Greenhouse floor heat pressure rise

sriche01

Good afternoon! I posted a few months ago about a project I was designing for heating the root zone of plants in trays in a greenhouse with in floor hydronic radiant heat. Originally planned for a micro-condensing boiler and a primary/secondary design, I changed the design to accommodate a Utica 175k boiler I picked up at an auction. Also, based on recommendations from this board, I scrapped primary/secondary and inserted a 4 way mixing valve into the equation, with 3 zone circulators. Here are some pics of my set up.



I am not a heating professional. TBH this was my first time soldering copper. But everything has worked how I would have hoped. I'm getting my temps where I want them. The boiler is keeping up. In fact it's over sized - I plan to add a fourth zone in the future.

I do have one issue with pressure. I've played with temps at the boiler and with the mixing valve. I've kind of settled at 160F boiler setting and setting the mixing valve at 5 or so my zones get between 120F and 140F at the circ. That's a nice temp because I don't run into extremes in my zones.

Since I've started mixing down though, my pressure on the boiler side of the mixing valve has been consistently rising. It had stayed near 15psi when I had the valve wide open. Now it rises to near 30 psi every few hours.

Pictured below is the mechanical pressure reducing valve above the expansion tank which is also on the boiler side of the mixing valve. I don't believe new water is being let in to increase the pressure, at least I never hear it filling. So I've wondered if the mixing valve is restricting the access the circulating pumps have to the expansion tank. I don't have a pressure gauge on the pump side of the valve but I suspect it's lower. I'd appreciate your thoughts on this.

pecmsg

That’s high for in floor!
possably too small of an expansion tank for 175K BTU and the volume of water. 

WHO advised against primary/secondary piping?

sriche01

pecmsg said:

That’s high for in floor!
possably too small of an expansion tank for 175K BTU and the volume of water. 

WHO advised against primary/secondary piping?

I changed my design to this suggestion by Hot Rod, although I am using a manual mixing valve rather than a computerized unit.


I suspect you are right with the idea of too small of expansion tank. I used the one that was in the system this boiler was on as a starting point. Is there a good resource for sizing expansion tanks?

sriche01

So I did a little calculating. I have my estimated specs in a SS below from Taco's calculator



My current tank in a filltrol Amtrol model 110 which has a total volume of 4.4 gallons and an accepted volume of 2.5 gallons so in size I should be good. Maybe I should remove the tank and double check it's charge?

hot_rod

Was the precharge in the tank adjusted?
Amtrol and Wessel have online sizer, the most accurate method requires you know system capacity.

https://www.amtrol.com/resources-rewards/selection-tools/

A 4 way mix balve is fine for that, however it should have an actuator and control to watch return and supply temperature and respond

With a manual adjustment just keep an eye in the boiler return

You can add another #30 ecpandion tank, check pressure on both when you install them.

sriche01

hot_rod said:

Was the precharge in the tank adjusted?

It may have been. It's a used tank that came with the boiler.

hot_rod said:

A 4 way mix balve is fine for that, however it should have an actuator and control to watch return and supply temperature and respond

With a manual adjustment just keep an eye in the boiler return

We check the system 3 times/day. I may eventually go to an auto valve but I'd rather have a little more control right now.

hot_rod said:

Amtrol and Wessel have online sizer, the most accurate method requires you know system capacity.

You can see the screenshot I used above from Taco. I did Amtrol's with similar results. Even when I add the fourth heating zone down the road, a 4.4/2.5 should work. Current capacity is 63gallons. Future capacity is 86 gallons.

hot_rod

I would check the tank pressure. If it is a used tank, even more reason to suspect an under charged tank. Over time some air migrates through the butyle diaphragm. So a check every few years is a good idea. Adding a Webstone expansion tank valve is a huge help.

sriche01

hot_rod said:

I would check the tank pressure. If it is a used tank, even more reason to suspect an under charged tank. Over time some air migrates through the butyle diaphragm. So a check every few years is a good idea. Adding a Webstone expansion tank valve is a huge help.

So I turned the system off and removed the tank. It was full of water. Once it drained I checked pressure - it had none. I was able to pressurize it back to 12psi. The Schrader valve appears a bit sticky, but the cap seems to seal it. If the pressure rises again, however, I'll repeat the process and replace the tank if it has lost all air pressure again.

EdTheHeaterMan

Not a fan of the Filltrol system. That tank has special threads so you do not try to install an Extrol tank on that pressure adjusting valve. It works by the air pressure in the expansion tank. If the pre-charge air pressure in the tank is 12 PSI, and the water pressure in the boiler is less than 12 PSI, the bladder in the tank is fully engaged to the valve end of the tank. (the threaded connection to the valve) The threaded opening of the bladder has a metal valve actuator that will force the pressure adjusting valve to open when the bladder is in contact with it.. Once the water pressure in the system is just above 12 PSI the water pressure starts to compress the air in the tank and moves the bladder away from the valve actuator. Great concept but has some problems because most installers don't understand them. And they end up getting the wrong replacement tank when the tank fails. Don’t make that mistake 10 years from now when you need a replacement tank.

So if your tank has 20 PSI air pressure in the bladder, then your system will fill to 20 PSI before the bladder moves away from the valve actuator. If it has only 6 PSI air pressure in the bladder then your system will stop filling at 6 PSI. So it is important to disconnect that tank from the system and measure the air pressure charge. Adjust it to 12 PSI in order to get your system properly filled with 12 PSI water pressure. You must be disconnected from the system because the water pressure in the system will change the air pressure in the tank to the system pressure and you will get a false reading. Second reason I don't like them

Another reason I don't like the Filltrol system… No Fast Fill override for purging the system of air. They were a good idea for old cast iron gravity systems where you vented the system with a vent on each radiator. Not so good for baseboard or radiant floor loops when you want more than 12 PSI for forcing sir out of the higher pipes in the system.

sriche01

Yeah the filltrol valve threw me a bit when installing. I had already plumbed in a fill valve, then realized that the tee above the tank was also a fill valve. I did disconnect and drain completely before testing pressure which was nil. Charged to 12, but I think the schrader valve is leaking. System pressure back up to 25 this morning. Thinking I'll replace with the valve Hot Rod mentioned and a different brand tank.

Sol_Brother

If the Schrader valve is leaking (test with a bit of spit or dish detergent) you can replace its core for a few cents. Some metal valve caps have the special two-pronged core removal tool on their tops, or you can get one at a bike shop or hardware store. But if the tank is losing air through the bladder it is time for replacement.

sriche01

So just wanted to update… I removed the filltroll tank because I was still getting elevated system pressures (25psi and higher). I got a watts tank of the same size and one of the webstone expansion tank valves suggested above. When I removed the tank, it still had 10 psi. Before installing the new tank I made sure the charge was at 12 psi. So far for several days I have not seen system pressures greater than 15 psi. Not sure what was wrong with the other. Possibly the fill troll valve wasn’t working correctly. But this set up is. Thanks for the help.