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Community and Q&A

Mod-con boiler: help!

metalanton | Posted in GBA Pro Help on

Hi,   I have been reading past questions on Mod con sizing and heating system design for months  and have learned so much from archived discussions on GBA. THANK YOU.  I  am now ready to ask a direct question.

I am building a 1500 square foot house in Montana.  It is a slab on grade with ICF perimeter and R-10 under slab.  6 loops of pex and I plan on one zone for heating.  compact floor plan.  I get moderate solar gain and feel the house is performing fairly well not freezing on the interior even with -10 degree days.  R-23 walls and R-60 ceiling, so nothing extreme on the insulation for this build.  the mechanical room is not huge, 28″ by 5 feet, so that is necessitating the mod-con vs. something like an HTP Phoenix with heat exchanger.

I have a plumber here that I have used in the past and he does a good job but a few red flags came up for me on his bid.  I was interested in putting a 55k btu mod con with a reverse indirect water heater for dhw and buffering.  When I mentioned the idea to the plumber, I had to explain to him what the reverse part of the water heater was since  he had never installed one… first red flag.  then when I got the bid it was for a lochinvar whn 85 and a lochinvar squire indirect water heater.   l know this boiler is oversized, but I see the squire seems to require a 100k btu boiler minimum, so maybe that was his train of thought.  Unfortunately in my region it seems like I cannot find anyone that is savvy on really trying to maximize mod con performance and the plumbers want to just stick to what they have always done and had few callbacks on.  I initially wanted to install the heating system myself but i have plenty of other things to do and their would be quite the learning curve for me to get it dialed in.  So my questions are:

1. DO you think I am on the right track with a 55k modcon such as the whn 55 paired with a reverse indirect.
2. If yes do you think I will be satisfied with the DHW production of this combo (it will only be about 2gpm) family of 4 but we can take turns running the shower. it seems like DHW production is the downside when paired with a small boiler.
3.  should I just ask him to downsize the boiler and go with his direct water heater or trust his sizing. ( I really dont want to do this)
4.   It seems the lochinvars are well supported here with navien being second.. I really like the htp UFT-80 but again oversized and no one seems to use them here.  I am ok with waiting for parts in the event of a breakdown.. The lochinvar looks great but the pricepoint is double that of htp or navien.  Any boiler suggestions?

I know this is a bit scattered starting point and I need to do a more accurate heat loss assessment but I really appreciate any guidance.  thanks in advance

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  1. Expert Member
    Akos | | #1

    This might not answer your questions, just add more variables.

    I'm a big fan of condensing tankless water heaters for space heat. They are cheap, compact and once installed, if anything goes wrong, simple to swap out for a new one. They also have much better turndown than most boilers (no need for buffer tanks).

    After being shocked by the price of a modcon for my place, decided to try the tankless option. For the price, even if I have to replace it every 3 years, it is still cheaper than the modcon, it has been running for 7 years now. The warranty on the tankless units is reduced when used in a space heating application, good to check the fine print of various manufacturers.

    I find getting DHW off an indirect tank pretty inefficient. When the "boiler" needs to fire up to make up for standby losses or smaller draws, the return temperature is very high well above condensing temperature, it almost never runs in condensing mode unless filling a tub. For the price, I think it is simpler to stick to a standard water tank.

    I'm also using a tankless for heat and DWH at a triplex in an open system. I find this is the most efficient, compared to a similar triplex with the indirect tank, the summer time gas consumption is about 40% less. It does mean a bit more maintenance as you have to deal with scale buildup.

    Two items. Tankless units are pretty high pressure drops, make sure your pump can provide enough head for the entire setup. They also have a very small inlet strainer which tends to clog easily, good idea to put a larger fine mesh Y strainer.

  2. Yupster | | #2

    Go to this link
    There you will find the installation manual for a Squire indirect tank. At the bottom is a performance chart listing indirect first hour output with different size boilers. A 50 gallon indirect with a 50,000 btu boiler seems to output about 2.5 gpm (150 gal/60 minutes). That's enough for a standard shower to run for an hour. You can use that table to decide.

