Expansion tank on Tankless System

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djdavenport

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I have a fairly new combi-mod unit which has been working great. When they installed it, they put in a 10 gal electric buffer tank (supposed cold sandwich cure) that the recirc system taps off from. Yesterday, while doing some routine poking around I saw that the T&P valve on the electric HW was dripping. I threw on a water pressure gauge (a cheap one and probably not very accurate) and it showed the DHW side had risen to about 110 PSI.

Normal water pressure is +/- 70 psi--which it was at the time I checked.

My questions are: 1.) Sloppy T&P valve weeping 40 psi early. Should I just ignore it (or replace it)?

2.) I know there's a lot of opinions on the internet about not needing an expansion tank with an on-demand system. (What triggers the hot water is the very thing that releases the pressure. Cracking open a faucet.) But I haven't found much about on-demand systems coupled with a small storage tank. And, would a bump from 70psi to 110psi be normal for that kind of set-up?

I don't mind putting in a tank if that's the smart thing to do. Will an occasional pressure spike up to 110psi hurt the system?

Thanks, as always, for your advice.
 

Reach4

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You have the hot from tankless providing water to the input of a 10 gallon electric tank type WH. That tank WH, near the point of use, is to give quick hot water for washing hands before the tankless gets fired up and the hot water arrives through the long pipe. The fact that you measure 110 PSI and show dribbling thru the T+P valve shows that you should add a thermal expansion tank.
 

Jadnashua

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At 110#, the T&P valve should not be opening. But, the instantaneous valve may not show the peak. A gauge with a tattle tale hand would show the peak, and it probably would show it peaking at about 150psi. Normally, a tankless system does not need an expansion tank, but when you add an actual tank, that could be an issue.
 

djdavenport

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Thanks, Jim. A couple of things I've discovered. The static water pressure is actually higher than I thought. It's 82psi, caused by a failed pressure reducing valve (it won't adjust down. Rebuild kit is on the way). I also was able to watch the pressure spike, and it is when the electric WH cycles on. Now, once I get the pressure back down to something reasonable 50-60psi, the spike (providing things are linear) should be well under 100psi, and I might just live with that for awhile without doing surgery to add an expansion tank. Not sure. Depends on how ambitious I'm feeling at the moment.
 

Jadnashua

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Water and copper pipes are essentially non-compressible. IOW, when you heat the water, the pressure will rise quite rapidly. Unless something balloons (supply hoses, washing machine hoses, maybe a dishwasher supply, toilet supplies, etc.), or, something leaks (a common one that leaks under pressure is a toilet fill valve), the pressure WILL rise and if everything in the house is tight, open the T&P valve. So, to prolong the life of those flexible components, install an ET. Properly sized, the pressure will not vary from the PRV except very slightly. The pressure rise will depend on how much water was used, then needs to be reheated. Keep in mind that in the winter, the delta temperature will be greater, thus, the expansion will be greater too, and raise the pressure more until something gives.
 

djdavenport

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It's a good plan--and a minimal investment. I'll head down the big box tomorrow and pick up an tank. You know, what's amazing to me is that I'm just a curious HO that likes messing with stuff. The guys, who are long-time, respected plumbing pros here in town, apparently didn't figure it out. Go figure.
 

Lifespeed

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Like @djdavenport, I thought I didn't need an expansion tank when I installed my tankless 6 years ago. But I do notice the pressure relief valve has been weeping for quite a while. The water heater is above the wall between the two bathrooms, so I only see the pressure relief pipe when I crawl under the house. Out of sight, out of mind. But the blue corrosion on the pipe tells me it has been dripping occasionally.

So in spite of our logic telling us a tankless doesn't sit there expanding hot water into a closed system, an expansion tank may well be required? I do have a recirculator which requires a check valve ahead of the cold water inlet to the heater, and there is a PRV at the inlet to the house from the city main. So it is a closed system right at the water heater cold inlet, no expansion to the cold side or the city main.
 

djdavenport

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The relief valve which was kind of just weeping was certainly my first clue as well. And when I threw on a pressure gauge, I was a little amazed at what the pressure actually was (both due to a malfunctioning PRV, which I rebuilt, and then the thermic expansion on top of that).

Putting in the tank completely solved my issue. Whereas I was getting about a 30 psi bump whenever the electric buffering tank kicked in, now I see a couple psi change, and it's always hovering around 60 psi. So I'm a happy guy. I did puzzle a little about where to put the expansion tank. The "cold side" versus the "hot side" debate. As it turns out, with a tankless system feeding a buffering tank, isolated on the supply side to the tank with a check valve between it and the Navien, there really wasn't a "cold side." Since they operate in series, it's hot coming in and hot going out. That said, by the time I teed it off, the line to the ex. tank barely gets warm, so I don't think it will compromise the life of the tank (which is what the "cold side" proponents often claim.)

So we're cruising. Outdoor temperatures in the 20s. Long burn times. Ambient indoor temperature doesn't fluctuate more than a degree or two--and that's probably just solar gain. Endless hot water. Stable water pressure. Life is good.
 

Nakopf

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Tankless with recirculation technically require a thermal expansion tank. I imagine it'd be even more necessary with the described buffering tank. Glad the OP found a solution; thanks for sharing.

flexcon-ph1.jpg
 

Lifespeed

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I added an expansion tank to my tankless with external recirculation pump and check valves. I no longer get that burst of high pressure from the first faucet turn on in the morning when the recirculator has heated the pipes from cold to hot before shower time begins. I expect this would be maximum pressurization time. I think this will fix the pressure relief valve nuisance opening, not to mention overstressing the schedule L copper pipes. My pressure relief drain pipe runs from the attic-mounted water heater down through the wall under the house, so this is not particularly easy to verify.

Moral of the story is tankless with a one-way check valve, especially a recirculator, do indeed have a mechanism to pressurize the pipes even if it is not as severe as a tank water heater.
 

Michael Young

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I have a fairly new combi-mod unit which has been working great. When they installed it, they put in a 10 gal electric buffer tank (supposed cold sandwich cure) that the recirc system taps off from. Yesterday, while doing some routine poking around I saw that the T&P valve on the electric HW was dripping. I threw on a water pressure gauge (a cheap one and probably not very accurate) and it showed the DHW side had risen to about 110 PSI.

Normal water pressure is +/- 70 psi--which it was at the time I checked.

My questions are: 1.) Sloppy T&P valve weeping 40 psi early. Should I just ignore it (or replace it)?

2.) I know there's a lot of opinions on the internet about not needing an expansion tank with an on-demand system. (What triggers the hot water is the very thing that releases the pressure. Cracking open a faucet.) But I haven't found much about on-demand systems coupled with a small storage tank. And, would a bump from 70psi to 110psi be normal for that kind of set-up?

I don't mind putting in a tank if that's the smart thing to do. Will an occasional pressure spike up to 110psi hurt the system?

Thanks, as always, for your advice.



because you have a buffer tank, yes, expansion device is required by code, unless you have well water.
 

Jadnashua

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The bursting pressure of copper pipe is WAY over (thousands of psi) the pressure relief on the heater. The seals in faucets, the heater's integrity (heat exchanger seals), and supply hoses are the most likely casualties of an overpressure situation. A proper soldered connection can handle it, too.

A system with a properly installed expansion tank is subject to much milder thermal expansion issues, whether it is technically needed or not, than one without an ET.
 
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