Why must a Hot water heater have an expansion tank?

Tom_7

I don't understand why a hot water heater must have an expansion tank on the cold water inlet. I heard its something about Boyles Law but I'm not sure how it works. I read in a hot water heater manual that if the relief valve fails the expansion tank will blow before the hot water tank will.

masterplumb

X-trol

Barring any codes, you really only need an expansion tank on a domestic h/w heater when you have a Pressure Reducing Valve or the like on the water main. It "closes" the system and does not allow any expansion as the water in the h.w. tank gets heated. Hope I stated it clearly for you.Chris

Tony Conner

If You Have...

...a backflow preventor on your municipal water service, you need an expansion tank for your water heater. If you don't, this is the sequence - you use hot water. Cold water enters the tank as the hot is drawn out. You finish using hot water, and shut the tap off. The water heater does what it's supposed to, and heats the water in it back up to whatever it's set for. This water expands as it's heated. In days gone by, it would just expand back into the municipal system. But now, with a backflow prevention device installed, it can't. Now, the pressure in your water heater often increases to the point where the relief valve lifts. Many people automatically assume the problem is with the relief, and replace it. Over and over and over. They change brands, lift pressures, sizes, etc. The relief isn't the problem, it's doing what it's supposed to.

Do your DWH relief valve a favour, and install an expansion tank rated for potable water.

Mark Eatherton1

You don't have to have an expansion tank...

unless the expanding water has no where to expand to, in which case you WILL know.

An expansion tank is not meant to replace a relief valve.

If there is no check valve anywhere between the water heater and the city water main or WellTrol tank, and you don't need a check valve, and consequently wouldn't need an expansion tank.

Now if the expanding water creates a nuisance problem by backing hot water into your cold water line, and you decide to install a check valve to keep this from happening, then you WILL need an expansion tank.

My good friend and fellow columnist Dave Yates wrote an excellent article on this at Contractor Magazine, which can be retreived from www.contractormag.com

Enjoy, and Merry Christmas, Happy Chanukah, Happy Quanza and Happy Holidays!

ME

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chuck shaw

When cold water

heats, it expands.

When the water expands, it must go somewhere. Frequently, it will go up the cold water line. In areas that require a backflow preventer on the cold water inlet from the street, this process cannot happen. Many towns also use what is known as a self checking meter. Meaning that there is a check valve built into the water meter. As the water heats, and expands, it would cause the temperature and perssure relif valve to blow, this is what it is supposed to do. However, this can also release very hot water, with the risk for scalding. Codes covering this may vary from state to state. In Massachusetts, the code reads, "Where water meters, or metering devices with check valves are installed, which creates a hazard or nuisance, an expansion tank shall be installed as close as possible to the water meter or metering devices. The expansion tank shall be of adequate size and constructed of approved materials."

The pressure in the expansion tank must be adjusted according to the manufactures instructions for a proper installation.

Hope this helps.

Chuck Shaw

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kevin

I thought ...

that it was best to install the potable water expantion tank as close as possible to the water heater? As least that what all the manufactuers' say on the install instuctions. kpc

masterplumb

Placement

It's where I've always put it.

chuck shaw

That is where

I put it too. I have never had an inspector question it. I was just copying word for word from the Massachusetts code book. Section 2.14.4.j. Next month at the Plumbing/Gas Inspectors Association meeting, I will bring it up. See what they say.

Chuck Shaw

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Jack_23

Expansion

When water is heated from 32F-212F it will expand approx 1/23 or 4.3% of its vol. You need the expansion tank to keep your T&P from going off.

Greg_4

What about when 'heat traps' are installed?

I was wondering about heat traps - the devices which act as a thermal check valve that can be added on to water heater inlet/outlets. Do these block water expansion enough to cause concerns? Greg

kevin

i think...

that it a lesser issue.the little plastic balls are easily displaced to allow some expantion. Think about it if it can make a t&p valve weep/open a little thermal trap won't stop expantion. kpc

Heatermon

Heat trap nipples

are NOT check valves and are designed to allow for thermal expansion through the nipple. They build in a "bypass" to allow water to go around the "ball" and equalize pressure on both sides of the nipple. The amount of water they can flow is alot less backwards than forward.

Heatermon

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chuck shaw

I spoke with

the chairman of the local chapter of the Plumbing/Gas Inspector's Assoc this morning. The explantion he gave to me for the way the Massachusetts code is written was, placing the tank close to the meter protects the whole house, allowing the whole water main to be used for thermal expansion. Water heater companys reccomend placing it near the water heater to protect their product. We have probally gotten in the habit of doing it that way, because the problem we see is at the water heater. I personally don't see where it would make a big differance. But that is what he had to say. I will bring it up for discusson the thrid Wendnesday in January, when we have the next meeting.

Chuck Shaw

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John G. Merritt

Heat traps on HW Heaters?

Hi fellas!
What exactly is a Heat trap, why is it used, and what does it do?
When do you need one? Never heard of this.

John

Greg_4

Perhaps I should have

put quotation marks around the ... act like a "thermal check valve". Didn't mean to imply they were check valves. I didn't know if these nipples allowed enough expansion through design to not be an expansion problem or not - just an afterthought.

To answer the final query - these devices are nipples with some sort of polyethelene (or other synthetic material) which are a thermal break from cold water to the water heater. Test it by touching the inlet piping several inches or even a foot away from a water heater - it is warm or even hot to the touch. Heat trap nipples help keep waste of energy down by lessening the heat loss at near water heater piping. They're in most any water heater product catalog. The inlet ball 'floats', causing somewhat of a "seal" at the inlet water port, when hot water usage stops (a hot water faucet is closed following use). The outlet ball 'sinks' causing somewhat of a "seal" at the hot water outlet of the water heater. Not a hermetic seal, just sort of acting like a "thermal check valve" (improper explanation, I know, but it's my mind's understanding of how to explain their working). Therefore they do not allow hot water to mix with cold water at the inlet and vice-versa at the outlet. In theory, they should stop a considerable amount of heatloss caused by thermal mixing at these points and heat loss from the near water heater piping into air in the mechanical room.

Jackchips

Heat traps

can be a mechanical device or just a few fittings that loop down a ways before supplying hot water to the system. The main purpose is to keep hot water from circulating when there is no draw. The majority of water heaters have the hot tapping at or near the top of the tank and if there was no heat trap the water (as hot water wants to do) would just migrate up through the system and waste energy.