    I'm not sure how we can help you with knowing whether your boiler is going to be too big without any kind of heat loss calculations. Best do that before you start getting quotes for a system. Design first, then you have something concrete to get quotes on.
    If your house load comes out under the 51,000 btus provided by the Lochinivar WHN055 and you set the DHW up as a priority load and you are satisfied with the DHW output of 2.5 gpm, then you should be able to use that size. If it's bigger, go bigger. But there is no way to know without doing some proper design. Otherwise your guess is as good as his.

    As a side note, if you are going to fit a boiler, an indirect, a manifold, boiler piping, maybe an HRV, water equipment etc. into a 60" x 28" utility room, you'd better be one heck of a organizer! That's extremely small for a boiler room. You might have some Hvac/plumber guys swearing at you behind your back....

  3. joshdurston | | #3

    Reverse indirect and infloor radiant aren't a good mix. The reverse indirect approach requires that you maintain your tank hot enough to produce DHW on demand. This temperature is way higher than what is necessary for in floor heat in a concrete slab.

    Lochinvar boilers are top notch, I like IBC and Viessman too. Navien is more of a value product but seems to have a pretty good distribution and support so I wouldn't discount them.

    I would get your own heat load calc down by yourself or a third party ( Note that the 55 and 80 kbtu Lochinvars have about the same minimum fire rate so there isn't a huge downside to oversizing to 80kbtu.

    With high mass infloor, you can probably avoid a buffer altogether if you can setup your zone layout to ensure each zone is about 7000-8000btu, or even less as long as the flow rate is high enough. You can combine zones if necessary.

    An indirect tank setup will run at greater than 80% versus probably like 65-70% for a standard tank. But indirect setups are expensive.

    If you go with a combi and want to keep the size down, consider appropriate flow rate fixtures, and drain water heat recovery, it can be very effective during showers. You can also pipe a buffer tank and pump to many combi's to give a bit of storage.

  4. Expert Member
    Dana Dorsett | | #4

    >" ...I see the squire seems to require a 100k btu boiler minimum..."

    An indirect doesn't "require" a particular sized boiler for anything other than to hit it's hot water delivery specs. When you consider that a typical 50 gallon standalone's burner is good for about 30KBTU/hr, it should put it into some perspective.

    A reverse indirect an unnecessary and expensive piece of kit for combi system that has high mass/ low temp radiation like a radiant slab, a relatively low load, and is operated as a single zone. The best uses for reverse-indirects are in micro-zoned mid to high temp systems and low-mass radiation, which is the polar opposite of the what has been described.

    A Phoenix Light Duty (76,000 BTU/hr burner) or the Westinghouse labeled version thereof with an exterior plate type heat exchanger is probably the best choice here. Your space heating load is probably less than 25,000 BTU/hr @ -10F and your design water temp to deliver that is likely going to be less than 115F, which would be easy to deliver through a plate type heat exchanger from a tankful of 130F water.

    Size the tank for the biggest tub you have to fill. Remember that the Phoenix LD's burner is more than twice the output the typical 50 gallon standalone's, and designed correctly the heating system won't be able to draw more than a third.

    With a 4 person family and a low-load solar tempered home it's likely that water heating efficiency will be more important than space heating efficiency, so even operating the heating system at a fixed temperature knocks a few percent off space heating efficiency, the integrated burner of a water heater more than makes up for it compared to the losses incurred in using an indirect (of any type.) The plumbing to the heat exchanger is lossy, but only loses that heat to the conditioned space, and only in use when there is a space heating load. The plumbing for an indirect loses heat to the conditioned space every time the indirect is heated up, even when there is no space heating load.

    On a slab on grade there is unlikely to be a suitable drainwater heat recovery unit for you. The best performance versions are tall vertical gravity film types, and even the horizontal versions require some vertical height to work at all. If the slab is already poured and the shower drain is under the slab, that ship has sailed.

    If going with a boiler + indirect either the Lochinvar WHB-056 or WHB-086 with the 10:1 turn down ratios area reasonable choice, not so much the WHN-085 or 055. Even the WHN-055's minimum firing rate of 11,000 BTU/hr is substantially higher than the -xxB-086's 8500 BTU/hr. The WHB-056's minimum of 8300 BTU/hr isn't much lower than the -86, so if going with the WHB series rather than the WHN, the higher hot water recovery rate using the bigger -086 is arguably "worth it", with effectively zero down-side on the minimum modulation numbers.

