Boiler knociking and reducing valve tripping

Hello all…

This is my first winter in my first house. I’ve got a hot water boiler (Dunkirk PWB-5D). There’s a knocking sound inside the boiler that happens every couple of seconds when it’s running. It sounds like a little kid hitting the pipe with a hammer and it’s driving me nuts. Also, the pressure release value is tripping almost every time the boiler runs. The heat output is good and I can hear the pump running. I’ve only got 1 zone and the boiler is in the basement

I tried to check the Ex-Trol expansion tank (model 60). During the run, it gets hot on top only, the bottom stays cool. I tested the pressure in the tank when the system was cold. I turned off the valve on the feeder pipe and relieved the pressure in the boiler to 0. The pressure in the expansion tank was low (barely read 5 psi) so I pumped it up back to 12 psi. Then I turned the valve on the feeder back on, the system refilled to 12 psi so the pressure reducing valve seems fine. When I started the system, the pressure rose fast again (reaching 25 psi almost right away) and then slowly crept up to >30psi and the valve tripped again.

I think the problem was present along time before I bought the place because the pressure release value has started to corrode and has accumulated rust, and there’s a rust spot on the floor right below the valve. Oh, and there appears to be two pinhole leaks on the top side of the expansion tank as I can see a little droplet of water (it appears rusty) in the same spot during and after the boiler runs. I’m guessing it needs to be replaced along with the pressure relief valve.

What I think has been happening is that the system has been cycling each run, increasing pressure and tripping the release valve and the system refilling with fresh water once it cools. Perhaps this has led to lime buildup in the heat exchanger and is the root cause of the knocking. But I don't know anything... which is why I'm here.

Is replacing the expansion tank and pressure release value reasonable DIY jobs? Any thoughts on how to get rid of the lime build up if it is indeed the source of the knocking? Any thoughts?

 

If you get a chance, post some pictures that depict the overall system and closer ones that show greater local detail from different angles.

Check the temperature range of the burner cycle. It might be cycling to a temp too extreme and the high end needs to be adjusted downward. The fact that the expansion tank is hot on the top and cool/cold on the bottom......could your system be forcing steam into the tank causing the hot upper portion while the water in the expansion tank does not heat up very fast and keeps the bottom cooler.

 

Sure, I'd be happy to do anything that might help the problem. I think this link should work, let me know if you need more pics or higher resolution.

My crazy boiler overview

Boiler big pic 1

Boiler pipes

My crusty pressure release valve

My expansion tank with leaks

Also, I should add, there's been an interesting development. I turned down the gas at the manual shut off valve. I dialed it down just enough to reduce the flow slightly ad the knocking stopped completely and the system still heated fine. I'm pretty sure the this means something, but I don't know what that might be. I suspect the automatic valve might be the culprit

As for the expansion tank. I checked it again this morning and the pressure I had put in last night held. I should clarify that the top doesn't get boiling hot or anything close. It looks to me like the water from the system is getting in and circulating a bit, but nothing extreme... if that makes any sense. It doesn't have an air vent so I don't know if there's any air/steam in there, but it doesn't seem to get hot enough to be steam

As for the temp, I don't think I've ever seen it above 180... but I haven't checked it specifically at it's hottest. I'll watch for it tonight.

The troubling thing is the that the pressure still goes up higher than I think it should. I included a pic of the gauge to show the point. Before taking the pic, I drained the system a little to reduce the pressure down to 0 psi, then opened the feeder valve to fill the system back to 12 psi, then I shut the feeder valve and closed the pressure reducing valve in an attempt to completely close the system. Then I turned the boiler on and a few minutes later, the pressure was >25 even though the water had barely started heating (110 F). What could be causing this... I'm so puzzled. I know water expands when it heats, but should it be expanding this much so quick?

Oh, and Is the circulating pump in the right place?

Cheers and thanks for any input or advice! you guys are awesome!

 

Can't answer in detail at this time, but there are some 'givens' here...

If your relief valve is all crusty, it can not be trusted and must be replaced.

If your expansion tank is Holy, it needs to be replaced.

Can you do it yourself? Probably... not rocket science!

more later...

Oh... almost forgot to chant my mantra...

"never trust a boiler gauge... never trust a boiler gauge..."

 

Please open that valve again! That is a dangerous practice!

