GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

North Idaho ranch boiler sizing

user-6258531 | Posted in Mechanicals on

The plan for the N Idaho ranch is nearing completion. The house will be slab on grade with hydronic floor heat. There will be a natural gas fired boiler supplying heat to the floor tubing and an indirect water heater. Zone 6, heating 1800 sq ft interior plus garage, Attic R 60, Slab R 20, Walls R 23 blown fiberglass, Windows triple pane U .24. My calculations put the design heat loss at 28,000 BTU/hr using outdoor temp of -10F.

The first contractor has proposed using a Lochinvar 85,000 BTU/hr input modulating condensing boiler. This boiler also comes with a 55,000 BTU/hr input. Both have a 5:1 turn-down.
I’m no boiler expert but it appears to me that they have proposed an over-sized boiler.

Before I have this discussion with them, can someone please confirm the smaller boiler will work or explain to me why they may have proposed the right boiler?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    You'll get a lot of different opinions on how to size a boiler when there is an indirect water heater. It's true that you need a lot of BTU/h when two or three appliances or fixtures simultaneously call for hot water. But this situation is solved with controls, not a bigger boiler. When there is a demand for domestic hot water, the boiler controls can prioritize the indirect water heater zone and ignore space heating demands for a while. Don't worry -- your house won't cool off.

    Once the domestic hot water demand is satisfied, the heating zones can be prioritized. As I said, this is all accomplished with controls, not a bigger boiler. The 55,000 BTU/h boiler is plenty big enough.

    -- Martin Holladay

  2. Dana1 | | #2

    The most important aspect for sizing a mod-con is the minimum-fire output relative to your design heat load. Ideally the min-fire output would be less than half the design heat load, to ensure that it can modulate almost continuously for the entire heating season. The average wintertime heat load is usually less than half the load at the 99% outside design temperature, and if the min-fire output is more than half it means that it will be cycling even during mid-winter conditions, rather than modulating.

    Staying within Lochinvar's lineup the "right" boiler for your loads is the Lochinvar CDN040, which modulates between 9000-40,000 BTU/hr in, 8,500- 37,000 BTU/hr out. With a max-fire output of 37K you have a 38K/28K = 1.32 oversize factor, which is nearly the exact ideal 1.4x oversize factor per ASHRAE recommendations.

    See also:

    As Martin points out, even a CDN040 has as much or more heat output than the burner on a standalone tank heater, and with the water heater zoned as the "priority zone" it will have no problem managing both. With the high mass of slab radiation you will NEVER see a drop in room temperature during the tank's recovery time when the heating zone's call for heat is being suppressed.

    It's your house, your decision, but since you've run the load numbers you don't need to listen to sizing recommendations from the contractors. Tell THEM the load, and tell them which boiler you'd like them to bid.

  3. user-6258531 | | #3

    Thank you Martin and Dana.
    One part of my heat loss calculation that I'm not clear on is what delta T to use for the slab floor to the earth below. I figured that the slab would be 80 degrees and the earth below my R 20 foam would be 40 degrees, so I used a delta T of 40 for floor loss calculation. Does that seem reasonable?

  4. GBA Editor
    Martin Holladay | | #4

    Modeling heat loss from a slab on grade is complex. The soil near the perimeter of your home will almost certainly be colder than 40 degrees F for much of the winter -- especially in December and January in winters with little snowfall. However, the soil under the center of your slab may well be warmer than 40 degrees.

    For more information on this topic, see Can Foam Insulation Be Too Thick?

    -- Martin Holladay

  5. user-6258531 | | #5

    Thanks Martin. A google search shows that there is no accepted answer to my question, at least that I can find. It seems that using a 40 degree temperature difference between slab slab and earth is most likely overstating the heat loss. But one needs to have an estimate for design.
    FYI, the economics discussion in the link is pretty out of date. Installed solar is now $3/peak watt, not $8.

  6. Expert Member
    Dana Dorsett | | #6

    In some parts of the US PV is closing in on $2/watt, 75% cheaper than typical pricing circa 2009 when that article was drafted, which makes the financial case against excessive sub-slab insulation (and the somewhat arbitrary standards of PassiveHouse) even more compelling. Of course, using reclaimed roofing EPS at 1/4-1/3 the price of virgin stock goods changes the financial model too.

    By the time a new PV system installed in 2017 is at end-of-life (25-40 years) the replacement equipment will be even cheaper and more efficient. At some point even the low prices of even reclaimed insulation no longer has a financial rationale.

  7. user-6258531 | | #7

    Success. After some discussion, the first contractor agreed that the boiler could be smaller. His motivation to size it larger was to reduce likelihood of me complaining about hot water tank recovery. Really helped to have all the good information available on GBA along with Dana and Martin's coaching. And the second bid came in with a reasonable sized boiler. Thanks.

    Now if I could just find a source of recycled roofing foam on near northern Idaho.

  8. Expert Member
    Dana Dorsett | | #8

    Oversizing the boiler for better domestic hot water performance is a common error, and rarely necessary. The "right" solution for higher hot water performance (99% of the time) is to up-size the volume of the indirect, not the boiler.

    Nationwide Foam will ship pretty much anywhere, and has multiple depots across the country. (They supplied reclaimed foam for the St. Alphonsus Regional Medical Center in Boise.)

    You can also contact commercial roofing companies in Couer d' Alene or Spokane or Moscow/Pullman about buying reclaimed foam. I'm sure somebody is sitting on a warehouse full of reclaimed goods in pretty-good condition that they would be willing to share, for a price.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |