Pulsing pressure-Hot Water Boiler

sd_4

Recently moved into a new house with an 1972 American Standard gas hot water boiler which produces heat fine, but has one problem. When the circulator turns on without the boiler, there are the normal flow noises through the radiators, but when the gas burner kicks on to keep water temp, after about 5-15 seconds, the pressure gauge starts bouncing between 12 and 18 or so pounds. It does this until the burner turns off at which after a few more seconds it settles back down to about 15 or so pounds, which is where it is at when not in operation or when just the circulator is running. The furnace does not run other than when called as there is a hot water heater. The noise it makes while this is happening is quite loud and travels throughout the house. The furnace is 165k btu's and notes around the boiler show the gas company testing it at 164k burn rate. It has a 30 expansion tank which the top only gets hot when heat is operating, so it seems to be normal. No idea what the pressure in the tank is other than the tank looks original. There are 3 original zones on the boiler and the issue happens with each independtly. With some help we flushed the system and zones for air which helped the radiator noises and settled the bouncing to a swing of about 2-3 pounds. Also replaced the water pressure reducing valve and added a backflow preventer. When the old backflow was taken out, it was mostly blocked with gunk and we were attributing the bouncing to air in the system and figured the flushing took care of it. It didn't as over the next week the bouncing came back to its current state where it bounces the 4-6 pounds while firing.

Trying to figure out where to go next with this. If anyone can help me with a direction it would be appreciated.

Mike T., Swampeast MO

Sounds like the compression tank may be almost completely full of water (e.g. waterlogged). This will cause pressure fluctuation when the water heats and expands.

In normal operation the compression tank should not get hot--or even warm--as no water is supposed to be circulating into or out of of the tank.

Presume you're talking about a #30 diaphragm expansion tank. If so, a good chance that the air side has either lost its charge or the diaphragm is leaking.

If you find a hand valve in the pipe leading to the tank, shut it off. (Heating system OFF of course.) Attach an air pressure gauge to the air side of the tank. Drain water from the tank. The air pressure you read should drop to about 12 psi and stay there. If it continues to drop much lower than 12 psi keep draining until you have essentially zero pressure and then pressurize the air side to 12 psi. If a really bad leak in the diaphragm, the pressure will drop very rapidly.

If it [seems] to hold pressure and if at all possible, leave the system off and the valve leading to the tank off as well for a day or two. If you detect any drop in pressure on the air side, the diaphragm is bad and the tank needs to be replaced. (Once you open the valve leading to the tank and let in water you'll no longer be able to read the pressure of just the air.)

If it holds pressure nicely (or if the air stayed at 12# when you bled water), open the valve and turn the system back on.

sd_4

Bouncing pressure-Why??

Yes, it is an old Extrol 30 expansion tank on the boiler. There isn't a shut off between the boiler and the tank though. Only black pipe from the boiler direct into the fitting that holds the tank. There are what appear to be valves in the zone lines after the tank, but nothing before other than the water supply shut off before the pressure reducing valve. Though the top section of the tank gets as hot as the copper heating pipe, the tank sounds empty when tapping either while running or at idle. It actually starts getting warm to the touch about half way up from the bottom and gets pipe hot slighly above the first band near the top. There also doesn't appear to be a way to drain. There is a red plug at the very bottom which is not big enough to take a hose.

One additional thing. While the pressure is bouncing, the boiler sounds a little like a percolator.

Mike T., Swampeast MO

sd:

I'm sort of a dinosour who only works on old original gravity systems using plain (non-bladder) compression tanks.

That said, a defective or improperly charged diaphragm compression tank is very frequently described here as the culprit when the pressure "bounces" during firing. I won't venture to say exactly why other than pressure is somehow being relieved somewhere but the expansion tank is highly suspect. In fact, I'm rather at a loss to expain what else could cause the problem in any reasonably "normal" system.

Since you lack the shut-off valve to the tank, you'll have to drain the system and remove the tank to test it. I [thought] that at least some diaphragm tanks have built-in way to drain water, but perhaps I'm mistaken.

Regardless of if you attempt to DIY or have it done by a pro, I would highly suggest that provision is made to both isolate and drain water from the compression tank for this very reason.

Steamhead (in transit)

I don't think it's the tank

this sounds like an accumulation of dirt and/or sludge on the bottom of the boiler sections. This is more common on atmospheric gas boilers like yours than it is on wet-base units, since on yours the burners are placed below the cast-iron sections instead of inside them. It is also common on ill-maintained water heaters.

The sound you hear is steam bubbling up thru the sludge. The formation and then condensation of steam would cause the bouncing pressure gauge.

The cure is to thoroughly flush out the boiler sections with a hose to get the sludge out. You might want to have a pro do this.

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Mike T., Swampeast MO

You just heard from someone with FAR more experience than me!