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Old 06-06-2019, 02:01 PM   #1
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Does Thermostatic Mixing Valve at Water Heater Require Check Valve? Why?


I have a basic conventional 50gal gas water heater and a pretty standard copper plumbing setup. I want to add a thermostatic mixing valve at the water heater. I've heard I need check valves on the hot and/or cold lines into the mixing valve? Why? I don't understand why I would check valves with the mixing valve, but I don't need them now.

I've also heard that you only need check valves for mixing valves that are right at a fixture and that makes more sense to me.

Thanks
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Old 06-06-2019, 02:39 PM   #2
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Re: Does Thermostatic Mixing Valve at Water Heater Require Check Valve? Why?


When there's a pressure difference between hot and cold, and theres no check valve (or it's leaking by), you'll get cold water running into the hot supply, or vice versa. It's very noticeable on high pressure drop appliances, like instant hot water heaters. You end up with warm or cool water instead of hot water at fixtures that don't use the mixing valve. Eg. Dish washers, washer machines, and sometimes the kitchen sink, depending on code and setup.

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Old 06-06-2019, 03:24 PM   #3
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Re: Does Thermostatic Mixing Valve at Water Heater Require Check Valve? Why?


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Originally Posted by supers05 View Post
When there's a pressure difference between hot and cold, and theres no check valve (or it's leaking by), you'll get cold water running into the hot supply, or vice versa. It's very noticeable on high pressure drop appliances, like instant hot water heaters. You end up with warm or cool water instead of hot water at fixtures that don't use the mixing valve. Eg. Dish washers, washer machines, and sometimes the kitchen sink, depending on code and setup.

Cheers!

Everything in my house would use the mixed water. I'm not running special lines for appliances that could use super hot water. Are check valves still important for me in this case?
Also before someone comments I know adding the valves would not be much trouble or expense, I'm just curious and want to learn the reasons.

Last edited by riccobo4; 06-06-2019 at 03:27 PM.
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Old 06-06-2019, 05:35 PM   #4
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Re: Does Thermostatic Mixing Valve at Water Heater Require Check Valve? Why?


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Originally Posted by riccobo4 View Post
Everything in my house would use the mixed water. I'm not running special lines for appliances that could use super hot water. Are check valves still important for me in this case?
Also before someone comments I know adding the valves would not be much trouble or expense, I'm just curious and want to learn the reasons.
It's not critical then. Of course, if you add a dedicated hot line to something, then you'd want to add them if you already don't have them.

Do note that many models from the various brands have integrated check valves.

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Old 06-07-2019, 03:50 PM   #5
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Re: Does Thermostatic Mixing Valve at Water Heater Require Check Valve? Why?


You need an ASSE 1017 tempering valve. They usually have checks built in.
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Old 06-07-2019, 03:54 PM   #6
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Re: Does Thermostatic Mixing Valve at Water Heater Require Check Valve? Why?


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Originally Posted by Ghostmaker View Post
You need an ASSE 1017 tempering valve. They usually have checks built in.
They very often don't have checks built in. The question is why do I need checks. Could you explain what could happen without them and why that scenario cant happen already right now without the mixing valve?
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Old 06-07-2019, 07:24 PM   #7
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Re: Does Thermostatic Mixing Valve at Water Heater Require Check Valve? Why?


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Originally Posted by riccobo4 View Post
They very often don't have checks built in. The question is why do I need checks. Could you explain what could happen without them and why that scenario cant happen already right now without the mixing valve?
I believe that in your very specific case, i don't think you have much to worry about.

The only thing you'd need to worry about is any cross mixing. For example, if the water supply is restricted for some reason (small supply and/or the strainer in the meter gets plugged.) and you have a large load, such as a well plumbed toilet, you could have hot water mixing into the cold as the tank draws down. (they usually have 1 or 2 integrated check valves preventing back flow.) This wastes precious hot water. It's a hypothetical that may happen to you.

Cheers!
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Old 06-08-2019, 09:13 PM   #8
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Re: Does Thermostatic Mixing Valve at Water Heater Require Check Valve? Why?


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Originally Posted by riccobo4 View Post
They very often don't have checks built in. The question is why do I need checks. Could you explain what could happen without them and why that scenario cant happen already right now without the mixing valve?



You want to put money on your last dumb statement?
https://media.wattswater.com/ES-LFMMV.pdf


Features• Lead Free* cast copper silicon alloy body construction• Solid wax hydraulic principle thermostat assures dependable mixing of hot and cold water• Thermostat controls both hot and cold water• Solder, threaded, PEX or CPVC, Quick-Connect end connection or w/press• Adjustment cap with locking feature• ASSE 1017 listed• ASSE 1069 listed• ASSE 1070 listed• IAPMO cUPC listed Integral filter washers and check valves

Specs say they have checks.


You need checks to prevent hot entering your cold.

Last edited by Ghostmaker; 06-08-2019 at 09:16 PM.
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Old 06-08-2019, 09:44 PM   #9
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Re: Does Thermostatic Mixing Valve at Water Heater Require Check Valve? Why?


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Originally Posted by Ghostmaker View Post
You want to put money on your last dumb statement?
https://media.wattswater.com/ES-LFMMV.pdf


Features Lead Free* cast copper silicon alloy body construction Solid wax hydraulic principle thermostat assures dependable mixing of hot and cold water Thermostat controls both hot and cold water Solder, threaded, PEX or CPVC, Quick-Connect end connection or w/press Adjustment cap with locking feature ASSE 1017 listed ASSE 1069 listed ASSE 1070 listed IAPMO cUPC listed Integral filter washers and check valves

Specs say they have checks.


You need checks to prevent hot entering your cold.
Watts has a very nice lineup of valves. Quite a few of their model lines do not have check valves though. Just of their residential level versions, I think it's only 2 out of 5 or so model lineups. Some of the standard require the check valve, some don't. It'll depend on local codes. I personally would spend the extra $5 to get the integral check valves, there's little reason not to......

Cheers!
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