Expansion Tank & System Pressure After New Water Heater (Rising Pressure)

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cloves

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Hi everyone,

I recently installed a new water heater (vertex 50 gallon), expansion tank (2 gall), ge water filter (whole house with built in check valve), and 2 pressure gauges. Pressure gauge 1 is directly after the city water main and reads 60 PSI. Pressure gauge 2 is placed after my water filter.

Now the expansion tank is placed on the cold water supply line right before entering the water heater. Before putting the water on, I filled the expansion tank to 59 PSI. Then I filled the system with water.

Here is my question and problem. I have noticed that the water pressure gauge (the one after the water filter) has been rising all the way upto 70 PSI (after a full day of the water not being used). The city gauge maintains 60 psi. Is this normal? Or do I have to put more air into the expansion tank?
 

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Hardt

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FWIW, recently I got fed up with the low volume of water in my shower so I decided to adjust my pressure reducing valve to 60 psi (I read where Terry said he routinely tweaks prvs to 60 psi from the nominal setting of 50 psi). Much to my surprise, my water pressure was at 120 psi, or at the service line pressure. I replaced the bad prv and because I was curious, I started monitoring the pressure at my 120 gal solar hot water heater tank. During the course of a sunny day, the pressure would peak several times to 120 psi! Any routine water usage would cause a drop to 60 psi. In fact, if I drained only 1/4 cup water, the pressure would drop 10 lbs.

At this point, I decided I needed an expansion tank and so I installed a Watts ETX-30 and pumped up the bladder to 50 psi and I also re-adjusted the prv back to the default 50 psi. Now after monitoring the pressure for about a week with a tell-tale water gauge, with temperatures peaking around 170 F., the pressure has not reached 70 psi. Happy days...

So based on my experience, cloves, I think your system is working correctly.
 

Jadnashua

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If your water pressure rises above the PRV setting, one of two things is happening: the expansion tank has failed, is the wrong size, or is not precharged properly, OR, your PRV is defective. It doesn't sound like it could be the PRV, since (if it had a bypass valve, it wouldn't open until the differential pressure was higher) if it does, it would allow street pressure to bleed through, and yours is at 60psi. Unless your street pressure varies a lot (it could) and exceeds 80psi, a PRV would not be required. An expansion tank IS required when you have a closed system, and now, you have two of them in the house: effectively one in the PRV and the checkvalve you have for the filter.
 

cloves

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Hi guys wanted to follow up. Turns out the expansion tank was defective! It was losing pressure over a few days, it was brand new too.

I have another question that just popped up. We installed a Therm-X-Trol ST-5 Thermal Expansion as pictured above. Question does the expansion tank have to be rated for potable water? I found there online brochure which says that the thermal X is for potable closed systems. Why wouldn't they include that in there general description?
 

Jadnashua

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There are ETs designed for hydronic heating systems (their bladder can withstand the heat), and those designed for potable water...their bladder will not survive very long with hot water. Many heating systems don't actually have all that much water in them, but may have bigger temperature differences, but still their ETs tend to be smaller.
 

hj

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Potable water tanks are either galvanized or glass lined. A "heating tank" can be made out of uncoated steel which would rust with potable water usage.
 

cloves

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Thanks for the info everyone. I looked up the specs on an amtrol RX-15 (Designed for radiant heating) and it says "Utilizes internal plastic liner for corrosion resistance in system with high oxygen content or non-barrier tubing". So doesn't that imply that this is for radiant systems that don't use oxygen barrier (evoh) pex tubing? Obviously you need oxygen to get rust.

Thought I would also put this out here since we are discussing expansion tanks. The radiant heating plan I am working with calls for a potable water expansion tank. The system is "closed loop" and very similar to the below diagram. This one is from the phoenix install manual. I will be using a watts 9D atmospheric back flow preventer.



Q: If the system is "closed" why is there a need for a potable water expansion tank? An atmospheric backflow preventer for example should stop any sort of backflow. The rest of the system to the right of the heat exchanger has no contact with the domestic water supply since it is going through a heat exchanger. Even in the notes of the phoenix it says suitable for potable water.
 
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Jadnashua

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In a closed hydronic heating system, once you fill it up, you normally do not get any new oxygen into the system. Whatever is there upon fill, does cause corrosion, but once it is all used up and bound in the 'rust' of the metal you essentially end up with pretty inert fluid. This is why if you use pex, you want one with a built-in O2 barrier so you don't get more O2 in there.

But, if your system is going to be using potable water that is being constantly replenished, you'll constantly be introducing new O2 into the water as it goes through, and one designed for hydronic heating systems will not work very well long term. Note, at least on some of them, those designed for potable water do not handle quite as high a temperature. But, at least with Watts products, they do handle 200-degrees, and your pex should not exceed that either. ALso note, if you're going to be using potable water, you need bronze or SS pumps and valves...steel ones will just end up rusting away and make the water funky to use - IOW, it won't be potable any more. Not a big fan of using potable water in a heating system.
 

cloves

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Hey jad, I should have said "indirect" closed radiant system. Attached below is the plan I am working with. The temperature of the water should be around 120 degrees and as you stated once the water is in the system, its rare that the fill valve should ever put more water into the system. The water used by the radiant heating system will never make it back to the water heater. Everything runs through the heat exchanger. The circulator used on the water heater side and flanges are stainless steel as you mentioned.

Still trying to understand why a potable water expansion tank is specified in a closed indirect radiant system.
 
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Jadnashua

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Those for potable water are built more like a water heater with a glass or ceramic coating on the inside...hydronic ones may have some protections, but nowhere near what one for potable water has, since they are NOT expected to see continuous addition of O2. A closed system, as long as you keep the temperatures down, a potable one will probably last longer, and put less rust particles into the system in the interim before it fails. The thing that fails is often the bladder, but...
 
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