 

Thanks, I'll open the gas valve up all the way again. Is there a way to regulate the gas pressure more safely with the ignitor? The manual shows a pressure regulator adjustment that looks like it could be turned with a screwdriver but there's nothing in the manual about how to adjust it.

I'll get a new expansion tank and pressure relief valve and put those it.Can I replace the tank just by closing it off from the system and putting in the new one? I'm thinking I'll have to drain the system to replace the pressure relief valve so maybe I'll do both at the same time

 

As NJTrooper suggested, adjusting the gas flow to the combustion chamber by feathering the main shutoff valve is not the proper way to achieve the result you are looking for. I do not know where the proper adjustment is. Would the gas company have a service person take a look at it for you (perhaps at no cost).

Regarding the expansion tank...the expansion tank functions by having a compressible gas inside. When the liquid in the system expands, water enters the tank and eliminates system over-pressure; however, if the expansion tank is water logged and has no air inside, pressures increase. If you have a valve to isolate the expansion tank and a second valve to drain the expansion tank, do this. Shut the boiler off, isolate the expansion tank, and try to drain water from the expansion tank. Be sure to close the drain valve and open the isolation valve before restarting the system.

 

With the ignitor? No. Checking or adjusting the gas pressure takes a special gauge - don't try to do it yourself without knowing how.

Can't hurt to ask - but without a service contract, my guess is No, they won't work on equipment that you own. It would be a bit like asking the electric utility to check your dishwasher.

 

I think you are correct.

First thing though is to get the pressure under control.

When you did this, did you happen to notice if the boiler gauge indicated a pressure increase also? If it did, it means that there is still water in the tank. Here is a step by step that should be followed when recharging tanks... but when they are in the condition that yours is, don't bother... just replace it.

1. Shut off boiler and allow to cool to under 100°F.

2. Shut off water supply line to boiler.

3. Drain only enough water from the boiler drain to drop the system pressure to ZERO. REPEAT: DO NOT COMPLETELY DRAIN THE BOILER! ONLY ENOUGH TO DROP THE PRESSURE TO ZERO!

4. With an ACCURATE tire pressure gauge, check the air charge in the tank on the air valve opposite the end of the tank that's connected to the system. If ANY water comes out of the air valve, the bladder inside the tank is shot and the tank needs replaced. If no water comes out the air valve, and the pressure is less than 12-15 PSI, continue to step 5. If the pressure is OK, turn the water supply to the boiler back on and repressurize the system, turn the power back on to the boiler, no service is necessary.

5. Using a bicycle pump, or a small air compressor, add air to the tank until you have 12 PSI air charge.

6. Check the boiler pressure gauge again, and if it has risen off ZERO, drain some more water from the boiler drain until it is again at ZERO.

7. Check the air charge on the tank again. If it is below 12 PSI, add air to the tank until it is at 12 PSI.

8. Repeat steps 6 and 7 until the boiler stays at ZERO and the tank stays at 12 PSI. At this point, the tank is properly recharged and the water supply can be turned on to re-pressurize the system, turn the power on to boiler and return to service.


Not sure if this got answered...

Can you change the expansion tank ?

Yes, easily.

First, you need to shut off the water supply to the boiler, the main shutoff valve from the city water to the boiler.

Next, you need to drain ONLY ENOUGH water from the boiler to drop the pressure to zero.

Then, CLOSE that valve above the tank.

It should unscrew fairly easily, but BE CAREFUL! I suspect it will be VERY HEAVY, because my guess is that it's full of water. BE PREPARED for the weight of it! There could be as much as 8-10 gallons of water in there, and at 8 pounds a gallon... you do the math.

You have a larger than 'normal' expansion tank... I presume that you have all cast iron radiators and a large volume of water in the system. That would be reason for the larger tank.

 

I would wait until the heating season is over to change the relief valve. There's no point in draining the system now... and getting all that air into the pipes...

Plan on changing that PVC pipe to COPPER when you change the relief valve.

The others are correct about the gas supply. Don't mess with it... you need to first have the correct equipment, and second the know-how to use it.

Can you take more pics? all around the boiler... all the valves and such?

I think I can see your 'aquastat' to the left of the expansion tank... the gray box... I would like to know the setting of the temperature limit switch.

TURN POWER OFF! and slide the cover straight off. There should be a whell with temp markings inside... let us know what the setting is.