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Heat Exchanger size

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nate379
nate379 Posts: 161
edited 4:02PM in THE MAIN WALL

In the process of setting up an outdoor boiler which will tie into my indoor combi natural gas and radiant floor system.

Boiler will run 160-180*, insulated buried pipe to shop, then heat exchanger and thermostatic mix valve to get it to around 130* (what the combi heats the floor water to)

Trying to size the water to water heat exchanger. I've spent quite a while searching online and haven't really found good solid info for some reason. I get numbers all over the board.

The combi boiler is 120k BTU and it's never had to run non stop to keep the place heated. THOUGH future plans may include adding about another 900 sq ft to the building (currently is 1240 sq ft)

The outdoor boiler setup to keep it simple is in theory around 300k btu. It'll be heating another building as well eventually.

I ended up with a used 4x12 30 plate a while back. Best I can tell it's way too small. I think they had used it for heating shower water.

I found a 5x12 80 plate on sale for $200, would that be large enough? Next option is a 5x12 100 plate, it's about $325.

Comments

  • EBEBRATT-Ed
    EBEBRATT-Ed Posts: 21,089

    Your best bet would be to pick a MFG and call them and let them size it for you. That is what they do. No reason to guess.

    But you need to give them the correct information to do this. You need flow rates pipe sizes heating load and temperatures. guessing doesn't work.

  • nate379
    nate379 Posts: 161
    edited 4:16PM

    None that I found do that. They said "what do you want to order, we don't give advice"

  • hot_rod
    hot_rod Posts: 28,424

    How exact of an answer do you want?

    A manufacturer, rep, and a knowledgable supplier would use a software to size the exact HX.

    \There were some free online sizers, but the some, maybe most manufacturers want you to do a training now before they give you access.

    Here are the questions they will ask:

    Fluid

    Temperatures

    Flow rates

    Acceptable pressure drop

    Or this program will allow you to enter Load BTU/ hr and do flow calcs for you.

    Is 300,000 the actual boiler output? Do you need to move 300,000 btu/ hr?

    Use purge/ fluh valves so you can descale it, at least on the OWB side. Or unions so it can be easily removed to flush and clean.

    If you have a steel open type OWF, a Y strainer might be a good addition.

    Screenshot 2026-10-10 at 12.39.49 PM.png

    The print out looks like this

    Screenshot 2026-10-10 at 12.40.21 PM.png

    But I doubt you would find a quality, speced plate HX for that price.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream

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