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Indirect water heater as a buffer tank.....

Starch Member Posts: 102
Looking at a job with mostly radiant in-slab, a couple small high temp zones also (fintube). The owner wants to zone the radiant into some rather small areas, so I'm thinking a buffer tank will be needed, and I'm considering an indirect water heater to serve this purpose, as well as provide the domestic hot water. This will be an oil fired boiler, and the existing water heater is electric.

The total BTU load on the house is a little over 83,000 btu/hr at design conditions. The question is, how do I figure the domestic water heating requirements over and above the heating load, or is it even an issue?? The house is about 3700 sq ft, and will have 4 full bathrooms. There's only two people actually living there, the extra space is for potential guests and will be used sparingly.

Originally I was considering the Burnham V83 with 107,000 net output, but then I started confusing myself with the domestic water requirements.

Suggestions, as always, appreciated!!



  • Starch
    Starch Member Posts: 102

    Reposting this to bring it to the top.

    Would really appreciate some advice.

  • PJO
    PJO Member Posts: 140
    How about...

    an Ergomax (26 gallons with coils for domestic) or Phase III (less buffer gallons, but "tank in tank"). These might suit you. From what I've seen here on the Wall, they both have good performance.

    I wouldn't get too caught up in the "additional" BTU's for the domestic, but I don't do this for a living either. My own situation was to get a medium (13 gallon) mass boiler and I sized it by heating load alone with a tankless. I don't know if that helps or not, but just thought I'd try it.

    Take Care, PJO
  • hr
    hr Member Posts: 6,106

    I don't usually add boiler size for DHW indirect, UNLESS the dhw load is larger than the heatload. In my area we have fairly warm water source year around 60 degree is typical. Watch your recovery rate if your input water gets real cold. We had an issue with this in Utah where the incoming water could get into the high 30's sometimes :) Usually just before the water mains started freezing in the streets! That temperature really slows DHW production.

    I usually go with a 50 gallon. 60 or 75 with lots of teen agers! Priortize the DHW, of course.

    A buffer would be a good idea. Many ways to do this. If you buffer the mixed down temperature an indirect for DHW probably would not be hot enough to supply much recovery. Use a basic insulated electric hw tank.

    The best way to buffer the high temperature side would be a Ergomax type indirect which presents more boiler water capacity.

    Phase 3 work next best as they hold more "boiler side" water than a coil type indirect.

    Does answer your questions?

    hot rod

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  • Boonierat
    Boonierat Member Posts: 58
    Sizing the boiler

    I do this every day...............don't get caught up in adding the DHW to the heat load.
    Look at the BTU requirement of whatever brand domestic water heater you and you're client selects; then make sure you can hit that number. For instance a popular 40 gallon indirect we sell has a requirement of 127,000 to get it's maximum yield. Most of the time (not always) your DHW load will exceed your heat load. With the advent of consumers buying whirlpools and soaking tubs that number flies upwards real quick. Coupled with priority zone control you should have a happy client. There ain't much you can say to a client who has a five gallon bucket and a stop watch under his tub spout and says to you............."Why doesn't this heater deliver what it's rated for"
    Stay loose; stay well,...................Nels
  • I guess you saw

    John Siegenthaler's article in P & M; if not, here's the link (tip of the hat to hr):,2379,78589,00.html

    I've always considered a buffer tank as a lot more work, but sometimes it's necessary for proper system operation. It's not good to go into a mechanical room and hear the boiler short cycling or starting to kettle because of inadequate flow.

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  • Larry Ticknor
    Larry Ticknor Member Posts: 38
    One consideration

    I would be hesistant to add boiler capacity for domestic hot water following what HR has already said.

    Last time I asked Ergomax, they suggested that I would need at least a 100,000 btu boiler to produce enough hot water with the Ergomax. I was looking at an 80,000 btu and they suggested not to use an Ergomax. So check with them before you use a smaller boiler.

    Also, last spring HR posted on how to hook up a buffer tank so as to not damage your boiler and get the best benefit. It was a short post with two or three hand drawings included. I can't find mine right now, but that post was a gem. He alluded to a couple key points in his reply to you. If you go with a buffer tank, ask HR to post that again.

    Good Luck,

  • hr
    hr Member Posts: 6,106
    a buffering we will go

    Here is how I am buffering the MZ for my shop. My shop and office load are a mere 45,000. The MZ is a 90,000, so I need to let it have some volumn to exercise.

    I made some custom adapters to adapt 1" to the back of the MZ and put the buffer in series.

    Series is simple and works well for condensing equipment. Draw backs...on cold start up this arrangement would keep a non condensor in a cold run mode for a longer period of time. Probably not desireable for cast boilers. Also the time lag for heat to the emitters would be slowed. Also when the boiler is not fired it becomes a cooling tower of sorts as it would strip heat from the fluid stream through it and run it up the chimney.

    Parallel gets better as the emitters are not running through the boiler. Slow response and extended "cold run boiler cycles" are still possible.

    Primary secondary solves all the problems. Using a V/S pump from the boiler loop would allow you protection for the boiler preventing a cold system or buffer from overwhelming the boiler :)

    Here is the formula for sizing a tank. Using a 100,000 boiler as an example with the smallest load at 15,000 BTU/ hr. Cycle the buffer from 140-180 degrees. So a small "micro load" would still allow a 10 minute boiler run time.

    Play with some numbers in this formula to suit your system. Of course a very well insulated tank would be wise. Maybe wrap an additional insulation jacket around the tank you chose.

    A heat source with an output that would modulate from the smallest load up to the largest would be a simple way to do this, of course :)

    Hope to see the Weil McLain Ultra unvieled at ISH this week. Gotta scam a ticket for that black tie event somehow!

    hot rod

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  • Starch
    Starch Member Posts: 102
    Thank you, thank you, thank you....

    Thanks to all who replied! You guys are the best!

    I think I was just thinking too hard and confusing myself.

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