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Community and Q&A

High Efficiency Boiler with Indirect Water Tank

tom_dresser | Posted in Energy Efficiency and Durability on

I want to replace an old Burnham boiler rated at 96,000 BTU with a high efficiency Navien unit and an indirect water heater.  I have received several proposals for 80,000 BTU boilers with 40 gallon indirects.  The house is a 2 story Cape style house of 2000 s. f. with 2 full bath rooms.  It is insulated with fiberglass batts and recently blown in cellulose from Mass Save.   The knee wall is also insulated.

One contractor has proposed a 150,000 BTU boiler.  We both agree that 80,000 BTU would be sufficient for heating, but he believes the bigger boiler would give faster hot water recovery.  He indicated that because this boiler has a 15 to 1 turndown ratio it would modulate and run at a lower rate.  He is planning on installing this same configuration in his own 2000 s. f. house.

Does this proposal make sense or should I go with the 80,000 BTU boiler?

Thank you.
Tom Dresser

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  1. CramerSilkworth | | #1

    The <100 mbh boilers are often 10:1, so similar low end fire rate to the 150's 15:1.

    But how much hot water do you really need? Typical stand-alone gas tanks are 35-40 mbh, and they do fine.

    See also:

  2. joshdurston | | #2

    A couple considerations that may or may not be factors:
    -The recovery will likely be limited by the indirect's heat exchanger surface area. The boiler will heat up to it's DHW setpoint, and then maintain that by modulating the burner. To absorb 150mbh you'll need a bigger indirect tank (or colder water).
    -More BTU can mean bigger gas lines depending on distance.
    -Personally I would probably look for a firetube style boiler that is easily serviceable. Navien's have a reputation for tight packaging and being a pain to service (particularily the combi's, perhaps the heating only boilers are better).
    -My preference is to keep the DHW separate from the heating. Indirect's can be quite expensive by the time they are piped and pumped. If your house is suitable consider a heat pump water heater, or even a standalone power vent natural gas tank. Not a huge combi fan, but would do that before an indirect for cost (probably a IBC SFC/DC, or a Lochinvar).
    -Have you done a heating load calc? Or analyzed past fuel use?
    -The difference between a 8000btu and 10000 btu low fire doesn't seem like much but it's 25% more, and can mean more cycling in the shoulder season. Consider the min firing rate, not just the turn down ratio. Lower is better almost always. I'd take a tightly sized boiler with a lower firing rate, everyday over a generously sized boiler with a higher min fire rate.

    Here is a method to look at past fuel use to measure instead of guess your heat load.

  3. gusfhb | | #3

    150k is nuts, period

    ther is no reason to ovesize it at that level

    I have a 79k boiler with an indirect and you can run out of hot water with a family of 4 but it is rare, and basically silly[start laundry, start the dishes, run a bath, get in the shower]

    Don't do that

    1. bubbleboba | | #10

      I agree with this. 150k is crazy big.

  4. tommay | | #4

    Well considering a basic hot water heater is around 40000 btus, the boiler size should take that into consideration when used in conjunction with your heating load. What are you using as fuel, gas, oil.....if gas, get a separate HW heater so you boiler won't run year round, then you can downsize the boiler a little if you have insulated or go tankless if your HW demand is not that great. Even if modulating, the heat and HW demand does not change.

    1. gusfhb | | #6

      YOu do not need to add capacity for hot water unless the hot water load is higher than the space heating load.

      90 percent of the time you will never even have a conflict.
      Your boiler is sized for peak load, 99 percent of the time you are not at peak load.

  5. CraigRo | | #5

    It was -15F. The house was warming up from 64F to 70F in the morning. My 1978 home is likely 5+ ACH (working on that...). I have a 105MBh 87% cast iron boiler and 60Gal indirect. 2000SF 2 story with attached 2 car garage (10+ ACH, very leaky).

    With all 4 zones running: upstairs, downstairs, garage UH, and one shower down, my shower up next, the boiler supply temps were still climbing, as was the indoor temperatures. And my shower was hot.

    I live in CZ7. You do not need to oversize. I doubt you even need 80,000, as unless you turn priority off (I have), the boiler focuses all heating at the water. Go even smaller. Don't waste energy. Don't listen to contactors, most use rules of thumb to size and poor logic for validation.

    Most indirect tanks have a GPM and temp table for recovery. Example is from mine and how I'm currently s running: 7GPM @160F = 120F @4.5GPM DHW, or something like that.

