Hydronic Heating System Pressure

gonzotech

Have a hydronic heating system that was over pressurizing causing a 125lb relief to blow. We replaced a bladder for the expansion tank and the pressures somewhat stabilized but the system is still experiencing somewhat high pressures. the fill pressure is 30lbs. Once the system is filled to 30lbs we start the boiler up and after a day or so the system on the low side of the pump rises to 80lbs and the high side to 120lbs. The city water make up is only 40lbs so why are would it climb from 30psi to 80psi? could the expansion tank be undersized? Take a look at the picture provided and let me know what you think.

the_donut

Pump suction is drawing water into system.

JUGHNE

What is the nameplate pressure rating of the boiler?
How tall is the building?

gonzotech

I don't have the name plate pressure rating at the moment, boiler is located on the roof of a 2 story building.

JUGHNE

Do you know if this always had some problem with pressure?
I had a troublesome set up, I got things set and then shut off the water fill valve. But I do have a LWCO in place.

gonzotech

Donut, I fill the system to 30lbs, start the pump, suction drops to say 20lbs, the make up water fills some more till it reaches 30lbs during operation, I still don't understand how it climbs to 80lbs.

gonzotech

We've been on the site for a couple years and have always had these pressures. We recently have taken a closer look as to why the high pressures due to leaks in the building. Trying to nail down the cause. I may shut the fill for a few days and test it that way. The unit does have a LWCO.

the_donut

gonzotech said:

Donut, I fill the system to 30lbs, start the pump, suction drops to say 20lbs, the make up water fills some more till it reaches 30lbs during operation, I still don't understand how it climbs to 80lbs.

Displaces more air in tank. Water doesn’t expand much. I would isolate makeup water and see if issue goes away. If it doesn’t, compression tank is more than likely problem. It wouldn’t hurt to pull lwco and trindicator/gauges and check for clogs. Also check wye strainers and clean if you have them.

Mark Eatherton

What kind of distance is between the expansion tank and the pump? A few feet, many feet, many floors?

ME

hot_rod

Any indirect tanks piped into then loop? Could city water pressure be getting into the system other than the fill valve?

Is the boiler rated for a 125 psi operating condition?

gonzotech

MAWP is 125psi. Distance between expansion tank and pump is less than 10 feet

hot_rod

On the top of a two story building? 10 psi should be plenty operating pressure.

ratio

hot rod said:

On the top of a two story building? 10 psi should be plenty operating pressure.

This. The static fill pressure needs to push the water up to the highest point as referenced from the location where you are taking the pressure. Assuming you're taking the pressure at the PRV, unless it's in the basement you should be good with nearly anything, 10# is fine and will give you ≈23 feet above the PRV.

the_donut

ratio said:

hot rod said:

On the top of a two story building? 10 psi should be plenty operating pressure.

This. The static fill pressure needs to push the water up to the highest point as referenced from the location where you are taking the pressure. Assuming you're taking the pressure at the PRV, unless it's in the basement you should be good with nearly anything, 10# is fine and will give you ≈23 feet above the PRV.

Boiler is at high point, only pressure to get overcome friction needed. Static weight should be enough to overcome friction.

Jamie Hall

I'd shut the fill off for a few days, indeed -- but have someone keeping an eye on the whole system.

Have you checked the pressure at the fill when the pump starts and is running? Right after the isolating valves for the reducing valve? What I'm suspecting is that that pressure is dropping low enough to allow water in. Now in theory the expansion tank shouldn't allow that -- but it is possible that the tank bottoms out and can't hold the pressure up. Or, of course, that the line to the tank is plugged -- but I assume that you have checked that.

jumper

hot rod said:

On the top of a two story building? 10 psi should be plenty operating pressure.

Yes emphatically.

It's said that NPSH doesn't matter for a closed loop. Maybe when pump size is correct and the pressure is correct. Difficult to do after the fact but pump should be sufficiently low so that pressure is not an issue. The PRV should also be at bottom so that pressure reduction is minimized. That is if you absolutely insist on automatic make up.

Reliable LWCO is always a good idea. Does a flowswitch qualify?

Mark Eatherton

I've seen this happen where there was more than one expansion tank in a system. Remember, trapped air CAN act like an expansion tank...

ME

EBEBRATT-Ed

You only need 10 psi at the top of that system. you state you replaced the bladder. Did you re pressurize the tank with air? To what pressure?

Set the prv to 10-15 psi. Isolate the expansion tank from the system and remove any water pressure on the tank. Pressurize the air side to 10-15 psi same as the fill pressure. Bleed the system and get it running remove all air and leave the feedwater off and see what happens

ratio

Wait, are those valves drawn correctly? As drawn, to close the drain you also isolate the tank from the system.

Have you left the feedwater valves closed for a while?

Jamie Hall

ratio said:

Wait, are those valves drawn correctly? As drawn, to close the drain you also isolate the tank from the system.

Have you left the feedwater valves closed for a while?

Egad. Missed that. You're right @ratio -- I hope that drawing is wrong! Because that would surely do it...

gonzotech

Revised pictures with valves in the correct location.

gonzotech

EBEBRATT-Ed said:

You only need 10 psi at the top of that system. you state you replaced the bladder. Did you re pressurize the tank with air? To what pressure?

Set the prv to 10-15 psi. Isolate the expansion tank from the system and remove any water pressure on the tank. Pressurize the air side to 10-15 psi same as the fill pressure. Bleed the system and get it running remove all air and leave the feedwater off and see what happens

Yes we replaced the bladder and pressurized to 30lbs same is fill pressure. The fill valve is adjustable between 25 to 75psi. We are not sure why such a high pressure fill valve was installed. We quoted to replace with 12lb fill/35lb relief combo valve pending approval.

gonzotech

Mark Eatherton said:

I've seen this happen where there was more than one expansion tank in a system. Remember, trapped air CAN act like an expansion tank...

ME

Trapped Air? How would I determine if there is air trapped somewhere in the system. And if there was, how would that cause the system pressure to go up 50lbs? I had some one else suggest the same thing, Im just not understanding the concept.

JUGHNE

If you have CI tall rads, air can be trapped in the upper part adding to the expansion air available.
I purposely have not bled air from 2nd unused floors to avoid overheating.
If your pump ran for several days that air could be swept out of those rads and you lose that extra cushion.
Then with your fill pressure set so high, the pump could activate the fill valve bringing in more water than you want.

SuperJ

gonzotech said:

EBEBRATT-Ed said:

You only need 10 psi at the top of that system. you state you replaced the bladder. Did you re pressurize the tank with air? To what pressure?

Set the prv to 10-15 psi. Isolate the expansion tank from the system and remove any water pressure on the tank. Pressurize the air side to 10-15 psi same as the fill pressure. Bleed the system and get it running remove all air and leave the feedwater off and see what happens

Yes we replaced the bladder and pressurized to 30lbs same is fill pressure. The fill valve is adjustable between 25 to 75psi. We are not sure why such a high pressure fill valve was installed. We quoted to replace with 12lb fill/35lb relief combo valve pending approval.

I assume you had the pressure tank isolated from system and open to atmosphere when setting the bladder pressure? If not, the system pressure can compress the bladder so that a 30psi charge, is actually a fraction of that when the bladder is allowed to expand. This means you won't have much cushion at all and you pressure may spike up as things heat up.

SuperJ

gonzotech said:

Donut, I fill the system to 30lbs, start the pump, suction drops to say 20lbs, the make up water fills some more till it reaches 30lbs during operation, I still don't understand how it climbs to 80lbs.

On a properly setup system I don't think your suction pressure should change if that's where your tank and makeup is at.

This implies either:
-your tank isn't set up correctly (charge pressure),
-or isn't big enough,
-or you have compress-ability (trapped air) on the discharge side of the pump somewhere (when the pump starts trapped air compresses, dropping system pressure below makeup threshold.) I think you would have to be running a lot of differential pressure on your pump for this to cause a an increase all the way to 125psi though.
- Unlikely but a check valve between the tank and makeup could cause this too I suppose. (Basically preventing the expansion tank from pressurizing the system.

Note that there is no such thing as too big with expansion tanks (just takes up more space and costs more $$$). But too small is a big problem, definitely confirm the sizing (you might even be able to temporarily connect a second one to your drain line to test).

This video is excellent, I've set the link to start at a relevant point.
https://youtu.be/F8rlAUmb-pM?t=36m43s

delta T

how big is your expansion tank? 3 million btu boiler should require a pretty good size expansion tank, will depend on your system volume though

JUGHNE

And how big is the building......3 million is a lot of heat.
What type of emitters?

Mark Eatherton

gonzotech said:

Mark Eatherton said:

I've seen this happen where there was more than one expansion tank in a system. Remember, trapped air CAN act like an expansion tank...

ME

Trapped Air? How would I determine if there is air trapped somewhere in the system. And if there was, how would that cause the system pressure to go up 50lbs? I had some one else suggest the same thing, Im just not understanding the concept.

Testing is simple. Finding and eliminating requires you to think like air and water, which isn't so easy.

Testing: Turn off boilers and pumps. Isolate expansion tank and make up. Close all automatic air vents.

Connect a hose to the system, and point the end of the hose into a good bucket.

Open connected drain cock and see how much water flows into the bucket until water pressure drops to zero. If you get an initial SPURT, and then nothing, you are good. If you get a substantial amount of water into the bucket, this is the air bubble decompressing, pushing water out of the system.

Now comes the hard part. Thinking like water and air. Air has a tendency to trap in locations of low velocity, usually at high points of the system. Upright cast iron radiators are notorious for holding air. Find and eliminate any air pockets and or other expansion tanks in the system.

The expansion tank is what is referred to as the Point of No Mechanically Induced Pressure Changes. Formerly PONPC. This expansion tank connection needs to be as close to the pumps inlet as is physically possible. If it is, then your pump can not create any negative pressures that would cause the PRV to release water into the system. Unless as noted, there is another expansion tank somewhere in the system, or air bubbles acting like expansion tanks. There should only be one (1) expansion tank system per fluid in the system. So if there was a plate and frame heat exchanger, with 2 fluids, then there would be 2 different expansion tanks. One for the boiler side,and one for the non boiler side of the HXer.

Lastly, I agree with the others. Lower the operating pressure to around 12 PSI. More pressure isn't always better.

ME