Intermittent Boiler overpressure

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Old 12-18-08, 08:47 AM
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Intermittent Boiler overpressure

Hello,

I have a Vaillant oil-fired boiler. I have been having intermittent over-pressure problems this fall. I suspect that the problem is a bad reducing valve -- but I would like to rule out the possibility of a bad expansion tank before I replace the valve. I don't suspect a leak inside the tankless hot water as the problem sort of comes and goes. (Last time I went for a month without a problem) I have a Extrol 30 expansion tank. After allowing the boiler to cool somewhat, I isolated the tank and attempted to figure out how to bleed any excess water from the tank. I am assuming that the valve on the bottom is strictly the fill valve for re-charging the tank with air. So therefore I also assumed the the small red screw valve on the top was the way to bleed any water out of the system. After opening this valve nothing happened. How should I interepret this? Is the tank functioning properly or is this a indicator that the tank is bad. Also the tank is cold on the bottom and hot on the top when up to temperature. I read somwhere that this should be the opposite if the tank is working. Thanks Ron
 
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Old 12-18-08, 02:25 PM
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How old is the house, what kind of heat emitters do you have (cast iron radiators or baseboard convectors) and what size are the pipes from the boiler to the heaters?

Pictures always help. To post pictures you need to first upload the pictures to a photo hosting site such as photobucket.com or villagephotos.com. and then post the public URLs for the pictures (or album) here. More pictures are always better than fewer. Please have CLEAR pictures and have both close up pictures and ones from a far enough distance that we can see how the various parts are interconnected.
 
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Old 12-18-08, 03:54 PM
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Ditto the pics request... they will answer some/most/all of my questions.

When you say that you isolated the tank, I'm assuming that you have a valve on the pipe between the boiler and the tank?

The tank you are describing sounds as though it may not be a bladder type expansion tank. Those generally have a water connection on one end ... and the air valve on the other end. Most often they are installed in a hanging position from the piping. This would put the air valve on the bottom. So, I am a bit confused by your description...

If you have an older steel 'compression' tank, it will be strapped into the joists above the boiler. It may have different fittings/tappings on it for various valves...

Let's have a look.

BTW, what model Vaillant ya got?
 
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Old 12-19-08, 01:10 PM
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Hey,Any updates with your situation? I have exatly the same problem, thinking about changing expansion tank,have doubts since it's cold on a bottom ,and warm on a top,do your know way to check if expansion tank no good anymore?
 
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Old 12-19-08, 03:56 PM
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Allright, here are some photos of the boiler.

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I should give you some history here. We moved in about 2 years ago. House was built in 1964. I have no idea how old the boiler is. 3 days after we moved in the boiler began shooting water out of the relief valve quite rapidly. Plumber replaced hot water coil and that fixed the problem. I have had constant problems getting the 3rd zone to heat the upstairs part of our house consistently. I have bled the return pipes multiple times and the only thing that seemed to finally resolve the air problem in the line was actually increasing the pressure at the boiler a little bit. Normally the boiler ran at 15 - 18 psi. I would use the fast fill to increase it to about 22 -24 psi and that would do a better job of getting the air out. I bled the pipes again this fall before the heating season and right after that is when I started having problems. I was always very careful not to let the pressure get above 25 psi in the boiler -- but I'm wondering if I screwed something up by doing this. The problem lasted about 3 or 4 days then went away for a month. Now I'm having problems again. The frustrating thing is I seem to be getting different results every time I try to do different things to narrow the problem down. Last night I closed the valve prior to the fill valve (reducing valve) to prevent any water feeding into the system. However, I am still seeing the pressure rise in the system. I had it set just below 20 psi and by this afternoon it was above 30 psi. Is this indicative of a another leak in the heating coil?

My Vaillant model is F75-w-55 PP
 
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Old 12-19-08, 04:06 PM
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Sorry forgot to answer some of the other questions. I have baseboard pipes. 3 zones with 1 feed line and two return lines for each zone. Pipes are 3/4 inch copper.
 
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Old 12-19-08, 06:08 PM
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OK, pics ARE worth a thousand words!

You have a bladder type tank ... I thought you meant that the red cap was on top of the tank! well, it is, sorta ... more like above the tank, on a tee ...

Top Hot, Bottom Cool, NORMAL. The way that tank is mounted, which is also normal, the top will always be hotter than the bottom. The air is in the bottom, and the water on top. The BLUE CAP on the bottom of the tank unscrews to reveal a tire valve. That's where the air is checked/adjusted.

The RED CAP up top should normally be open to allow air to escape. That device is an automatic float type air vent. There's a float inside that drops when it's full of air, opening the valve and letting the air out... sometimes the valve doesn't seat all the way, and they leak. If you didn't get any air out, it means that either there is no air in there, or the valve is plugged up.
 
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Old 12-19-08, 06:29 PM
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Okay - which way would you lean towards (valve plugged up or no air) based on what I've described so far?