    500*GPM* delta of water temp = btus out. That's usually in the indirect tanks manual as well to help size your heart source. Example: 2 showers, clothes washer all running at the same time may be 7GPM of DHW demand. Find on the chart where that is and size accordingly.

  6. tom_dresser | | #7

    Thank you everyone for all the help. I performed the heat load calculation and got just under 50,000 BTU/hr, and under 70,000 BTU/hr after applying the 1.4 ASHRAE 99 factor.

    I found the Amtrol spec sheet for the 41 gallon WH-41 indirect. The sizing chart specifies a minimum of 60,000 BTU for a three bedroom house with two baths. The chart indicates it can go as high as 140,ooo BTU.

    So for the average heat load, 50,000 + 60,000 equals 110,00 BTU/hr.
    For the ASHRAE 99 load, 70,000 + 60,000 equals 130,00 BTU/hr.

    So the 80,000 BTU/hr boiler appears to be too small. The boiler also comes in 110,000 and 150,000 BTU/hr sizes so the 150 meets the ASHRAE 99 load with a little capacity to spare.

    Both burners have a minimum firing rate of 10,000 BTUs.

    I did consider just changing out the boiler and not the existing gas water heater. I am under the impression that if you remove the boiler from a chimney you must remove the hot water heater as well. If this is not true then just replacing the burner with the 80,000 BTU/hr burner and leaving the existing hot water heater in place might make some sense.

    Has anyone seen a GBA article that discusses this subject?

    Thanks again for all the help.

    1. Expert Member
      Akos | | #8

      When it comes to sizing hot water and house heat load don't need to add. A typical household hot water averages out to around 2000BTU, which barely budges your houses heat calculation.

      The water heater can consume up to 60000 BTU but those are for very short intervals during recovery. Even if you have it on priority and the house heat shuts of during that time, the tempearture of the house will barely budge because of the thermal mass of your heating system and house structure.

      Unless you are looking to heat a pool, there is absolutely no reason to over size the boiler.

      P.S. Even the 50k number sounds too high. I'm heating a 1900sqft mostly insulated century house in zone 5 with 24k.

      P.S.S. I stick by my statement in the thread Josh linked to. Indirects don't make financial after you pay for install/plumbing/pumps/fittings. A heat pump water heater is much cheaper to buy and install with similar operating cost.

      1. CraigRo | | #9

        Indirects don't make sense if you pay more than $25 for a 5y.o. tank and install labor isn't free. And if gas costs are fractional of electric. That is my case, it paid for itself as soon as the boiler fired the first time.

        Every region and climate is different, one size fits all isn't the answer.

        1. joshdurston | | #12

          Gas versus electric is a different comparison than indirect versus NG power vent/combi. I agree if the alternative is electric you want to look closer, but I suspect it would be wash over the next 8-10years.

          A NG standalone direct vent tank will probably be a 1/3 the cost of an indirect setup.

    2. joshdurston | | #11

      The ASHRAE 1.4x is an up to number, not a minimum. Keep in mind you want to size off the output (not input to the boiler).
      You don't need to add capacity for DHW since the load is large put only occurs a small amount of time. Feel free to cross post at, there are a lot of good pros there, who will tell you the same thing.

      80,000 is certainly not too small for a heating load of 50,000 + DHW. In fact it is oversized with a large safety margin.

    3. tommay | | #16

      Thomas, you should be able to leave the gas hot water tank hooked up to the chimney no problem. Natural venting is always best in my opinion. One benefit of keeping your gas HW heater is if power goes out, you still have hot water.
      Your findings are correct about the sizing for using a boiler for both heat and hot water. Many here think that just because a HW heater only has to run for 15 minutes to recover, that is only used 1/4 of the BTU input it is designed for. They don't realize that the input value is what the tank is designed to "Input" into the tank regardless of how long it has to run. A 40000 BTU burner on a gas HW heater draws a flow rate of 40000 BTU worth of gas as soon as it turns on which is why you have to have proper gas pipe sizing. You can't use a 3/8" inch supply on a burner that requires 3/4" just because it's only going to run for 10 minutes and only use up 2000 BTU. Same applies to a boiler that is supplying the BTU's through a coil that is designed to dissipate a certain heat rate at a given temperature. It's akin to saying I can install a 10 amp breaker on a circuit that has a 6 amp and 9 amp load because I am only going to be using those appliances together sparingly.

      1. Expert Member
        Akos | | #17


        There is a big difference between a gas feed to a DHWT vs an indirect. With an undersized gas feed, the burner will be starved and the hotter flame will wear out the burner in no time.

