Mixing valve requirement

[Keith Bellairs]

Is there any downside to taking the cold water mixing valve out? The code seems to require tanks to run at 140 and then a cold water mixing valve to take the supplied water down to a safe 120. Apparently this is to prevent storage tanks from growing legionella.

For a tankless heater that does not store water exposed to air, this does not seem to make sense. In addition it requires wasting energy. I hate to pay to heat water and then mix cold water into it. Just saying.

Finally, I live with hard water. If the tankless is run at 120 and not mixed down it should not get scale on it as fast as if it is run at 140.

The contractor who installed our heater did it to code. We have an adjustable mixing valve. But even when it is set to provide max temp it drops the outlet by 20 degrees.

The code in our state does explicitly require the mixing valve on tankless water heaters. Anyone know why?

 

[Reach4]

As I understand it, the code reasoning would be to prevent scalding.

 

[Sponsor]

 

[Standardairconditioner]

Keith, you're right about the storage tank. When a storage tank can be made as hot as possible, it can prevent extremophiles that can survive up until 120°F.

But 140°F isn't all that vicious either. Many homes do not have TMVs, have their water heater tanks set at 140°F, and they are fine for any household. Upon turn on a tap, it takes some time for water to reach the faucet, plenty of time to realize the faucet handle is on hot.

Why would a TMV be placed after a tankless heater? Probably as a fail safe mechanism. This way no heater, tankless or not, can go rogue and just spit out lava. In a society of lawyers and lawsuits, the mixer and energy waste is cheaper than a settlement.

 

[Jadnashua]

A properly working tempering valve should not arbitrarily drop the temp 20-degrees unless it is adjusted incorrectly or not working properly. If, say, adjusted to 120-degree output, at 120-degree input or below, it should essentially shut off the cold water and pass through the hot.

A typical tankless, at full flow in the winter time often would have a hard time exceeding 120-degrees.

Where I live they are required to pass code.

 

[Reach4]

Finally, I live with hard water.

How hard is the water, and what WH? If you read the warranty info for your WH, you may find that it will not honor the warranty if the hardness is more than 7 grains.

How long has this WH been in service?

 

[Keith Bellairs]

A properly working tempering valve should not arbitrarily drop the temp 20-degrees unless it is adjusted incorrectly or not working properly. If, say, adjusted to 120-degree output, at 120-degree input or below, it should essentially shut off the cold water and pass through the hot.

A typical tankless, at full flow in the winter time often would have a hard time exceeding 120-degrees.

Where I live they are required to pass code.

My mixing valve is marked MIN to MAX. MIN appears to cutoff the warm side. MAX appears to be a 20 degree differential. At max if I set the WH to 140, I get 120. If I set it to 130, I get 110. So the valve appears to be calibrated by differential not by actual temps. It would be great if I could change it out for one that could actually be set to 120. Then I could set the WH to 120 and get 120 water.

Where we live, too, it will be hard for the WH to get to 140 once the ground gets cold. So when it gets cold enough for the WH to only get to 120 we will only get 100 out of the tap. That was our experience last winter.

Yes, everyone is required to follow the code.

 

[Jadnashua]

What brand and model tempering valve is installed? Most of them are a thermostatically controlled variable valve that is continuously adjustable to get the desired output temp.

 

[Standardairconditioner]

A TMV shouldn't be knocking off 20°F from hot water if it isn't over the max. Basically if your water supply is something like 115°F, then a good TMV should not be mixing any cold water at all, and still give you 115°F.

 

[Keith Bellairs]

What brand and model tempering valve is installed? Most of them are a thermostatically controlled variable valve that is continuously adjustable to get the desired output temp.

Its a Caleffi Mod 521. Their spec sheet says you are right. I have it set at MAX, which should be good for 150. So now I don't know what to think. Of course the output temp should keep going up when I raise the temp in the WH because I have not gone above 150 (thank goodness).

Could the digital readout on the Rinnai be off by 20 degrees? I think I'll set the TMV to 120 (4 on the dial), set the WH to 140 and see what I get for output temp. then I'll crank the WH down to 120 and do it again.

Fun and games.

 

[Keith Bellairs]

This thread was very helpful. Once it was explained to me how the TMV was supposed to work I did some more tests. It turns out that the thermometer supplied as part of the Caleffi kit (installed at the output of the TMV) reads about 20 degrees low. When it reads 100 the tap water reads 120 on my candy thermometer. Since it is in a well in the copper pipe with only indirect contact with the water, I think it is not surprising that the copper pipe cools it. But it comes off as useless or dangerous - if I got the water reading up to 120 on it I'd be supplying 140.

So the energy waste I thought I was seeing turns out to have been an instrumentation problem.

Thanks for the comments and help.

 

[Dana]

With a tankless there's no point to running it at a temp hotter than you actually need to use, which is typically no more than 115F. The entire heat exchanger is purged with every significant hot water draw, and water is not being stored at a 115F (which would be a potential legionella hazard), so the only rationale for a mixing valve is scale prevention. A mixing valve that establishes a delta-T is a lousy choice for the application. A mixing valve designed to regulate the output to 120F (or whatever else it's set to ) should pass 115-120F tankless output undiluted with cold water, but would mix down if the tankless output went over temp.