Boiler pressure at 30 psi - can't get it down!

Reply

  #1  
Old 02-14-10, 09:40 AM
Member
Thread Starter
Join Date: Feb 2010
Posts: 6
Boiler pressure at 30 psi - can't get it down!

I have a four-unit building with an older ack-o-matic hot water boiler, 4 taco zone valves, 1 B&G 100 series pump. A month ago, I noticed when the boiler was fired up, the pressure would get up to 25-30 psi and the pressure relief valve would start to drip quite a bit. When the burners turn off, the pressure comes back to 12-16 psi and the relief valve stops dripping. This happened no matter what heat temp the boiler was set at (150 - 180). Fortunately, all 4 zoned units were getting good flow and heat. I first drained the expansion tank (closed the valve to it first and then opened it when I was done). That would make the pressure stabilze for a few days, and then the same thing would happen. There is no hole or drip in the expansion tank. I called an HVAC pro, who did what I did and said it looked good and left, but of course a few days later the problem was the same. I did the same thing 3-4 times, same outcome and eventually even after the expansion tank drainage, the pressure would still get too high now just a few minutes later. I then drained each unit separately and there was no gurgling and the flow was good. Then I checked the feeder/fill valve and it would always fill up to 12 psi when I drained some water out of the system. The HVAC pro returned and he tried draining separately from the expansion tank, relief valve, and the bottom side of the boiler itself and in each case no air but the high pressure remained when the burners come on. We also tried bleeding the baseboards in all four units (each room) and there was no air. He kind of gave up and left. Since then, it has been about the same but now there is a small knocking sound and with that I can see the guage on the pressure meter jump every few seconds. I also tried to let the boiler cool, drain some water, let the fill valve get it up to 12 psi, close of the fresh water supply before the fill valve and let the boiler run. It would still go up to 18-20 but then when the burners were off the pressure would come back down to 6-8 psi. The expansion tank seems to be fine, the fill valve is doing its job right, and there is no air that I can find, so I am dumbfounded Right now, I left it the way it is but I am very nervous about the high pressure and I am sure it will destroy the boiler eventually. I've had a few HVAC pros and they don't know why it is happening and are looking to replace any and all parts but without any working hypothesis. Any ideas about what might be wrong or what else to try? Thanks...
 
Sponsored Links
  #2  
Old 02-14-10, 10:08 AM
Member
Join Date: Aug 2009
Location: USA
Posts: 4,947
What type of expansion tank? Is it a bladder type or one that is up in the ceiling?
 
  #3  
Old 02-14-10, 10:12 AM
Member
Thread Starter
Join Date: Feb 2010
Posts: 6
Hello - it is one up in the ceiling, steel, probably 15 gallons
 
  #4  
Old 02-14-10, 11:20 AM
NJT's Avatar
NJT
NJT is offline
Member
Join Date: Oct 2006
Location: USA
Posts: 23,539
Upvotes Received: 1
Let's see it... free account / Image hosting, free photo sharing & video sharing at Photobucket / upload there / drop a link to album here.

I have little doubt that the tank has ever been fully drained... if you've been reading posts, you surely have heard of the "Drinking Straw Analogy" applied to expansion tanks?
 
  #5  
Old 02-14-10, 11:29 AM
Member
Thread Starter
Join Date: Feb 2010
Posts: 6
Hey there. I will have to run over there later to get some pics. I have read your posts and I think u are right on regarding the vacuum suction. When i got 4-5 buckets full out, I figured that was pretty good. It was barely dripping afterwards. The other reason I wasn't too concerned with waiting to completely drain was that it would just get filled up to almost half way with water the minute I open the valve anyway. I have drained this tank myself the last 7 years and never had this or any other pressure problem. But it doesnt hurt to try it again - will do that with i go to get some pics

Other than leaks or clogs (as far as i can tell, i have neither) , what else can cause a steel expansion tank to go bad?

The most bizzare part of this problem is that for a few weeks after the expansion tank draining, the pressure would stabilize for a few days and then gradually get worse again. But now it seems to get bad right away. It would seem the pressure reducing valve would be bad, but it seems to keep it at 12 everytime i remove water from the system.
 
  #6  
Old 02-14-10, 11:51 AM
NJT's Avatar
NJT
NJT is offline
Member
Join Date: Oct 2006
Location: USA
Posts: 23,539
Upvotes Received: 1
It could be a few things...

The pressure reducing valve might be slowly leaking through... when they do this, they will usually appear to function properly initially, but then that slow leak starts and eventually the system is overpressurized. If this is the case, you can test it by closing the manual shutoff ahead of the reducing valve after the pressure is at 12-15 PSI... if the pressure no longer rises, then it's a pretty sure bet that the reducing valve is leaking through.

In an attempt to remedy the situation, it sounds as though the system was 'purged' and there may have been air trapped out in the system that was acting as 'extra' expansion space. Air in a system will NOT cause over pressure ... it will in fact have the opposite effect.

wasn't too concerned with waiting to completely drain was that it would just get filled up to almost half way
True... to a point. If the tank is empty... FULL of AIR at atmospheric pressure, when you repressurize it to 12-15 PSI, the air will compress, and the tank will fill to 1/3-1/2 full of water. But this is with an EMPTY tank...

If you drain a few buckets full, and pull a vacuum at the top of the tank, and don't introduce any more air to the tank, when you turn the water back on, you will be right back where you started... there is no additional air in the tank to displace the water... all you are doing is 'changing' the water.

I believe you may have both a partially waterlogged tank, AND a leaky reducing valve.
 
  #7  
Old 02-18-10, 08:04 PM
Member
Thread Starter
Join Date: Feb 2010
Posts: 6
Thanks for the info everyone.

When I drain my tank I first close the shut off to it. The drain vavle has a cap right at the drain valve so I open that up. At first nothing came out. It had a small screw in the drain and when I turned that, it all came out gushing - so that is why I think air was able to get it in and water out. I let it drain all the way until it just drips lightly.

So on Sun when I left (and posted the first time) I had left the system at 12 and turned a shut off after the feeder to see if it was the feeder that was causing trouble. I went today (4 days later) and all of a sudden there has been no more leaks from the relief valve. But the pressure on the gauge reads 0-1! I checked all the returns from the five zones and they were all hot and the zone valves were all open. I thought for sure that the second floor would not be flowing but no one has called to complain about lack of heat and there is good flow from what i can tell. Needless to say, I have left it alone for now.

So I guess the pressure gauge may be bad? It was reading the high pressure good because only when it read 25-30 did it drip or pop. Also, I dont' know if I should open the fill valve again. Probably not. But then I never know if the pressure might be too low at some point since the gauge is at pretty much already at 0.

Also, I imagine I would have to drain the entire system to put in a new pressure/temp gauge right?

What would you guys do?

Thanks
 
  #8  
Old 02-18-10, 08:51 PM
Member
Join Date: Nov 2002
Location: Massachusetts
Posts: 1,816
too high pressure

If you have 0-1PSI in the boiler after shutting off the supply feed. Open it again to fill to 12PSIG, and shut it off again and watch it again every day and see what the pressure is doing. If it drops to 0 PSIG you likely have a water leak in the circuit and having the water off could crack your boiler, so be careful!
Make sure when you fill the boiler after draining the conventional expansion tank, you close the small screw in that airvent at the bottom of the tank. Pressurize the water circuit to 12 PSIG first THEN open the gate valve to the tank and the fill valve will pressurize back up to 12 PSI.
Another question, Did you add any thing onto this circuit like another zone or hotwater heater???
If your pressure goes up when shutting off the supply water feed, this can happen when a tankless HW coil springs a leak and water line pressure being higher starts to fill the boiler water circuit....
 
  #9  
Old 02-19-10, 05:56 PM
NJT's Avatar
NJT
NJT is offline
Member
Join Date: Oct 2006
Location: USA
Posts: 23,539
Upvotes Received: 1
As much as I hate gauges, the SAFE thing to do at this point is assume that it is correct... if there IS a leak, you could run into more trouble than you want. You could have a slow leak somewhere... you won't see water because the system is hot and the water evaporates as soon as it leaks out. But what you want to look for is the REMAINS of a leak... big old cruddy 'growths' of mineral at the joints...

Also, I imagine I would have to drain the entire system to put in a new pressure/temp gauge right?
Maybe... depending on how the system is valved, you may be able to isolate sections so you don't have to drain the whole thing... obviously, the less you drain, the better.

On the other hand, it's easy enough to go to the supply store and pick up a 0-30 or 0-50 PSI gauge, and fittings that adapt to a hose bib. Screw the gauge onto the bib, open the valve and voila! instant replacement pressure gauge.

But for now, assume that there is a slow leak somewhere, and take hvac's advice and run that pressure back up to 12 PSI.
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes