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Backup oil boiler choice?

Levi Member Posts: 3
I have a heatmor outdoor wood boiler. I am looking to install a backup oil boiler into my hydronic system. I have a 2000 sqft house with in floor slab radiant heat downstairs and baseboard radiators upstairs. I was originally going to put in a toyotomi oil miser om-180. Now I'm thinking this is not the best option because they are designed to run at 130F. My wood boiler runs at 180F. I am thinking I need to purchase a boiler designed to run at 180F otherwise I don't get heat out of my baseboards. I know there are other options such as changing mixing valves, installing heat convectors made to run at lower temps, etc..... I know people will prob need more info but hopefully I can get the ball rolling on a discussion. Thoughts?


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,506
    How much will you be using the backup?
    Caleffi https://www.caleffi.com/usa/en-us/technical-magazine has some great info in their Idronics series.
    But leave temperatures out of the equation in the sense that it doesn't really matter how hot the water the water gets, but getting it to the emitters at the right temperature. This will require mixing in both scenarios.
    I'm sure you're in floor radiant (I mean I HOPE) isn't getting 180° water.
    Ideally you would want both units feeding into a buffer tank, and sending the right temperature, based on your heat load and outdoor temperature, to the emitters.
  • Steamhead
    Steamhead Member Posts: 16,732
    Let's back up here. How much heat does the house actually need?

    The way to answer this is by doing a heat-loss calculation. Slant/Fin makes a free calculator available, here:


    There was a version for PC but I can't find the link now. If you need it, one of us can send it to you.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • Levi
    Levi Member Posts: 3
    The whole heating system is plumbed to accept 180 degree fluid. There's a mixing valve on the slab side to bring it down to 125-130 F. There is no mixing on the baseboard side sending 180F water to the baseboards. I could just slap in the toyo and run it full bore at 176 F and this would get me heat out of my baseboard. And the slab would be fine because of the mixing valve. I'm just afraid I'll lose a ton of efficiency because the toyo is designed to run at 130 F. I'm thinking I'm better off just running a good old fashioned well McClain cast iron designed to run 180F ? I'm no plumber, just looking for thoughts from somebody who hopefully knows more than me. I hired a plumber when I put the system in more as a design consultant. I did the labor myself. He basically designed the system around the wood boiler with a supply and return stubbed out for an add on oil boiler at a later time. There may be other design flaws in the system because it never quite worked as he advertised. That, or I just don't have enough knowledge to optimize it my self.
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
    edited August 2017
    I actually have an oil miser 180 for sale! It was my oil backup. I have switched to propane. Tank, dual filters, oil line, and boiler all are coming out. It will run to 176 degrees, should not be a problem on your high temp. The 180 is a boiler, the others are water heaters by listing.

    It has served me well as a backup and is verrrry quiet, and made for direct venting from the get go, unlike other oil boilers. Those are the reasons I bought it anyway.

    Let me know if you are I interested or would like more info.

    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
    Also, do you have a heat exchanger between the OWB and the slab/baseboard? I run a wood gasification boiler as primary heat source but everything is pressurized closed loop. I am on a 2200 square foot radiant slab, with cast iron rads on second floor.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • Levi
    Levi Member Posts: 3
    Yes there is a plate exchanger as the solid fuel boiler is atmospheric. Wanted a gasification boiler at the start. Ended up settling for non-gasification because of the price. It's not a bad one though. It cycles enough that there's really only alot of smoke when you reload the wood.
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
    Do you have a chimney if you wanted to run a cast iron boiler? I got the Toyo because of its direct vent already built into the design. I have been very happy with its performance overall.

    What kind of temps do you run with your current system? Are you worried that the 176 degree high limit is too low?
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!