DHW heating

Rollie Peck

I'm a homeowner doing research prior to replacing a 40 year old boiler with a more energy efficient one. Would like to produce DHW more efficiently also.
Most indirect DHW heaters seem to use 180 degree water from the boiler which is well out of the condensing range for high efficiency boilers.
Is it possible to start out with 130 degree water to handle small DHW draws and raise that temperature for larger or longer draws? Would that be too inefficient?
Thanks in advance for your help. The Wall has already been a tremendous help for other parts of this project.

Rollie

curiousity kills

Is it really though

Here in N.H we recomend indirects over oil. Long term indirects have a lifetime warranty with little to zero maintence and will give you plenty of hot water.An oil fired water heater could cost almost as much as an indirect but will need to be replaced aprox 8-10 years never mind yearly burner and flue cleaning necessary to keep it reliable and running efficient. I am a master plumber and an hvac service tech and have installed an indirect in my own home with no complaints. You also could install outdoor temp controls to run the boiler at a lower temp during none heat months and install a priority control for winter months. Hope this helps..

kevin coppinger_4

Well before...

jumping in and giving a long answer...there are a couple of questions i have. What type of heat emmiters do you have? Will they work with such Low water temps? If you want to do the indirect route( and I have one) you will need to get a lot of storage and a big HX to allow a long recovery time. Other option could be a dual fuel tank (electric back up with a hot water coil) or depending on your fuel you could use a tankless water heater like a Takagi, Rinnai or Bosch....kpc

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The Kid

indirect hot water

depending on the size of your boiler will depend on the size of the indierct water heater you should install..Example..Super stor SSU45 (45gal) recomened boiler size (in BTU's at 180*)is 141000 btu's. in addition to that prioirty controlling is a most,possibly with freeze protection.

jerry scharf_2

the 180 number and condensing

Rollie,

The numbers in the manufacturers charts are designed to give the largest number possible for first hour recovery rate (marketing.) The hotter the water you put in, the more DHW you produce. You can heat water with a lower temperature, just not quite as fast.

ME had an example a few months ago with a subject something like "pictures of my wife taking a shower". It's the PC display of the data from his munchkin T50. In that he ran the hot water to the indirect coil around 160, and had a slower pump to get a higher delta T. With these, he had the munchkin in condensing mode for much of the burn time (it's the return water temperature that counts.)

I'm not sure what the payback would be, especially if you had to increase the tank one size, but Mark showed it can be done.

jerry

Rollie Peck

DHW heating

Thanks, guys, I can see that keeping the return water below 130 degrees with a DHW temperature of 125 degrees would be tough.
Really wanted a Viessman or Monitor MZ, but the local Viessman dealer has shown me that neither one would be practical unless I replaced much of my baseboard with radiant panels to get the return water temperature below 130 degrees.
Can't afford to do that and the two highest loss rooms would not have space for enough panels.
Now working on Plan B.

DaveGateway

Don't just go by return water temps

Some manufactures claim their units will condense at higher temps since the heat exchanger can extract so much heat. Take a look at Dunkirk Quantum Leap boiler. Only comes in a 100K size, though. Even getting high 80% to 90% eff on a regular direct vent boiler might be more cost effective then tring to recoup the cost of a high end modulating boiler that gets slightly better efficency.