Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit

HTP UFT Combi boiler pipe size/btu output

styxplostyxplo Member Posts: 47
I purchased a HTP UFT combi floor mount boiler. I'm in the process of figuring out the plumbing and pipe size. It's the 140,000 btu model. In my research I keep coming across that 1" pipe is good for 7.5 GPM and 75,000 BTU. The boiler side inlet and outlet piping is 1". How can it deliver 140,000 btu's through 1" pipe?


  • mattmia2mattmia2 Member Posts: 1,619
    Greater temperature differential between the inlet and outlet. There is a chart in the manual.

    In your application you may need a somewhat higher flow through the boiler than the recommended flow for quiet flow in the system itself. If I recall the actual tappings out of the boiler case are bigger than 1".

    You may also need to set it up primary/secondary and run your primary loop at a higher temp than the secondary loops at design conditions.
  • mattmia2mattmia2 Member Posts: 1,619
    Oh, also the 140,000 is intended for DHW heating, you won't be able to get 140,000 btu/hr out of the DH side unless you design your heat emitters to give you a high temp differential. I missed the combi part.
  • styxplostyxplo Member Posts: 47
    My design BTU for the house is 55,000 btu’s I bought the larger boiler because it was a combo unit. My mechanical room is kind of tight already. The taps on the boiler are 1” NPT. The non combo unit is 1-1/4”. I’m trying to determine the pipe size for my primary/ secondary loops. I also have a small HTP buffer tank that I plan on plumbing in the 2 pipe config. I thought I was kind of close to the limit for 1” copper so I was going to use 1-1/4” for the piping. Are there drawbacks to oversizing. If the boiler only had 1” taps does it make sense to use 1-1/4”. Thanks Mark
  • DZoroDZoro Member Posts: 1,035
    You will only need 55,000 btu's for your house heating side, so 1" is more than sufficient. I see no need to go larger, would be a waste of money IMO.
  • mattmia2mattmia2 Member Posts: 1,619
    There is sort of a step in cost between 1.25" and 1" fittings where 1.25" fittings are a lot more expensive than 1" fittings which can get very expensive if you are building manifolds out of 1.25" fittings. I used a lot of iron pipe for my near boiler piping for this reason.
Sign In or Register to comment.


It looks like you're new here. If you want to get involved, click one of these buttons!