        An indirect is just a standard water tank with a heat exchanger in it. The rating of the unit is the max amount of heat that heat exchanger can transfer to the tank. The amount of heat you feed into this heat exchanger is entirely up to you, very easy to control by adjust the flow rate though the zone. There is absolutely no requirement to feed it the rated BTU.

        This is similar to a radiator. Just because it is rated at say 18000 btu, it doesn't mean that you can't flow either colder water or less water through it to get it down to heat load you actually need in the room.

        For example, my own home has rads capable of putting out 95 000 BTU at 170F. Since the house has only a 36 000 BTU heat load, I can get away with feeding them with 125F water. It doesn't mean that I have to connect up a 100k boiler to the system.

        P.S. I run my own indirect capable of 180 000 BTU at 20 000 BTU, mostly because I didn't want big lines and that is the pump I had kicking around. No issues with hot water.

        1. tommay | | #20

          Sure you can lower temperature or flow rate but the recovery time will be longer. You need a certain amount of BTu's to raise a certain volume of water from t1 to t2, that will not change regardless of how you modulate. What's so hard to understand? I can fill a tub at 2 gpm or 5gpm, it's still going to take the same amount of water but the time to fill it will change. If you can wait all day to fill it then I guess you could get away with 2 gpm. It all comes down to the user and how it is being used but the facts are the facts when it comes down to the overall energy required. Do you want high performance or can you settle for less.
          Your example of your heat load is off too. If your heat load is only 36000 then yes you would not need a 100000 btu boiler. Your baseboards don't care how big your boiler is as long as they get the temperature of water they need to work up to spec. But the amount of btu's needed to heat your house doesn't change. You lower the incoming temp. to your baseboards, or decrease the amount of btu's your system puts back into the returning water, the system runs longer.

          1. Expert Member
            Akos | | #21

            The point is that you don't need extra boiler capacity with an indirect.

            You can either limit the flow rate to reduce the BTU to the tank, have a bit slower recovery (which is not an issue if your tank is sized right), or you can put the indirect on priority and maybe on one of those polar vortex days with the morning setback recovery, your house might cool down a degree or two while the tank recovers.

            Neither option needs the boiler upsized.

  7. tom_dresser | | #13

    Some additional input:

    I expect domestic hot water demand to occur in the morning with a family of four going to work and school (in the future). Estimating four 15 minute showers at 3.5 gallons per minute. That is about 200 gallons of hot water. The boiler will also be bringing the house up ten degrees or so from the nightly setback.

    The first hour rating table indicates that it takes 120,000 BTU to get to this level of hot water output. At 60,000 BTU it delivers a little over 100 gallons.

    Am I understanding this information correctly?

    1. Expert Member
      Akos | | #14

      GPM at shower is not the same as GPM at the hotwater tank. Shower water is around 100F, so the 160F water in your tank is mixed with cold water to get the temperature down. With 50F cold water supply your 3.5GPM shower only uses 1.6GPM out of the tank.

      At 1.6GPM, an 80k boiler can just about keep up without using any of the buffer capacity of the tank. Even if you limit the indirect to 30k to satisfy the full heat of the house, you'll never run out of hot water.

      P.S. With slow reaction time of hydronic heat, your setback recovery time needs to start well before morning showers not to have cold bathrooms, the boiler won't be running at full tilt.

    2. joshdurston | | #15

      More BTU for hot water isn't necessarily bad on it's own, but depending on how your house is zoned, you want to consider the min-fire capability. You don't want to give up a low fire rate in the process.
      Also, with outdoor temperature reset (that you should be using with a modcon) you are reducing your emitter output which calls for lower min-fire otherwise the emitters can't get rid of the heat without running hotter. This reduction in output leads to better comfort since the heating cycles will be long and low instead of short blasts of heat. At 180f versus 120F you need a lot less baseboard to dissipate 50,000btu, but at 180f it is less efficient and less comfortable.

      How is your house zoned and controlled? Is it radiators, or baseboard, or infloor? Room by room control, floor by floor, or is the whole house on one loop?

      Your tank should be keep at 140f, but you're realistically not going to shower at more than 105F. Maybe one family member might go a degree or two hotter but 110F is getting really hot for a shower.

      Anecdotally, I have a 50gal 36,000 btu (input), hot water tank. It's at 140F with a thermostatic valve regulated down to about 110F, (and the shower controls are rarely at full hot). And I have a wife, and 3 daughters, and I never run out of hot water. But I think my shower heads are substantially less than than 3.5gpm (that's a lot, especially for a "greenbuilding" site).
      I also, live in Ontario where my winter time water temperatures are fairly low. If I ran out of hot water my first step (beyond reducing usage) would be to install drain water heat recovery.