If it is plugged up -- would that be enough to cause the overpressure?
 
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Old 12-20-08, 02:09 AM
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Sorry I didn't have time to finish my reply and answer the real question...

If you've got the manual feed valve closed AND it is actually closing, then it's possible that the coil is causing your pressure problem again. I see that it appears there are redundant valves... the gray plastic ones seem to have been added later... not sure I like them plastic valves in this application... but let's not worry about that now.

It appears that one of the plastic valves is on the water line into the coil... is that correct? Next thing to try would be to close that valve down and go without hot water for a while and see what happens.

The pics are difficult to follow because I can't tell where the various pipes start and end... for example, where does the water feed actually connect to the boiler? Try pulling back some with the camera to get wider angles that I can follow the piping. Might be able to help get that third zone bled too if I could see that.

Is your boiler serviced annually? It appears that you do woodworking in the vicinity... sawdust ... I'm willing to bet that the blower wheel in the burner is loaded... have you been present when the tech is doing the service? ever notice if the top cover is removed during cleaning/vacuuming process? Those boilers are kinda a pain to clean... I'm willing to bet that it's not.
 
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Old 12-20-08, 02:22 AM
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I think to check that tank pressure properly, you should close the feed valves, shut down the boiler, open a drain on the boiler and drain only enough water to drop the boiler to ZERO PSI. THEN close the isolation valve, and check/adjust the tank pressure. Even if it's not the direct problem, I'm positive that if it hasn't been done in years, the tank pressure is low... maybe VERY low.

So before blaming the coil, let's get that outta the way first.

While you've got the pressure to zero and the isolation valve closed, if you've got any doubts about that air vent valve, go ahead and replace it at that time.

Can't see the back of the boiler, I'm assuming that your pressure relief valve is on the hot water supply fitting that exits the boiler above the flue pipe?
 
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Old 12-20-08, 08:04 AM
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Okay -- I followed your instructions and drained the boiler to about 0 psi. (By the way it sort of seemed to settle on 6 psi and would go no lower so I assumed the guage is slightly out of calibration) I took off the float valve at the top and checked it out. I could hear the float moving up and down freely so it seemed to me that it was working properly. I charged the tank to 12 psi. It was definitly low. After reopening the valve next to the tank the float valve let out the excess air and then closed itself off. So it seems it is working properly. I have now isolated the hot water from the system so I will wait a while and see what happens. Here are some more pics of the boiler.

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The bleed valves for the returns are nowhere near the boiler. Each return line feeds into a 1.5 inch return line that runs along the length of the basement which runs to the back of the boiler and into the circulating pump. All the bleed valvers on the return lines are at various places throughout the basement. The ones pictured are in the greatest spot of all -- Between the rafters and above the main beam that runs along the length of the basement. A lot of fun to get to........
 
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Old 12-20-08, 08:23 AM
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Good... we'll await the results...

meanwhile, those 'thrush' valves have a shutoff on them. Assuming they were installed in the proper 'direction', the shutoff will be on the boiler side, with the drain on the system side. (still too many close shots to relate them to each other ... can't follow the piping layout easily) ...

Those shutoffs are sometimes used for 'balancing' the flows, but are very useful during 'bleeding and purging' operations. By closing them off, you can force the water to flow up and through the zone, rather than taking the path of least resistance straight through the boiler and out the drain. This may be why you can't get that 3rd zone completely air free.

So... how long has it been since the boiler/burner was serviced? If that is indeed sawdust I see, you would probably do well to somehow filter the combustion air into the boiler. Like I said, I bet your blower wheel is packed...
 
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Old 12-20-08, 08:40 AM
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6 psi and would go no lower so I assumed the guage is slightly out of calibration
"slightly" ? that's an understatement.

Yeah... gauge is probly whack... and something you should think about verifying ... I use one I made from junque that I found around the garage...



It really does help to know what the pressure really is!
 
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Old 12-20-08, 09:59 AM
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I think I have a pretty good handle on how to bleed the pipes. I have been bleeding from each zone rather than from the boiler and I think it is working properly now. I have found it necessary to partially close one of the thrush valves also in order to balance the heating out somewhat in the upstairs zone. But it takes quite a bit of work to get it right.....


I'll let you know on the results of our other test later. I want to wait as long as possible to make a good determination
 
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Old 12-20-08, 03:37 PM
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I let the boiler run with the hot water supply off all day. The pressure started at about 19 psi -- and within a few hours it was up tp about 30 psi. However after about 5 hours the pressure dropped back down to about 25 psi and has remained there since then. I had it off about 7 hours total. I just turned the hot back on and will see what happens. Not sure how to interperet this.

I will also look into the guage problem -- I just won't have any time the next few days to scrounge one up.
 
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Old 12-21-08, 08:45 AM
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I turned the hot water supply back on last night and the pressure creeped up to 35 psi by this morning. It seems all indications point to a leak in the hot water coil. Is there anything else that could be causing this?
 
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