Radiant Heat System - Inconsistent Pressure

DJ42069 · October 2019

I have a radiant heat system that fluctuates in pressure between 0 and 40 and can't figure out why. To give some background:

System is 10 years old. Original tank was replaced in 2016.
A leak was discovered in the system in February after some flooring work was done. Source of leak was determined in March and fixed by heating company. Lines were flushed at the time.
ST-5 tank, pressure gauge and valves were all replaced in February.
Interior manifold, located in the upstairs master bedroom closet, was reconfigured in February.
System maintained pressure for a couple of months after leak repair and then started to fluctuate in spring. Heating company came out, flushed the system again, but could not determine source of problem. Pressure remained stable throughout summer.
Pressure started to fluctuate again starting last week as colder temperatures in the area kicked in.

I've checked the indicator at the bottom of the ST-5 tank and it appears to be functioning normally (I also tapped it and seemed fine). I've attached some pictures of the system for reference. Any ideas on what else I should check for?

JohnNY · October 2019

The ST-5 is precharged to 40 PSI. What is the operating pressure of your radiant system?

DJ42069 · October 2019

JohnNY said:
The ST-5 is precharged to 40 PSI. What is the operating pressure of your radiant system?

It varies. Most of the time it's in the 30-40 range but it will dip as far as zero before going immediately back up again.

hot_rod · October 2019

The tank has a coil for the hydronic side? How much water in that side, could be the expansion tank is too small for the water volume and temperature swing it works through.

JohnNY · October 2019

DJ42069 said:

JohnNY said:
The ST-5 is precharged to 40 PSI. What is the operating pressure of your radiant system?
It varies. Most of the time it's in the 30-40 range but it will dip as far as zero before going immediately back up again.

Then it is set to zero and the temperature rise is bringing up the pressure to that of the expansion tank at which point, the tank bladder stabilizes it. I'm guessing all the radiant heat tubing is on the same level of the water heater?

DJ42069 · October 2019

JohnNY said:

DJ42069 said:

JohnNY said:
The ST-5 is precharged to 40 PSI. What is the operating pressure of your radiant system?
It varies. Most of the time it's in the 30-40 range but it will dip as far as zero before going immediately back up again.
Then it is set to zero and the temperature rise is bringing up the pressure to that of the expansion tank at which point, the tank bladder stabilizes it. I'm guessing all the radiant heat tubing is on the same level of the water heater?

The tubing is on two levels: the ground floor and the second floor. When you say "it" is set to zero, what are you referring to?

DJ42069 · October 2019

hot_rod said:
The tank has a coil for the hydronic side? How much water in that side, could be the expansion tank is too small for the water volume and temperature swing it works through.

Sorry, being a novice to this I'm not sure what you mean exactly. How would I determine how much water is flowing through the hydronic side?

JohnNY · October 2019

Well, the fact that the tubing is on the 2nd floor too kind of negates my theory but by "it" I mean the pressure regulating valve that fills your radiant floor heating system. I assume there is some kind of device attached that brings the pressure down to a certain level for this system. The pressure from the water main is frequently too high and so a reducing valve is common.

Zman · October 2019

You seem to be lacking some key components. Is there a pressure relief valve and air eliminator somewhere?
I suspect @JohnNY is correct and that the precharge on the expansion tank is too high. You need to remove the tank and set the pressure on the schreader valve (Under the black cap) to 15 psi. The air release valve on the top right will likely get the air out eventually but really is not a substitute for an air eliminator. You could probably remove the hose bib on the top left and replace it with a 30# relief valve.
If all that does not solve your problem, you must have a pinhole leak in the tank coil.

HomerJSmith · October 2019

As Zman says, get a decent micro-bubble air eliminator. Set the air charge on the bladder tank & the pressure regulating valve to 12-15 psi and the pressure relief valve on the radiant section with a 30 psi valve. All assuming you have a heat exchanger in the tank servicing the radiant heating. In other words, the water going to the domestic hot water side is separated from the water in the heating side.

You are pumping into a tridicator, When the pump turns on there will be an increase in pressure as read on the gauge.

Your manifold doesn't have any balancing valves, so, I suspect you will have uneven flows in your 5 loops.

You have a Honeywell mixing valve on the tank. What is the tank temperature set at? What is the model # of your B/W heater?

DJ42069 · October 2019

Zman said:
You seem to be lacking some key components. Is there a pressure relief valve and air eliminator somewhere?
I suspect @JohnNY is correct and that the precharge on the expansion tank is too high. You need to remove the tank and set the pressure on the schreader valve (Under the black cap) to 15 psi. The air release valve on the top right will likely get the air out eventually but really is not a substitute for an air eliminator. You could probably remove the hose bib on the top left and replace it with a 30# relief valve.
If all that does not solve your problem, you must have a pinhole leak in the tank coil.

I added a picture of the Grundfos booster pump which is the only part of the system that I think wasn't pictured previously. The only pressure relief valve that I'm aware of is the one on the side of the tank which can be seen in the second picture down.

Is there a way to test for a pinhole leak in the tank coil?

DJ42069 · October 2019

HomerJSmith said:
As Zman says, get a decent micro-bubble air eliminator. Set the air charge on the bladder tank & the pressure regulating valve to 12-15 psi and the pressure relief valve on the radiant section with a 30 psi valve. All assuming you have a heat exchanger in the tank servicing the radiant heating. In other words, the water going to the domestic hot water side is separated from the water in the heating side.

You are pumping into a tridicator, When the pump turns on there will be an increase in pressure as read on the gauge.

Your manifold doesn't have any balancing valves, so, I suspect you will have uneven flows in your 5 loops.

You have a Honeywell mixing valve on the tank. What is the tank temperature set at? What is the model # of your B/W heater?

I'm not sure how I can tell what the temperature is right now given that the Honeywell thermostrip expired three years ago and I'm not sure how to read the mixing valve. Model number for the heater is CDW2TW50T10FBN.

Zman · October 2019

I think a pinhole is unlikely. Since you don't appear to have an autofill. If the pressure is constantly rising when the system is not running, you have a pinhole.
I think if you fix the other issues I mentioned, the the system will run correctly.

Radiant Heat System - Inconsistent Pressure

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