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hot water circulation problem

i have a forced hot water circulation system. 3 story building. i am getting excessive pressure build up when the heat is running.i have already changed the water main, the auto fill and reducing valve assembly, the pressure temp. gauge on the boiler, bled all the radiators, the expansion tank, even added another one, but it still builds up too much pressure, and bleeds off through the releif valve on the boiler at 30 psi. which leads to air in the system. i cant maintain the 12- 15 psi required, because the auto fill takes it right back to about 20, then when the heat kicks in, it goes over 30, and its just a vicious loop.

Re: hot water circulation problem

Sorry about the cost for the items you've replaced, but it's commendable that you've attained the expertise to diy on your heating system; keep up the good work; don't worry, you can fix this.

Watching that gauge needle move toward and past 30 psi is nerve-wracking.

But HW boilers are designed so that you can ISOLATE the components one by one from the house piping & eventually find the culprit.

As you no doubt know by now the HW system is isolated from the house water supply, except for 2 exceptions:

1) the 50 psi fresh water feed & inlet/reducing valve which knocks down the psi from 50 psi to 12 psi;
2) the 50 psi fresh water feed that supplies the hot water domestic coil inside the boiler, or the indirect HW heater, if you have one.

You also probably know that water EXPANDS ~5% when it is heated by the boiler, so this could also cause the pressure to build up, especially if the expansion tank is the old style & is waterlogged, or the new style & the inside bladder has collapsed.

Could you reply back & advise if you have the old style expansion tank, or the newer bladder-type; also advise if you have a domestic coil or an indirect HW heater for your hot tap water supply.

Since you have 3 floors the cold psi on the gauge should actually read ~15 psi, so the water pressure can lift itself up to the 3rd floor.

Try closing the water inlet supply valve temporarily to isolate the 50 psi house water supply from the boiler supply.

Place a bucket under the pressure reducing valve (PRV) and open the valve by pressing or lifting the handle to vent some water until the gauge reads ~15 psi.

Run the boiler by turning up the t-stat & see if the pressure still goes past 30 psi.

If it still pops the PRV the only other 2 possibilities is that there is a leak in the domestic coil inside the boiler that's allowing 50 psi water to mix with the boiler water (or the indirect HW heater if you have one), or the expansion tanks are not working.

Check the dial on the house main water meter for any needle movement.

Instruct everyone in the house not to use the faucets, or flush the toilet for 5 minutes, & then check the red needles on the water meter dial.

Most meters have a HALF-FOOT red needle off to one side that measures the slightest movement of any water thru the meter.

The half-foot red needle shouldn't move at all for two minutes.

If it moves, there's a leak somewhere & you'll have to further isolate the boiler from the 50 psi system.

If there is still a problem, shut off the 50 psi feed to the domestic coil, or the indirect water heater, if there is one; if there isn't a shut-off valve to these, shut off the main house water supply temporarily.

Bleed the PRV of water again to get down to 15 psi, & see if the problem clears up.

Since you have two expansion tanks, including a new one, a problem there is not likely, but there are ways to check this.

Please post back with the info requested & the results of your tests.

Re: hot water circulation problem

ok. here goes. we replaced the water main because from the water meter to the boiler the pipe was galvanised and the pressure in the house was only about 20 psi and we were having problems with the overall pressure in the house anyway but it was also affecting the auto fill as it did not have enough pressure on the 50 psi side to close it comletely, so water was constanty leaking into the boiler. the potable hot water is from a seperate hot water heater. the expansion tanks i mentioned are brand new (extrol 12 psi charge). i noticed the old one was waterlogged, and changed it (theyre cheap so i just changed it rather than try to screw with it). once i saw that diddnt work, i added another thinking maybe one was not enough, because the original steel drum is still in the basement ceiling (not connected of ourse.).
this has me scratching my head and i have to do alot of running around, up and down in to the basement because i have to check the water all the time, and if i let the system get up to that 30 psi mark, i have to bleed the radiators because air then gets into the system.
i checked for leaks when i did the 50 ft. of 1 inch copper for the main and came up with nothing so thats ok as far as i can tell.
i am beginning to think i maybe got a bad auto fill.
thanks for the info and hope to hear back soon.

Re: hot water circulation problem

regarding the replaced parts, they needed to be replaced anyways because this place has been neglected lol

Re: hot water circulation problem


One step at a time:

Have you tried any of the things I suggested.

1) Did you temporarily shut off the water supply to the boiler, dump some water from the PRV & see if there is any improvement?; 2) Did you check the small half-foot red needle on the water meter for any movement?; 3) is the separate hot water heater for the tap water connected in any way to the boiler??

Please post back.

