Heatload and boiler pump

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mamuel
mamuel Posts: 10
in Gas Heating

Good afternoon. Got a heatload calc done on house for getting a new boiler put in sometime this year yet. Install guy is down and I'm not around so haven't discussed the details too much but just wondering a few things for discussion. Heatload summary shows total load at 46.5K btu total load. Includes basement that has built in emitters but isn't sealed and finished as well to retain heat so will be doing windows and such shortly). The calculation was done with design day being 0 degrees so most most of the time, around 20 degree winter. Just reading the summary photo i have with measurements and btu's and gpm, the GPM required on all emitters is mostly less than 1 gpm. What size pumps would i even consider with a boiler that is 50k or less BTU and such low gpm than usually what comes with pumps?

Screenshot 2026-07-07 153259.png

Comments

  • Steamhead
    Steamhead Posts: 18,989

    What type of emitters are you using (radiators, baseboard etc)?

    Is everything on a single zone? What piping will you use- series-loop, Monoflo, home runs to a manifold, type and size of pipe?

    Co-Founder, All Steamed Up, Inc.

    Semi-retired

    Baltimore, MD, USA

  • GroundUp
    GroundUp Posts: 2,572

    The resistance in the piping and emitters are 95% of what determines circulator choice. Flow rate and BTU are only a small piece of the puzzle

  • mamuel
    mamuel Posts: 10
    edited July 7

    Everything is baseboard. I have to pull off the basement ceiling to trace and map out the piping but I assume series. I dont remember if its 1/2 or 3/4 at the moment. I'm overseas right now so will have to verify when i get back hopefully soon. I thought that if I split it up diagonally, then total length of emitter in both zones will be just over 70ft (which i think is ideal max lenght?) so I was curious on doing zones that way or just have them parallel but 1 zone if that works. House isn't large or anything. Was suggested a HTP or Loch Knight 55kbtu mod con for the turndown. Even know its high temp emitter, shoulder season would help with modulating at least.

  • hot_rod
    hot_rod Posts: 28,606

    Fin tube generally designs around a 20° delta. A 46,500 BTU/ hr load would require 4.6 gpm of flow. Seems odd that the fintube lengths are in inches. Add them all up, 65- 70" of 3/4 fin tube is about max on one loop.

    3/4 fin tube will have a low pressure drop so most any of the common residential circs will work Grundfos Alpha 15-58, Taco 007E, etc. I like the newre electronic circulators, especially if you have multiple zones.

    The Knight comes with a variable speed circulator for the boiler itself. Usually that boiler is piped with a primary secondary of better yet a hydraulic separator.

    Download the installation manual for the Knight, or other boilers you are considering, for their suggested piping options.

    As for piping the fin tube, series, or individual home runs are a couple options. Depends on how many zones you want? Fig. 6.2 shows a home run with pex runs to each fin tube. This allows every room to have it's own temperature control, which may be overkitt for your project. But home run allows you to add zone control later on.

    I would suggest at least an upper and lower zone for best control.

    Screenshot 2026-07-07 at 10.32.40 AM.png Screenshot 2026-07-07 at 10.31.52 AM.png
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    HVACNUT
  • mamuel
    mamuel Posts: 10

    Alright, finally got back and was able to do some research on the house. Not enough time to change boiler out but that leaves me another winter to plan ahead for spring i guess. The heating oil fill price wasn't exciting though…. I did notice that the entire house is pipped in series. Starts in kitchen and goes to each one along the walls, room to room until the bathroom then back to the boiler. So started looking at the idea I had before and even the knight manual shows that it can be prim/sec piping or single temperature, full flow which is where I thought maybe this split loop might be good or might be better as current series plumbing to get water temps down when not needed since it is a small house and the load calculations were for design temp of 0 degrees and its usally 10-30 most of the time in WI. The image I'v attached it the house but the blue line represents the split zone. Not a home run per say but tying the 2 splits together where they are located in the basement would be pretty simple. 1" pipe from boiler split in to 2 legs of 3/4" and divided the house shows that one side would be 38ft. of emitter and the other would be 40' of emitter. Pretty nice balance (maybe some balancing valves since the piping would be longer on one side of house?) Otherwise as series, just a big loop of 78'. I left out the basement emitters but if i can tie them in, then some free heat for the basement when main floor is calling for heat anyways. So that brings me to the gpm understanding and such. Knight manual shows can be prim/sec or full flow single temp. The manual also shows the minimum flow for the knight is 2.1 gpm for high fire and 1 gpm for low fire. So trying to put this all together, is this a good plan that i have? Also want to have IDWH which may or may not affect this with flow or something. With ODR and the turn down it has and sensor on pipes with the even lengths, hopefully that can lower the temp needed in the baseboards enough to help shoulder season water temps lower a bit and also be pretty healthy and efficient. Thoughts please?

    split house.jpg
  • mamuel
    mamuel Posts: 10

    I even been looking at Combi boilers but they all seem on the larger side and have always just been told to shy away from them.

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