Radiant heat system pressures

[Sukhoi]

I'm trying to get the system pressure and expansion tank pressure set properly in this radiant system that has come into my responsibility. The boiler and the rest of the hardware is on the third floor of a three floor house. The boiler is a standard hot water heater with a closed-loop propylene glycol/water mix coolant. There is an expansion tank installed at the highest point in the entire system, with the tank inlet facing down (facepalm).

Cold system pressure at the water heater inlet is about 10.5 PSI. The bladder fill valve on the expansion tank reads about 9 PSI, and the two gauges are 3-4 ft apart in height. Upon turning the water heater temperature back up to "hot" the water heater inlet rises to 15 PSI.

I need to figure out if the expansion tank precharge is set correctly, but the tank is only installed with an isolation valve and no way to vent pressure on the tank inlet side when it's isolated. I know the correct way to do this is to bring the whole system to 0 PSI, but that'll be a PITA and I don't want to do that. Does the fact that the water heater inlet pressure went up 5 PSI when the water heater went from cold to hot tell me the expansion tank is water logged and the bladder must have no precharge? Otherwise shouldn't the tank have taken up the fluid expansion from the heating without raising system pressure 5 PSI? The whole system is only two years old so I doubt the bladder is at 0 PSI, but who knows. I've tried banging on the side of the expansion tank but I can't tell a difference between the top and bottom halves.

 

[Toastedlightly]

Sounds like it is working. The bladder works by being squished by the expanded water volume.

If you didn't have that cushion setup correctly the pressure would go up much much higher.

The 4 foot different should be 1.7 or so psi. Sounds like it's working as intended.

What's the cold vs hot temp?

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[Sukhoi]

Cold was around 80 °F. I ran it for a couple hours to dump as much heat as I could. Hot is around 120 °F.

Good to hear you think the pressure rise is appropriate. I figured the PEX would stretch to take a lot of the extra pressure meaning I could see just a 5 PSI increase from a waterlogged system, but I really have no clue. I guess regardless of the expansion tank functionality, a 5 PSI rise is fine. Likely the old school non-condensing water heater is going to get replaced with a nice new electric boiler intended for radiant systems in a couple years, which will drastically increase the system efficiency. I'll just keep an eye on the overall system pressure in the mean time and make sure it doesn't start climbing much higher than it is now in case the expansion tank starts to die.

 

[Scarpozzi]

Cold was around 80 °F. I ran it for a couple hours to dump as much heat as I could. Hot is around 120 °F.

Good to hear you think the pressure rise is appropriate. I figured the PEX would stretch to take a lot of the extra pressure meaning I could see just a 5 PSI increase from a waterlogged system, but I really have no clue. I guess regardless of the expansion tank functionality, a 5 PSI rise is fine. Likely the old school non-condensing water heater is going to get replaced with a nice new electric boiler intended for radiant systems in a couple years, which will drastically increase the system efficiency. I'll just keep an eye on the overall system pressure in the mean time and make sure it doesn't start climbing much higher than it is now in case the expansion tank starts to die.

If you can find some of the pex tubing, its operating ranges are often printed on the side. They'll say things like 200 degrees at 80psi...

You want to stay below those maximum ratings.

If you've got a 40 degree drop in temp as water travels through the system, increasing the PSI would actually make the heat transfer to the system less efficient unless you increase the temp to compensate. If your loops are too long that can cause issues as well.

I have loops installed in my garage floor, but never got a boiler or pump system. I just ran the tubing and planned on getting some evacuated solar tubes on a copper circuit to run to a heat exchanger before converting to pex. That's just so I could control the heat as those arrays can product 500 degrees and melt pex. One day, I may play around with it when global warming kicks us into another ice age.

 

[Sukhoi]

There are exposed lengths of the PEX at the valve access points, and yep it's 80 PSI @ 200 F. So I'm definitely nowhere near the PSI limit even on the bottom floor.