Reasonable \"trick\"

paule

I need a boiler to supply hot water for DHW and an air handler with a hydronic heating coil. One of the proposals I've received uses a Triangle Tube Prestige Solo boiler and a Smart Series 40 indirect. The "trick" is that for DHW they heat the water to about 160 and then use a mixing valve to get the appropriate temperature. By doing this they claim to effectively double the capacity of the indirect. Does this seem like a reasonable thing to do? Is this inefficient (as compared to a larger capacity indirect)? Are there any disadvantages to doing this?

Thanks,

Paul

Darrell_4

Yes.

While the actual numbers may be subject to interpretation it is common, and even advisable, to run the water heater at a higher temperature and then mix it down to the desired output temperature. The advantages are several. It takes less 160 degree water to make 120 degree water than it does if you use 140 degree water to make 120 degree water so in this sense it does make the stored 160 degree water last longer. The water heater will stay cleaner. The mixing device is a more precise device than a tank full of water if closely controlled output temperatures are the goal. The boiler probably won't know the difference between heating your DHW to 120 or 160.

Some jurisdictions are beginning to require the use of a mixing device on indirect fired water heaters because of the potential for 180 degree water.

JohnNY

Yes,

Paul, either way it's about storing energy.

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Bob Bona_4

I'm not

quite so sure about doubling capacity, (mileage may vary), but yes, it's a good set up to keep the tank hot enough to kill bacteria-140 minimum-and mix down to tap temp post tank. 160 seems kind of high to maintain 24/7 though.

It's not too much more to bump up to a Smart 50 if you are worried about capacity. I have never spec'd anything smaller than that, and offer the 60 or 80 for those that have walk in carwash showers. Don't forget your dump loads like tubs that can deplete a 40 REAL fast

Jamie Hall

Another advantage

of storing hot and mixing down -- Legionaires and other nasty bugs don't like it hot, but they sure do like it warm.

paule

Ahh, I should have mentioned that there's a height constraint -- so the 40 will definitely fit, but the 50's a bit iffy.

Thanks,

Paul

Bob Bona_4

FYI

Buderus makes a horizontal 42, 53, and 79 gallon indirect, if that helps. They are 26" high, length varies w/ capacity.

Uni R_2

There is a downside...

With the 40 at those temps you'll be calling the boiler more often (and in the off and mild seasons stranding 2½ gallons of 180° water in the HX each DHW call) and you'll also do very little condensing when doing those frequent DHW cycles.

A larger tank and a wider differential and mixing valve to keep output safe and constant would reduce the frequency of BTUs being stranded in the HX and allow condensing through more of the DHW cycle.

It's too bad that the Prestige can't spike the temp up to a safe level on a weekly preset basis. Does anyone know of a simple add-on controller that could do this?

Rich Kontny_3

Bradford White

Has a 25 gallon water heater they fire at 78,000 btu. It has a built in TMV on it and it puts out about 125 gallons/hour. I put one of these in on Monday. It has unbelievable recovery.

I would think that the indirect at 160 is the same concept. Less hot water needed when it starts so hot

Unknown

All else aside,

If your going to heat the tank to those temps all the faucets should be anti scald types for safety. Mixing valves can fail and send that 160* to the system.