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Indirect Sizing and Recovery


As you may have noticed in one of my other general posts, I'm going to have my oil boiler replaced with a Buderus 115WS and an indirect. I'm trying to make sure I get an appropriately sized indirect. I did a little research today and determined that my incoming well water is about 57 degrees and the shower heads we have us approximately 2.5 gallons/minute. Using 130 degree hot water, it works out that I need about 1.6 gallons/minute of hot water from the indirect. I was planning on using a SuperStor SSU-45 which I believe with a 85,000 BTU boiler will supply about 1.8 gallons/minute. This seems pretty tight and only assumes one shower head running at a time. In the future, my young girls I'm sure will discover the joys of showering and will be happy to use as much hot water as possible :) The girls are yound right now and use the bath tub..... any idea how much water it takes to fill a 6' tub?

Also, the boiler output is pretty tightly matched to the heat loss of the house. I'm concerned that in the cold weather the house temperature will have a hard time recovering from the indirect taking priority over the heat. Has anyone experienced this?

Many thanks.



  • lchmb
    lchmb Member Posts: 2,997

    Use a mixing valve on your indirect and run it to a higher temp. As far as the boiler, size it to the heat loss. I have never had an issue with a system making hot water on a cold day and keeping up with the building..Keep in mind your heat loss is done to the coldest couple of day's of the year. The rest of the time it's over sized...
  • Ironman
    Ironman Member Posts: 7,378
    Relax Brent

    It looks like this whole thing has you overly concerned.

    I have four daughters still at home, ages 12 to 25, plus a wife who fills the tub with hot water to soak in every night. Our clothes washer goes from 10:00am to 10:00pm every day. And my boy and I even take a couple of showers a week. Well, at least I do. We have a 50 gal gas water heater that has a 40k btu burner and many times we'll take three consecutive shows with no problem. Your indirect will have twice that input. You're gonna be alright with what you're getting.

    And, generally the indirect will hold the boiler in priority for 20 minutes or less. It's not gonna cause the house to fall behind where it would be noticed or where the boiler won't get it back. You're also forgetting there's 45 - 60 gallons of storage. Either one of those indirects has at least a 150 gal. first hour rating.

    I've put 32 gal. indirects with smaller input boilers in houses similar to yours and never had a complaint. Relax. :~)
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Brent H.
    Brent H. Member Posts: 161
    Higher Temp with Mixing Valve

    Doesn't the mixing valve only help with a dump load and not continuous flow?

    Also, the Logomatic can only be set up to 140 degrees for hot water.
  • Gordan
    Gordan Member Posts: 891
    Yes on the first one

    But anything is a dump load until you draw down the stored heat. In other words, it will give you a longer continuous draw until you're counting solely on the boiler and what it alone can provide.
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