Low flow mixing valve for domestic hot water--needed benefit?

HomeownerDustin

I'm installing a HPHW heater. Plan is to set the temp to 140F and install a mixing valve to 120F. In your experience, is there a real need for a low flow mixing valve? Deciding between these two:

• Regular: https://www.supplyhouse.com/Caleffi-521516A-3-4-Press-MIXCAL-3-Way-Adjustable-Thermostatic-and-Pressure-Balanced-Mixing-Valve-w-Temp-Gauge
• Low flow: https://www.supplyhouse.com/Watts-0559109-3-4-LFL1170M2-US-Lead-Free-Sweat-Lo-Temp-Mixing-Valve

Big Ed_4

For application you want the regular . The low range model max is 120* ….

hot_rod

This is a good option also. This is accurate down to .5 gpm, so if you have low flow fixtures it is ideal. It is fast responding and also will slow to a trickle if the cold water supply fails (scald guard)

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DCContrarian

May I ask why you're plumbing it this way? With a heat pump the efficiency is inversely proportional to the temperature rise, it takes a lot more energy to heat water to 140F and then mix it down to 120F than it takes to heat the same quantity of water to 120F.

hot_rod

it is not uncommon to run the hpwh to 140 . legionella protection for one, but also with the 120v units recovery is fairly slow, so the 140 buffers that. It also can leverage lower over night electric rates.

https://idronics.caleffi.com/sites/default/files/magazine/file/idronics_33_na.pdf

DCContrarian

The whole point of using a heat pump instead of a resistance electric heater is efficiency. The efficiency is entirely determined by the lift. With air temperature of 70F, at 120F you're lifting 50F, at 140F you're lifting 70F. That's a 40% higher lift, you're going to use 40% more electricity to get the same amount of hot water.

The higher temperature isn't going to give you a better recovery rate. The compressor is going to be limited by how much current it can handle, if it's using 40% more electricity it's going to take 40% longer to produce the same amount of hot water.

A higher temperature does give you a bigger first-hour capacity. If that's what you need you're much better off just getting a bigger tank.

hot_rod

The same amount of water?? If have put additional btus into the hotter tank.

Mixing the tank down in this example give you additional capacity. Sure efficiency drops.

But the reduced kwh costs off peak needs to factor in also. That could offset some of the efficiency hit.

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DCContrarian

"The same amount of water?? If have put additional btus into the hotter tank."

I feel like I've had this same conversation a million times with people.

With combustion fuel, a BTU is a BTU, it doesn't matter how you get there.

With a heat pump, a high temperature BTU costs more than a low temperature BTU.

With, say, a gas water heater, if what you want is 100 gallons of water at 120F, it doesn't matter if you first heat 87.5 gallons to 140F and then mix in 12.5 gallons at 50F, or just heat the 100 gallons straight to 120F. Gas consumed is the same either way.

With a heat pump, it makes a big difference. You use 40% more electricity first heating that water to 140F and then mixing it as opposed to just heating it directly to 120F.

It's easy to wave your hands and say "sure efficiency drops," but the whole reason you're spending $2500 on a HPWH instead of $300 on a resistance one is efficiency — and you're giving away 40% off the top.

As a general rule, with a heat pump, if you're mixing, you're doing it wrong.

hot_rod

Only a real sucker would pay $2500 for a HPWH. In some areas they are almost giving them away. Prospective buyers Google rebates offered in your area. Federal, State, local, and utility programs are available.

If the average homeowner pays 3- $ 500 a year for DHW, we are not talking big money differences for running 140F.

Hard to put a price on Legionella protection, unless you have contacted it. And lived to talk about it. Operating the tank at 120 is putting the welcome mat out for legionella parties :)

Incentive Program from Mass Saves

For the OP, if you can afford to run the tank at 140F, I recommend that.

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HomeownerDustin

Thanks Bob! I’ve noted all your links and info.