System Pressure fluctuation in Hydronic Heating system

gg2

My Amtrol Extrol expansion tanks do not seem to regulate pressure effectively. The system has been operating about 3 years and I still can’t get it right.

System Description: An OWB circulates water to the house through a heat exchanger, on the other side of which is an 80 gallon buffer tank in a closed loop system. Above that is a 30 gallon buffer tank heated through thermosiphon. Water from the 30 gallon tank is circulated through a radiant system. Total system volume 142 gal. When OWB not in use, water temp in the 80 gal tank drops, activating a modcon boiler and a diverter valve that steers return water away from the 80 gal tank directly to the 30 gal tank. When not is use, the 80 gal tank stays at about 60-65 degrees and is still part of the water system.

I calculated the size of the expansion tank at 8.6 gal based on Seigenthaler's formula. (see notes) Other simpler volume-based formulas come out to 3 or 4 gal. With the OWB in play, temperature in the 80 and 30 gal tanks stays from 150 to 165 degrees and the pressure stays between 10 and 22 psi, rise and falls with water temperature. When the modcon is in play, the 80 gal tank sits idle and the modcon keeps the 30 gal tank between about 140 and 160 degrees. However, with just the modcon in play, the system pressure varies with the water temperature from 12 psi to 28 psi. When transitioning between OWB and modcon, the pressure differential really gets ugly and I end up adding or relieving water from the system to regulate the pressure. At any given system temperature, the pressure stays constant regardless of activation of zone pumps so the changes in pressure appears to be temperature driven. I did size all my pumps.

The first Amtrol Mod 60 is shown between the buffer tank and the system pump. (see pic) The Model 60 holds 7.6 gal, but on further review, only accepts 2.5 gal of expanding water. Siegenthaler says you can’t really have too big an expansion tank and in one article discusses adding a second tank in the return side of the system. So I did. It didn’t help. I double checked the air pressure of both Amtrols at factory setting of 12 PSI with system pressure at zero. I learned from posts by MikeAmann and jesmed1 on this site that displacing air with water in the expansion tank still makes the pressure go up so maybe I need a bigger tank? Or is something wrong with my install?

Jamie Hall

You need a bigger tank, plain and simple. Keep in mind that all the calculators assume pretty normal sized systems -- and will give you the minimum size tank to avoid overpressure, starting at a normal lowish pressure, like 12 psi. The bigger the tank you use, the less the pressure will swing.

Further, be sure that the tank -- or tanks -- are connected to the system only at one point, and that that point is upstream of the intake of the primary circulating pump. More than one tank, if they are connected at separate locations, can result in some really weird behavour under certain conditions.

Zman

Where are the tanks in relation to each other and the circulators? They should be tied in at the same location on the suction side of the circulator.

Were the tanks precharged to your system pressure before they were installed? Isolation valves and purge ports are a good idea for any expansion tank.

You are a little tight on your sizing, but I am surprised it is causing the pressure swings you are seeing.

hot_rod

A gauge right at the expansion tank would be the best place to read. Any gauge downstream of the circ will see the delta P the pump adds at that exact gauge location.

It does sound like you need more tank capacity. You can add additional tanks, as long as pre-charges match.

Them #60 has about the same acceptance as a #30. Notice where the diaphragm crimps into that tank at the same level. However the air charge chamber is larger. So pressure swings are not as noticeable.

gg2

Thank you all for your insight. Very helpful. Both tanks were set at 12 psi. The first one is upstream of system circulator. The second was downstream past all the loads on the return side. I noticed no change in the pressure issue after installing the second one. My first Amtrol install was short-sighted on my part RE later servicing but it won't be hard to add an isolation valve, union, pressure gauge and purge valve to make it easy to service the tank and double check the pre-charge under zero pressure conditions. From all your comments, looks like upgrading the first upstream Ex-60 to at least an EX-90 which accepts 11.3 gal of water volume would get me closer to where I need to be.

gg2

I installed the Amtrol EX 90 in the system which has 11.3 gal capacity. The existing smaller tank is isolated for now. I set the tank pressure at 12 psi. At the moment, with the OWB in play and both storage tanks hot, my system psi is about 25 but I am only seeing 10 psi at the gauge above the expansion tank Why would there be such a difference? Right now my connection is through the base of the air scoop. Should I find another connection location upstream from my circulator that might better reflect the pressure in the system?

EBEBRATT-Ed

You can only have 1 expansion tan or multiple tanks connected together. You can't have a tank on the suction to the pump and 1 on the discharge. Pipe both of them to the suction side of the pump.

GroundUp

Where are you measuring "system pressure"? If the pressure above the tank is 10, the rest of the system at the same elevation will also be 10 when sitting idle. If you're catching 25 anywhere while the tank remains at 10, something is piped wrong. Assuming your direction of flow is left to right in the photo and that's the only circulator (right of the air scoop), that should be a perfect connection point to the system.

gg2

I have both expansion tanks connected together as shown in the photo, both going into the scoop and tied into the refill line. The one to the right is closed off for now. There are three other zone pumps downstream, one priority to circulate heat exchanger for DHW and one each for North and South pex zones for radiant floor. The pressure gauge reading 25 psi is located in a 1" D line between the 30 gal buffer tank and the Modcon which sits idle most of the time. The other pressure gauge is about 3 ft downstream of the circulator in the photo. As expected, it shows about 4 psi higher due to its location. Is it possible my expansion tank is to far away from its connection, thus the reduction in pressure?

Jamie Hall

Do the two pressure gauges read the same or very close when all the pumps are off? If not, one of them is lying to you...

leonz

Who drilled through those ceiling joists to run pipe, Larry, Daryl and Darrel????

I am glad I have a steel compression tank for my system.

gg2

I have both expansion tanks connected together as shown in the photo, both going into the scoop and tied into the refill line. The one to the right is closed off for now. There are three other zone pumps downstream, one priority to circulate heat exchanger for DHW and one each for North and South pex zones for radiant floor. The pressure gauge reading 25 psi is located in a 1" D line between the 30 gal buffer tank and the Modcon which sits idle most of the time. The other pressure gauge is about 3 ft downstream of the circulator in the photo. As expected, it shows about 4 psi higher due to its location. Is it possible my expansion tank is to far away from its connection, thus the reduction in pressure?

gg2

The downstream pressure gauge still runs3 degrees or so high even when the system is sleeping. (4-5 psi higher when the pump is on) I suspect that one is the liar. The gauge above the tank (showing 10) is brand new...but nowadays I guess that doesn't mean much considering where a lot of this stuff is made.

hot_rod

It really depends where the gauges are located, and the expansion tank tie in point. See a couple examples below. And of course gauge pressure willincrase as the water heats and expands.

Also gauge accuracy from one to another. Ideally you could read everywhere with the same gauge, with PT plugs for example.