For the not so long story, please skip to the next paragraph. So, I’m an avid do it yourselfer who is also cursed with the inability to make impulse decisions. I must have the best and most practical (which is sometimes contradictory) solution to whatever project I’m working on. I’m sure many on the forum can relate, since that is why we are here. One of my current projects is to replace my 30+ year old forced hot air oil furnace and hot water heater. With the opinions and data I have gathered, I’m getting ready (still on the dam fence) to install a Burnham MPO-IQ boiler, Weil-McLain Gold Plus indirect water heater, and Slant Fin baseboard heat emitters. During my exhaustive, mind numbing until I’m blue in the face research, I read about how hot water buffer tanks can help provide longer boiler run times when using low mass heat distributors such as baseboard or hydroair, thereby increasing efficiency of the boiler. It would not seem particle to my situation to purchase a buffer tank simply for my heat setup. Then it dawned on me, I will already have one, the indirect water heater.

I thought of this idea of using the indirect hot water heater as a hydronic heating buffer. My plumbing layout is very simple and only meant to convey the idea. Basically, the water from the heating system circulates through the indirect before returning to the boiler. The heat exchanger inside the indirect will act in reverse, pulling heat from the domestic HW. My choice of tank in tank indirect should not add much series resistance to the heating system flow. This setup is simply a plumbing layout change, it requires no additional parts. The only downfall I can see is that I can’t figure out how the domestic HW can have priority if the heating system return water temp stays low for too long. Outdoor reset may also be an issue.
Please tell me what you think. Am I missing something?

 

Reverse indirect. Ergomax or Turbomax.

Outdoor reset would indeed be an issue. Depending on supply water temperatures, you could go months without needing anything near what is required to heat domestic water.

 

The problem would only be determined by the minimum boiler water temperature you are using for outdoor reset.
The minimum on the ODR module is 130f. That would be more than enough for DHW. If the tank temp goes higher than you have the mixing valve for higher indirect water heater temps. You could still use the pump pre-purge feature on the IQ control.

 

Alternatively, you could pipe in a cheap electric water heater as a buffer on the space heating. Pull the indirect off the header before the buffer, obviously. It would be essentially a giant fat part of the pipe.

If you want more reading to go from blue to purple, get Modern Hydronic Heating by John Siegenthaler, and read his columns at pmmag.com. He's one of the best in the industry. Great stuff.