Pressure gain in hotwater boiler

Glenn Harrison

Also, that expansion tank line should be minimum 3/4",as per Dan's books.

Threadcutter

Pressure gain in hotwater boiler.

I am working on a hotwater boiler that services a ranch house with approx. 2000 square feet of living space and three zones of 3/4" copper and fin baseboard.
The system is zoned with 3 Taco zone valves. A 100 series B&G circ. mounted on the return side moves the heating water. Thre is a 30 gallon compression tank mounted above the boiler that is fed from 1/2" tubing off of the bottom of the air scoop on the 1 1/4" supply main. The air scoop is loacted just down stream of the 1 1/4' flow check and before the zone valves.
The boiler also supports an indirect hotwater heater.
The problem I am having is pressure gain in the system. The pressure builds over a few days or weeks depending on the season to a point where the relief valve opens.
After draining the expansion tank all appears well for awhile and then the pressure gain comes back.
I have changed the automatic feed and that did not cure the problem. There was also a problem with the triple aquastat and this too was changed This problem started after the system was changed from a single zone to three zones. Taco zone valves were installed for this purpose.
My thought is that the opening and closing of the zones valves is causing a momentary drop in pressure and this action in turn allows the automatic feed to open for a moment. This addition of excess makeup water then accumalates in the compression tank until the tank must be drained or the relief valve opens.
My thought is to install a low water cut-off in the supply main above the boiler and close the valve ahead of the automatic feed. This would provide boiler protection and stop the influx of water.
My customer wants me to pipe the compression tank to the return feed about 18 inches above the circulator intake. He feels the return line will have a lower pressure then the supply side, and water will move through the pump and not in to the compression tank.
Does anyone have any thoughts on this?

[Deleted User]

If'n I'm a readin' ya right...

Sounds like your customer had been hanging out at this web site...

The pump should pump away from the point of no pressure change (the expansion tank connection) and also make sure there are NO automatic air elimination vents ANYWWHERE in the system.

Others will chime in I assure you.

ME

To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"

John M.

pressure gain

If after closing off valve ahead of fill valve, the pressure continues to build, there may be a small leak in the heat exchanger of the indirect fired water heater. House water pressure is building up in the boiler.

Duncan_2

Possibilities

Any fittings on the compression tank that might slowly be leaking air?

A small leak in the heat exchanger of the indirect water heater is another possibility. The higher house pressure in the indirect DHW bleeds into the boiler side of the system and raises its pressure.

Interesting that it occurred after the zone valve install, though.

One by one, I'd test the compression tank (if a fitting air leak is a possibility), and rule out the auto fill and the indirect by temporarily isolating the house pressure side.

It never hurts to pump away from the expansion tank.

Duncan_2

Wow

You type fast, John. Saw your post after I typed mine.

Threadcutter

Thanks for the advice. Whats with the "no air elimination divices" in the system. My mentor of many years always placed a #7 Maid-O-Mist air vent over the top of the air scoop. Is this wrong? If so, why?
I should add that we always pipe to "power purge" for gross air elimination.
I again thank-you for all your help. This is a great site visited by some very talented people and friendly people.

kevin

if I ...

understand you right it is a conventional compresion tank.The air scoop you say connects to the tank from the BOTTOM of the air scoop? It must be the top of the air scoop or it won't work, it will eventually water log(air rises). The other option would to put in a diaphram tank. When you do the conventional tank you should have no other auto vents in the system because that air is the only way the tank keeps the air in the top.If you take it out somewhere else it will water log. kpc

sootmonkey

how

how can the expantion tank work when it is piped between two closed valves? (the flow check & the zone valve)

sootmonkey

hey

think of the expantion tank as an expantion/contraction tank. call for heat. zone valves open. burner fires. circ. starts. water heats up, and while the zone valve is open, water will expand into the tank. call for heat is over. zone valve closes. circ. stops. flow check closes. expantion tank is now isolated. water in system contracts as it cools. psi drops below feed valve setting. feed valve adds water. just a guess. do you need a flow check with zone valves?