    Of course the UFT-80W's minimum fire input is lower still at 8000 BTU/hr, value priced, and practically DIY-able. The Westinghouse branded version (WBRUNG-080W) has been carried in recent years by one of the box-store chains. They went out of the way to make installation & setup nearly idiot proof (not that the more creative idiots are easily foiled.)

    I don't know much about Lochinvar's low mass combi boilers. Combi boilers tend to be poor fit for most low-load houses, but with high mass slab radiation operated as a single zone that won't matter as much. Navien's have a track record, and with reasonable local support it would do fine, without local support walk away and look for something else.

  5. metalanton | | #5

    Thank you so much, all of these comments have brought me a great deal of clarity already. I stand corrected about the boiler recommendation of the plumber it was indeed the whb 86 not the whn85. So he was on the right track there. I really like the idea of using the HTP Phoenix. But I'm afraid the space constraints of my mechanical room have made that one difficult but I will ask the plumber about it. The HTP Phoenix seems to prefer a lot of room around it for Servicing which we would only have access to 1/3 of the circumference. The house was designed by my wife's aunt and it was a constant battle to properly design all of the Practical details of the house unfortunately. The mechanical room she had designed was actually half the size. But I changed the location and expanded it the best I could. I am very tempted to go with the UFT 80 even though the plumber is unfamiliar with it over the Lochinvar just for the cost savings. Thanks especially, Dana, I was hoping to hear from you!

  6. metalanton | | #6

    Is running a plated HX off the uft80 an option?

    1. Expert Member
      Akos | | #9


      If you are tight on space, can't beat the tankless for DWH and either an open system or plate HX for your floor heat.

      Everything fits into a 3x3 utility room under a stair with room to spare.

  7. Expert Member
    Dana Dorsett | | #7

    The UFT-80W is not designed for potable water (not that I know of- though in Korea where the thing was designed they might do that)- it's not a tankless water heater or combi. An indirect has the heat exchanger inside the tank , but there's no reason an exterior heat exchanger couldn't be used with some other tank. It adds another pump, but the potable side and system side would operate at the same time.

    The EFTC-199W combi boiler has decent specs using some of the same internals as the UFT boilers, but I have no direct or indirect feedback on how well they really do. The -199W has a lower minimum firing rate than the -140F, but at 19,000 BTU/hr it's not exactly "low". But with a high-mass slab it probably won't short-cycle.

    How many linear feet of what size tubing is in the slab, total?

  8. metalanton | | #8

    6 loops of 1/2" . average length 280 ft. Per loop. So around 1600'. I really like the idea of the Phoenix. I just checked service clearance in the installation manual and it only needs 3" on the sides. We have 2" but o think it might be worth the squeeze.

  9. Expert Member
    Dana Dorsett | | #10

    1600' of half-inch PEX contains about 135 lbs of water. A condensing burner at 19K min-fire delivers about 18,000 BTU/hr or 300 BTU/minute. So the temperature rise rate when the water is at the same temp as the concrete is about 300BTU/135lb= 2.2 degrees per minute. Set up for a 10F swing it'll be a 4.5 minute minimum burn time, which isn't bad.

    As the water temp in the tube rises heat will be going into the concrete at a higher rate, so the actual minimum burn would be higher.

    All of which is to say a wall-hung combi with a minimum firing rate in the ~19-20K range can probably be made to work in your system without adding a buffer tank, but if using a combi boiler that needs a hydraulic separator (like the Naviens- not sure about the Lochinvars or HTPs, since I haven't dug into them) it wouldn't hurt to use a 15-25 gallon high-mass hydraulic separator. With 6 loops at ~1 gpm each the flows are low enough to get away with using a small electric water heater (unpowered) as a buffering hydraulic separator.

    Enough with the back-of-napkin hydronic design... do the real math before committing to a solution (or hire somebody to do it for you.)

  10. metalanton | | #11

    Ok time to get out the calculator. Thanks again

  11. metalanton | | #12

    Well, I had a second plumber visit yesterday and their recommendation was for a Navien ncb-240e. When I asked them why not the ncb-180e they responded that they always use the 240 not because of the space heating load but for the DHW. Their bid was only 1500 less than the Lochinvar knight plus indirect water heater, so I can't justify the cost savings for the Navien. Looking at my space though I am leaning towards a combi unit. The eftc199 or the Lochinvar Noble seem like possibilities. Anyone have experience with the Noble?

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