Re: hot water circulation problem

so sorry jack i tend to get ahead of myself.
the water heater is not connected to the boiler in any way except for the water main.it has its own gas supply too.

i tried turning off the water feed and dropping down the pressure to 15 psi. after running for about 45 minutes, the water temp was 135 and at 25 psi. so the pressure in the system increased by 10 psi.

i checked for leaks and have none, using the method you suggested.

i also noticed that the 1/2 inch pipe from the flo control valve to the expansion tank shut off valve was hot but the pipe leading to the expansion tanks and the expansion tanks themselves were cool to the touch. i made sure the valve was turned on. do you think this may be a problem with that valve being stuck closed on the inside?

Re: hot water circulation problem

If nothing is showing on the main water meter dial, that's a step forward.

It still doesn't rule out a leaky pressure reducing valve.

Did you say you replaced this??

Another possibility is that the reducing valve is misaligned & allowing more than 12-15 psi to pass thru to the boiler.

Is there any chance of some photos???

Did you install the newer BLADDER-TYPE expansion tank(s), or do you have the older steel one propped up between the floor joists???

If you have a newer type ET, it may be necessary to close the valve at the expansion tank & unscrew it to test your theory to see if the valve works.

You would have to shut down the system, drain some water from the lower boiler drain, then remove the ET, then open the water inlet valve to test the valve at the expansion tank for free flow of water.

Pleas post back.

Photos if possible.

Re: hot water circulation problem

i did replaced the whole reducing valve auto fill assembly.

i will post back with some pics., but i do have the old type expansion tank in the floor joists BUT- it is NOT connected to anything. i have 2 "FLEXCON H2PRO HTX 30" with a 12 psi pre-charge expansion tanks now that are brand new.

Re: hot water circulation problem

here are the pics....

Re: hot water circulation problem

sorry it only let me do 5
in the pic with the et shutoff valve, the pipe that seems to go up to the old et is NOT connected. who ever did the change originally just left the pipe to rest on the top of the valve.

i have not gotten the chance to test the et theory though....

Re: hot water circulation problem


Yes, thank for the photos; they are very clear.

That looks like a B&G little red or red fox circulator pump; I couldn't make out the name of the boiler, & we would have to get its total btu/hr output.

The HTX 30 Flexcon ET's are listed in my catalog as having a capacity of 4.54 gallons each; hopefully, this will be enough capacity for the amount of water in your system.

The internal bladder prevents complete extension to the full volume of the ET, but the bladder ET's are allowed to be much smaller than your old steel ET because they are under 12 psi pressure.

I get the impression that in the case of your system there is a LOT of water; when you total up the water in the boiler, the associated piping & the radiators covering 3 floors it could add up to 100 gallons.
(You might try getting a ballpark figure of this total).

When this is multiplied by the expansion factor for when the water heats up to ~200 degrees, you're probably just at the capacity of your 2 new expansion tanks if they're charged to the standard 12 psi.

If this is true, then the ET's won't have quite enough capacity unless you add a few pounds of air in with a bicycle pump.

I don't see a shutoff valve near the new ET's; the first step would be to shut down the system, drain some water from the boiler, & install a ball valve in the 1/2" copper tubing just above the ET's.

Once this is done, you can simply shut the ball valve anytime you want without draining the system to adjust the air pressure in the ET's, or remove them for any other reason.

You should also attach some steel metal strapping right away to the body of the ET's and secure them to the floor joists above; they way they are now, if they should fill with water, they will fall to the floor from the weight of the water.

It would be necessary to remove the ET's by unscrewing them from their metal nipple.

You would then need a normal auto tire pressure gauge to press on to the little tire valve to check the psi charge (usually 12 psi).

The air pressure would be increased to 15 psi on both ET's by using a regular tire air pump and they would be replaced on the system with the steel strapping.

I've included some sites below that explain the usual causes of too much water pressure in the system & ways to resolve it.

The Amtrol site has a chart entitled "Sizing the Extrol"

If you can determine the btu output of your boiler (it may be listed on the housing, or you can check the internet via the model number) you can determine the size ET you should be using.

The output of your boiler is probably 150,000 btu/hr or 200,000 btu/hr (just guessing).

If you notice on the chart, this size boiler output requires a slightly larger ET (SX- 30V is listed at 14 gal max. volume) than the combined 2 that you have on there now.

This may turn out to be the problem, but make further checks on this.

The psi on the pressure reducing valve is set according to the height of the highest radiator on the 3rd floor, and its distance from the boiler.

Thus, if the highest radiator in your house is 25' above the boiler, the formula/calculation is:
25/2.3 = 10.87 + 4' = 15 psi (the cold psi pressure on the boiler dial).



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