      If your worried, get a bigger tank, not a bigger burner, and consider drain water heat recovery if your plumbing allows for it. Run the tank hot with a mixing valve.
      Maybe consider a DHW recirc pump so you don't dump the first 3-5 gallons down the drain while you're standing outside the shower in the bathroom waiting for it to warm up.

      1. tom_dresser | | #18

        Hi Josh,

        I have baseboard heat in three zones: First floor has two zones, one of approximately 100 feet for the original house, the other of 45 feet for a sunroom that was added on. The second floor has one 65 foot zone.

        I appreciate all the information you have been providing me. Just to make sure I understand you correctly, are you saying that a 150k boiler that modulates down to a 50K average heat load at the northern border of climate zone 5 will not perform as well as the 80K boiler which is also modulating? The contractor says that they would perform identically.

        And are you also saying that 30K (80K - 50K) BTU is enough capacity to run the hot water for 100 or so gallons per hour during the morning rush?

        If not, and I need to upsize the tank, how should I go about sizing it? One contractor that proposed an 80K boiler also proposed a 45 gallon indirect. How much of a difference would that make compared with the 41 gallon one?

        Thanks so much for your help,

  8. Jon_R | | #19

    Not that it makes a big difference, but I'd review the information here about the downside of large turndown ratios (say > 10:1). Even better would be numbers for the specific boilers you are considering.

    Also note that cycling is overly maligned - it often improves efficiency.

    1. Expert Member
      Akos | | #22

      You have to be careful with information provided by a manufacturer to justify their technology.

      The item that is missing here, is that a modcon on low fire feeding a heating loop running on outdoor reset will also be running at significantly colder RWT. Even with the excess air going through the flue you are still getting much more condensation and higher efficiency.

      When it comes to heating with modcons, the highest efficiency is to run them at low fire, without cycling with a very low RWT. No amount of marketing fluff will get around this simple fact.

      1. Jon_R | | #24

        Clearly at some point, the losses due to excess air lowering the exhaust dew point can exceed the gains due to lower RWT lowering the exhaust temp. But it would be interesting if you have data showing otherwise.

    2. tommay | | #23

      Jon, good links...

  9. joshdurston | | #25

    I would say you want to make sure you boiler is sized to cycle at a reasonable rate.
    I'll say somewhere between 5-10min as a minimum run cycle with a 15 min start to start time.
    If you have a high thermal mass system broken into only a couple large zones you could have a fairly large boiler with a low turn down ratio and have it run quite nicely. But if you're microzoned with low mass/water content radiators min fire rate become important.

    I have a non-ideal situation in my home. A 100,000 btu polaris tank feeding about 20,000 btu of radiation. Since I run a aggressive outdoor air temperature reset that 20,000 btu of radiation is probably more like 8000btu in the shoulder season. Luckily the Polaris has a 34gal tank to buffer the capacity. So I get relatively short burn cycles of about 5 minutes, but then the boiler stays off for quite a while. When I replace it I will be looking for a <=8000btu min fire modcon, or installing a buffer tank.

    So to OP Tom, it's obvious that DHW production is a top priority, and you can significantly enhance your safety margin by going with a bigger boiler. If you go up to the 110,000 btu boiler your min fire only goes up 2000btu to 10,000btu so it's probably not a huge deal. I wouldn't go to the 150,000 option even though it has the same min fire. It's tougher to get good combustion setup on such a broad firing range.

    A small buffer tank on the heating side, and a large indirect will be provide stable operation with capacity to spare for extreme DHW usage, and setback recovery.

    Here is a buffer tank calc:

  10. tom_dresser | | #26

    Thanks to everyone for all the help you provided me on this query. It was all very helpful.

    This week I installed the 80,000 BTU boiler with the 45 gal indirect. Just in time for the first cool fall weather.

    It's great having a team of experts to help out with important decisions like this.

    Thanks to everyone again,


    1. joshdurston | | #27

      Great to hear, it would be nice to get some updates as you get some runtime on the equipment.
      Don't be afraid to tweak your settings a bit to maximize efficiency. (sometimes the boiler's DHW production set point is unnecessarily high, or the outdoor reset set too conservatively).

  11. laylabloompro | | #28

    Burnham AL35SL Alliance SL Hydrastone-Lined is working good for me. I love this indirect water tank.

    you can check also :

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