable temperature. The highest achievable temperature is the maximum setting of the high limit cutout control, often as high as 240 degrees.

Bokonon has large pipes in the original system, at least 1-1/4 and maybe 1-1/2 inch nominal size. These pipes hold a LOT of water, far more than would 3/4 or one inch copper. If there are cast iron radiators installed they also will hold a LOT of water. It is the total volume of water and the maximum temperature differential that must be used when calculating the required USABLE volume of the expansion tank, the BTU size of the boiler is irrelavant.

An oversized expansion tank will not cause any problems in system operation whereas an undersized expansion tank will cause exactly the problems being experienced. The only downside to an oversized expansion tank is initial cost and the room necessary for its installation.

Bokonon, you need additional usable expansion tank volume. You already have paid for and installed a new tank. If you still have the old tank you may try to use it in addition to the one already installed after pumping the air side to the same pressure as the new tank and verifying that it is not leaking. If you do not have the old tank anymore then you need to purchase another tank. You may be able to "get away" with the smaller (number 30) tank or you may want to just make sure by installing an additional number 60 tank. The easiest way to install it would be to connect it into the steel nipple between the make-up water PRV and the return line. I would cut the copper line going into the PRV before the last elbow and install a union fitting. Remove the steel nipple and replace it with nipples, a tee and another union to the PRV to allow for easy removal when the PRV needs replacement. From the side of the tee install a ball valve and then piping to the additional expansion tank. You may use either steel or copper piping to the expansion tank and you may mount/install the tank where it best fits, even hanging it from the overhead is okay.

 

furd, if you don't have a little water in the tank when the system is cold, by setting the air just a tad below, the system pressure will drop like a rock when it cools off. The system cools, the tank empties, and then the vacuum on the system tries to pull the bladder out the little hole. (sounds painful!)

If you have more air, there is also less room for water, they both take up space.

How much water would the tank take if the pressure were 20 ?
How much if it was 25 ? or 30 ?

The tank will take the most water when the pressure on the air side is EXACTLY the same as the pressure on the water side, but to play it safe, in order to not draw a vacuum when the system cools, set the air pre-charge _just below_ the system pressure at it's coldest temp.

I'm not talkin' much... just like 1 PSI.
Just a wee bit of "pre-load" on the bladder.

 

Sorry but nope. The only time the system pressure will go to zero is when you drain the system. The expansion tank comes from the factory with a precharge and the diaphram is not being "pulled out the little hole" by the atmospheric pressure being lower than the tank pressure.

Think about it.

 

I really don't wanna hijack the thread, just lemmee say this and I'm outta here:

I think I get it now... and I think I understand why I saw this happen on my system (pressure drop to zero and below-)

Say you install a tank with a 15PSI precharge on a system that's got it's PRV set to 12 PSI. As the water gets hot, you will see rapid increase from 12 to 15, but then the rate of pressure rise will slow as the tank begins accepting the expansion water. Right?

And, if you run with the feed valve open, you should NEVER see the pressure drop below that setpoint.

I run with my feed valve closed, so if I were to bleed some air when the system was hot, my system pressure COULD drop as it cooled, because there isn't enough water in the system now... right ?

So, the best advice then is to set the pre-charge as closely to the coldest temp system pressure expected as possible, yes ?

I believe it's still correct that too much air pre-charge will also limit the admittance of the tank though ? If the pre-charge is at say 25 the system would zoom to 25PSI rather quickly, no ?

That "sucking the diaphragm out the hole" thing was a figure of speech BTW. Only meant figuratively.

 

Thanks for all the advice. I do still have the old tank, and since it was holding 8 psi, I will add some air and see it will hold a little more.

It doesn't sound like much work to add the second tank and since I have one laying around, the cost will be minimal. It certainly can't hurt to try. And yes, I do have cast iron radiators.

Question for curiosity and educational sake: Before I bled all the radiators, I was not seeing this problem (though to be honest, I wasn't paying so much attention to the cold and hot readings on the pressure guage back then, so it might have been happening and I just didn't realize it). When I bled the radiators, especially the ones on the 3rd floor that were stone cold, a LOT of air came out. Is it possible this air had been providing an additional "cushion" in the system that was moderating the pressure swings (making the cold radiators and pipes act almost like extra expansion space)?

 

One of Dan Holohan's articles over at heatinghelp.com talks about exactly that! The system he looked at had NO expansion tank...

 

NJ Trooper wrote:

"Say you install a tank with a 15PSI precharge on a system that's got it's PRV set to 12 PSI. As the water gets hot, you will see rapid increase from 12 to 15, but then the rate of pressure rise will slow as the tank begins accepting the expansion water. Right?"

That is correct.


"And, if you run with the feed valve open, you should NEVER see the pressure drop below that setpoint."

That is also correct.


"I run with my feed valve closed, so if I were to bleed some air when the system was hot, my system pressure COULD drop as it cooled, because there isn't enough water in the system now... right ?"

Also correct.


"So, the best advice then is to set the pre-charge as closely to the coldest temp system pressure expected as possible, yes ?"

Yes. It is also not a good idea to manually bleed air with the make-up water off. After you think you have bled all the air manually it is fine to close the make-up and allow the automatic air vents to function (or not, lol) as long as you know what pressure you should have at any given temperature and you periodically check that the pressure and temperature is what is expected.


"I believe it's still correct that too much air pre-charge will also limit the admittance of the tank though ? If the pre-charge is at say 25 the system would zoom to 25PSI rather quickly, no ?"

You understand it well.


"That "sucking the diaphragm out the hole" thing was a figure of speech BTW. Only meant figuratively."

I know. I put that response in because there will be people without your experience and understanding who will read these comments and I didn't want anyone to get the wrong idea.

 

Bokonon asked:

"Question for curiosity and educational sake: Before I bled all the radiators, I was not seeing this problem (though to be honest, I wasn't paying so much attention to the cold and hot readings on the pressure guage back then, so it might have been happening and I just didn't realize it). When I bled the radiators, especially the ones on the 3rd floor that were stone cold, a LOT of air came out. Is it possible this air had been providing an additional "cushion" in the system that was moderating the pressure swings (making the cold radiators and pipes act almost like extra expansion space)?"

Yes, that is exactly what was happening.

Ideally the expansion tank should be connected on the suction side of the circulator pump. By connecting the additional expansion tank in the way that I described you will have this second tank piped correctly.

 

Thanks again for so much advice. (And thanks, Trooper, for the heatinghelp.com reference -- good reading.) This place is a tremendous resource for newbies like me. A week ago I knew nothing at all about hot water heating systems (I had forced air in all my other homes).

I'll give it a shot and report back!