High loop pressure or bad gauge?

jeff4444

I’ve been in process of purging air and starting up my radiant floor heat system for the season. I’ve noticed higher pressures at the pressure gauge than in the past.

In the past I could get the system to run around 15 psi. I’m not sure if I have an expansion tank problem or just a problem with the pressure gauge itself.

This system consists of a propane water heater with supply and return heating taps, two circulating pumps, one for first floor radiant heat, and one pump for a small amount of baseboard heat. 

what I see this year is the pressure gauge with a cold tank shows maybe seven psi. As the system warms up pressure rises to 35 psi at about 110°.

This doesn’t make any sense to me so far. The auto fill line is valved off, there is a 12 psi expansion tank. I understand I can check the pressure of that tank, but I would have to drain down a section of the piping, which I can do if this is a probable cause.  Is this a likely culprit?

I removed five or 6 gallons of water from the system so far and the pressure hasn’t come down much, again this is confusing. I have attached a picture of the pressure gauge, in the larger system.

Do these gauges sometimes fail in such a way that the pressure seems to vary widely with temperature, increasing as temperature increases?

Anything else I can check? 

Bottom photo shows pressure, dropping again as tank temperature comes down.

Jamie Hall

Most likely is a problem with the expansion tank. Remember that to check the pressure in it it must be isolated from the system and drained.

jeff4444

I can’t figure out why letting 5 or 6 gallons out of the system barely changes the pressure at all (according to the installed gauge)

jeff4444

@Jamie Hall thanks for your note about the expansion tank. I did a little research and just did a quick check. I pushed in the Schrader valve and air does come out. Tapping the side it seems like the lower half at least is air, so not sure if there’s a problem there, at least so far. If nothing else is found, I’ll drain the pipe around it and verify the air pressure.

I’m going to get a pressure gauge with a garden hose fitting to compare readings against my existing gauge to see if they agree. I’ll post the result.

pecmsg

jeff4444 said:

@Jamie Hall thanks for your note about the expansion tank. I did a little research and just did a quick check. I pushed in the Schrader valve and air does come out. Tapping the side it seems like the lower half at least is air, so not sure if there’s a problem there, at least so far. If nothing else is found, I’ll drain the pipe around it and verify the air pressure.

I’m going to get a pressure gauge with a garden hose fitting to compare readings against my existing gauge to see if they agree. I’ll post the result.

Now you have to get the system to Zero PSIG and retest the x-tank.

jeff4444

I can drain the section with the expansion tank and leave it vented to air. Then I can see if air pressure is 12 psi as marked 

pecmsg

Than do that and know. 

EdTheHeaterMan

An observation:


What pressure are you operating the water heater at?
What relief valve are you using. P&T at 150 PSI or the boiler type at 30 PSI
How are you adding water to the system in order to fill and vent the system?

Enquiring Minds want to know

jeff4444

Thank you both for the comments. I will have a new pressure gauge tomorrow or Friday that I can put on one of the several boiler drain locations available, so I can get confirmation that the pressures I am reading are correct.  So I just want to confirm that.

Once I have done that, and it may take me a few more days because I’m tied up on some other stuff at the moment, I will check the pressure in the exp. tank once I drain that part of the piping down.

Good questions about the tank set up —-
  — there is a separate pressure reduction and fill valve like a boiler, set around 15 psi. I have the fill valve cut off right now, so no new water is being introduced.

— the pressure relief valve is at 30 psi. It has not popped even when I read 40 psi on the other gauge, that’s one reason I want to just confirm that I’m looking at real pressures.

— There is an air elimination system like a normal boiler set up, just before the two circulator pumps. However, you point out a very important item — I may have a cushion of air there at the top of the tank, I have vented from the pressure relief valve, but not from the nominal domestic hot water output at the top of the tank. I will try that! There may be a loud hissing sound, that would be great - Excellent point!  Is the hot water output truly right out of the top of the tank, I’m counting on that.

So, I’m also warming up to the idea that the expansion tank could be under pressure. Sorry, I don’t have the answer yet, but I’ll get back to you in the next couple days hopefully with more information, appreciate all the comments so far.

EdTheHeaterMan

— There is an air elimination system like a normal boiler set up, just before the two circulator pumps. However, you point out a very important item — I may have a cushion of air there at the top of the tank, I have vented from the pressure relief valve, but not from the nominal domestic hot water output at the top of the tank. I will try that! There may be a loud hissing sound, that would be great - Excellent point! Is the hot water output truly right out of the top of the tank, I’m counting on that.

Almost at the top. There is a slight dip in the Hot Water Out to create a small air cushion for expansion. This way the relief valve will not blow off from hydraulic pressure when the cold water heats up and expands, when it is used as a water heater. But it is nowhere near the 15 gallons you have designed.

By the way. It is not a bad thing to have that air cushion. You need not vent it. You can leave it there. Over time the air may get dissolved into the water and be expelled at the air vent. As a matter of fact, that may be what has happened over time. If your diaphragm expansion tank has the incorrect pressure (it should match the fill valve pressure) then you may be experiencing an increase in pressure. As the air leaves the system (thru the air vent), it will be replaced by water, as long as the fill valve was left open for some time.

You mentioned it is closed now, but was it open for a portion of time while the system was operating over the years?

jeff4444

The fill valve always seemed to over pressure the tank, by the pressure gauge I have anyway. I think it typically filled it up to 20 psi. So I generally left it off, I also didn’t want it open in case a leak developed in the radiant tubing. I am going to double check that pressure gauge though!  But, to answer your question, it was usually closed so I guess that means it didn’t normally introduce more water.

jeff4444

But, with that fill valve normally closed, the pressure was fairly stable. Probably varying between 15 and 20 pounds. So what changed this year? Maybe the expansion tank?

jeff4444

So, the answer is – – faulty pressure gauge.

Thanks for your suggestions, I finally got some time today to get back to this project. I drained down the pipe in the area of the expansion tank, so there was no pressure on it anymore.

I checked the air pressure in the exp tank and it was 12 psi, just what it was supposed to be. So I pumped a few more pounds in there for the heck of it, now it is at 15 psi.

Since that wasn’t a problem, I figured it probably was the pressure gauge, I went out and bought a replacement at HD, re-pressurized the system and it’s acting like normal now — it will stay between 15 and 18 pounds.  I may be able to get that down a few pounds when I finish purging the baseboards.

 From what I could see there was a little bit of gritty stuff on the business end of the pressure gauge when I pulled it out. This may have been at least partially plugging up the pressure port. So I probably have to replace this every 5 to 7 years in the future.

thanks for your good ideas, I did learn how to troubleshoot and maintain the expansion tank in the process. That may be handy in the future.