Increased pressure at boiler. So what’s going on?

[Heyjoe]

Burnham’s boiler manual says there are only three things that cause increased pressure,and that’s what I see posted all over the internet. So what’s going on? They don’t mention the boilers heat exchanger, old pumps,air eliminators,water velocity, water temperature, heat limit setting, old age or anything else, just three things. PRV, expansion tank and leak in a boilermate coil.Thats it period.

I haven’t tried it but it seems to me that after watching my boilermate /water heater run that I could hook up a garden hose to the drain and run hot water for hours and the boiler pressure would never get over 15 psi, but open a heating zone and in 5 minutes it’s approaching 30 psi. Burnham won’t talk to you and the only advise plumbing contractors have is buy a new one.

 

[Dana]

Running the hot water full blast won't likely change the peak pressures on the system. The pressure on the boiler will go up and down with boiler temperature, and the pressure at a given boiler temperature is about the same independent of which zone is calling for heat.

It may be that the heat exchanger inside the water heater takes heat at a higher rate than the zone radiation. How fast the temperature & pressure rises on the boiler may be slower when serving the water heater than when serving a heating zone, but that's about it. It's unlikely that the heat exchanger in the water heat is so large that it can dump heat into the tank at the same rate that the boiler is delivering heat- it will eventually hit the high-limit temp. Your water heater is probably small enough that the aquastat is always satisfied well before the high-limit is reached on the boiler.

 

[Sponsor]

 

[Jadnashua]

IF your expansion tank is failing or isn't sized properly, the pressure will rise more than by design. Most pressure relief valves on a boiler is at 30psi. A common running pressure in a residential boiler unless you have a 3 or more story building, is 12-14 psi. Your expansion tank should be set to the nominal cold pressure in the system. You need enough pressure in the system to ensure the lower pressure at the high point doesn't drop from ambient. That usually means about 0.43psi per foot from the boiler to the highest point plus at least a couple of pounds. That will ensure there's alway enough pressure so the boiling point of the water doesn't drop because the pressure drops to what is effectively a vacuum, lowering the boiling point.

If your ET is precharged to 60psi, it won't accept any expanding water until the system's pressure shoots to 60psi. A working safety pressure relief valve would have already opened. If it's a 30psi one, and it's not opening, it has failed.

 

[Fitter30]

To check the bladder expansion tank it has a tire valve stem on the end of the tank you will need a low pressure tire gauge 0- 50 lb, a tire pump or air compressor. With boiler and pump or pumps off depress valve stem water comes out tank needs to be replaced. If just air tank should be at system gage pressure. Like jadnashua wrote 12- 14 lbs system pressure.

 

[Jadnashua]

We're assuming you have a bladder type expansion tank. If it's waterlogged (i.e., the bladder has failed), if you wrap on it, it will be a thud. If it's got air trapped in there, at least part of it will sort of ring if you wrap on it and the part with water will sound dull. Water leaking when you check the pressure is always a sign it is bad.

Note, to readjust the pressure in the tank, you must first shut any autofill off, shut the boiler off, open a valve to relieve any pressure in the system, then, you can check and adjust the ET's pressure. Once it's good, shut any valves, and refill the system. An alternative might be to remove the tank then check the pressure, but depending, that might be more of a pain.