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Thread: Mixing Valve for Tempering water

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    DIY Member handyman923's Avatar
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    Default Mixing Valve for Tempering water

    I am looking to install a mixing valve on our current water heater so I can increse the temperature to 140 to 150 and not be scalded. I was looking at the Honeywell AMX300 mixing valve that attaches directly to the top of the water heater.

    The specs list the minimum flow rate as 0.25 gpm. My question is what will happen if you open a hot faucet just a trickle and have a flow rate less than 0.25 gpm? Will you get scalded with 140 or 150 deg water? Or is the mixing valve just not quite as accurate below 0.25 gpm?

    Thanks!

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    Forum Admin, Expert Plumber Terry's Avatar
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    If you're using that little water, you will have a hard time scalding anybody. The heat loss as it dribbles through the pipeing system is going to take a toll.

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    DIY Member handyman923's Avatar
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    Next question... Can a recirculating pump be added to this system? This valve has a cold water return inlet and the product manual siad this can be done if you add an aquistat and a temperature controlled pump on the return line. But then I read online that this will not work because the mixing valve does not mix unless there is flow. Therefore it would put superheated water in the hot-water lines.

    Thanks!

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    Retired Defense Industry Engineer jadnashua's Avatar
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    A recirc system draws water through the line, just like you opened the faucet, but instead of dumping it, it goes through the return line back to the WH. On a retrofit system, you may not have a dedicated return line, and then, they use the cold line and a thermostatically controlled crossover to allow flow when the hot isn't hot enough.
    Jim DeBruycker
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    DIY Member handyman923's Avatar
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    So it sounds like I could use the Grundfos Comfort System with the mixing valve and not have to add a dedicated return line. Is this correct?

    Thanks!

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    Retired Defense Industry Engineer jadnashua's Avatar
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    That should work and is reported to be a good system. FWIW, I have a RedyTemp unit (more expensive, easier, to me anyways to install) that works well.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Moderator & Master Plumber hj's Avatar
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    Without an actual "flow" there is no way for cold water to pass through the valve to cool the water down. A Comfort system does NOT remedy that problem. It will still produce full temperature water at the faucet until the "tempered/mixed water" can fill the pipe.
    Licensed residential and commercial plumber

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    DIY Member handyman923's Avatar
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    Is there any way to get a circulator system to work with a tempering valve (and not get super hot water in the lines)?

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    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by handyman923 View Post
    Is there any way to get a circulator system to work with a tempering valve (and not get super hot water in the lines)?
    A recirc with dedicated return would only work before the mixing valve so you would need multiple mixing valves close to the POU. A retrofit recirc that uses the cold side as the return along with temperature controlled crossover, would work downstream of a mixing valve.

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    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by hj View Post
    Without an actual "flow" there is no way for cold water to pass through the valve to cool the water down. A Comfort system does NOT remedy that problem. It will still produce full temperature water at the faucet until the "tempered/mixed water" can fill the pipe.
    The very limited amount of flow with a "Comfort" system would not likely present a scald situation. The slow moving water in the pipe would loose much of its heat on route to the POU. The temperature controlled valve in the "Comfort" crossover would stop the recirc flow long before it was anywhere near scalding.

    All that said, depending on how the piping is laid out WRT trunks and branches, there may be the remote possibility of a small slug of scalding hot water in the line getting to the POU, much like the "cold sandwich" does on a tankless.

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    Moderator & Master Plumber hj's Avatar
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    There are ways to use a tempering valve and circulation system, but they involve mixing the "circualted" water into the cold inlet fo the mixer.
    Licensed residential and commercial plumber

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    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by hj View Post
    There are ways to use a tempering valve and circulation system, but they involve mixing the "circualted" water into the cold inlet fo the mixer.
    Can you elaborate on how that would be plumbed? Would you return the recirc to the bottom of the tank or to the cold inlet side? Come to think of it, I probably should not have said "would only work before the mixing valve".

    Since the mixing valve adds water from the cold side into the stream that is circulated, essentially the recirc pump will put back more water (blended cold plus hot) than it took out of the tank if it returns to the bottom of the tank. The extra water then has to flow backwards out the cold inlet but since it is now blended, it will no longer be cold. Depending on how far this blended hot water has to travel (heat loss over distance) before it is reintroduced into the tempering valve, the re-blended hot water could conceivably be hotter than what the tempering valve is set to.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Okay, I must be missing something obvious...with a retrofit system, at the cross-over point, it is getting the hot water from the tempering valve at the WH. The pump is pushing (or pulling) that water back through the cold water line which is going into the WH on the cold inlet side. So, since this is not normally ever hotter than the setting of the cross-over, the 'cold' inlet to the tempering valve would never be above the 100-105 degree range, while the hot from the WH could easily be much hotter. Why would the hot water line end up hotter than the setting of the tempering valve? Worst case, it might be nearly 100% from the cold side, which wouldn't be more than the cross-over point's setting, or don't I understand the limitations of a typical tempering valve.

    What am I missing? I could see this being an issue with a dedicated return line, since returning the hot to say the drain or cold input might back up and feed the tempering valve, too, but not with using the cold line as the return controlled by the aquastat in the crossover.
    Jim DeBruycker
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    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by jadnashua View Post
    Okay, I must be missing something obvious....
    I was talking about a dedicated return line run to the bottom of the HWT where the drain is. That is a common practice.

    Returning instead to the cold line would mitigate that.
    Last edited by LLigetfa; 02-02-2013 at 04:41 PM. Reason: typo

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    DIY Member handyman923's Avatar
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    I found the installation instructions for the mixing valve I was planning on using: http://www.forwardthinking.honeywell...ll/62_3112.pdf
    This valve connects directly to the water heater and has a special circulating return inlet on the valve (see page 3). Also, Figure 5 shows just a aquastat and a pump on the return line going into the return inlet. So it looks like this could work.

    HJ, is this return inlet the same thing you were referring to?

    Thanks!

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