GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X 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

Combi boiler short cycles

cbut8995 | Posted in General Questions on

Hi, When purchasing a combi boiler, I know for smaller apts the heating portion will typically short cycle but will the hot water portion ever short cycle? I havent seen much discussion around the DHW portion short cycling, if at all.

Would this be a good calculator for a combi boiler for sizing a buffer tank. I found this online and according to this if the combi boiler I was looking at: Mascot FT Floor Standing Combi Boiler has a min output of 19K BTU and the minimum system load for my apt (smallest is the bedroom at 6000 BTU for 10 feet of runtal baseboard at 180 degrees F) and 10 min run time. I’d assume the 20 degree delta is normal for the runtal baseboards.

I am not so sure I am using the 20 degree delta correct as the variable but its seems any outgoing water temperature coming back is 20 for most systems and if the run time should be 10 min (average as suggested) and if the minimum system load is just finding the length of baseboard multiplied by the BTUH/ft I am using in the smallest zoned room at 180 degrees F. If so, I received these numbers below. 

((10 x (19,000-6000)) / (20 x 8.33 x 60)
= 13 gallon storage tank.

The smallest storage tank I have seen was around 20 gallons so would this be okay to use with this calculation?

I have another apt that had a calculation of 7 gallon storage tank, is sizing it up to 20 gallon work with the system as well or I should be getting as close to the number as possible.

Sizing a Buffer Tank

To properly size a buffer tank, you need to know the following:

  • The minimum desired run time for your boilers – typically 10 minutes for most condensing boilers

  • The minimum output (Btu/h) of your particular boilers. (Remember – this isn’t necessarily the lowest fire rate your boilers can produce, but the start-up rate, which can be higher than the lowest fire rate.)

  • The minimum system load (Btu/h) based on the zone with the least possible demand.

  • The system ∆T (°F) at minimum system load.

With these values you can complete the following equation, which will result in the gallon size necessary to satisfy your needs for Btu storage in water.

Screen shot 2015-03-11 at 9.47.42 AM.jpg

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. Expert Member
    Akos | | #1

    Looks right. Keep in mind that if you run outdoor rest on those unit (which you should), the heat output of the radiators will go down, which would require a larger tank, so the larger 20gal tank is a good thing. With that the boiler won't short cycle even during the shoulder season

    1. cbut8995 | | #2

      Thanks Akos. You have been super informative and cant stress how much I appreciate it. What is the "outdoor rest"

      1. chrisjri | | #3

        Outdoor reset has a sensor the checks outdoor temp and raises or lowers the boiler temp accordingly.

      2. Expert Member
        Akos | | #4

        Typo. Outdoor Reset.

        It is an additional sensor hooked to the boiler that adjust the water temperature based on outside conditions. All modcons have an input for this, just needs to be installed and configured.

        This lowers the water temperature when it gets warmer outside so you get more condensation and higher efficiency.

        1. cbut8995 | | #5

          I will have to talk to my GC about this. I am not sure where we would put 7 outdoor resets for our new condo and if we can in NYC. Is this a must or like an optional thing. The tenants will be paying their own utilities (gas and electric) that power the combi unit and they will all be in the basement of the building together so I'd imagine it would be super warm inside all the time as my other buildings that have a gas hot water heater and boiler.

          1. Expert Member
            Akos | | #6

            If they are in the basement it should be very simple. This is a small sensor about the size of a large matchbox wired with 2 conductor thermostat wire.

            It is not a must but it is a good thing as it keep the heating loop temperature more stable and you don't get expansion noise in your heating pipes and radiators. With a well configured outdoor reset and TRVs, you can skip the thermostat in the apartment.

            Also a modcon supplying 180F water won't be condensing, at that point the efficiency is the same a mid efficiency boiler. With outdoor reset the temps are lower so you get more efficiency.

            I guess for a low load place, this won't matter but if the cost is minimal, it should be put in.

  2. Deleted | | #7


  3. cbut8995 | | #8

    So I will probably need 7 of these outdoor resets.

    I was reading into it and probably will get one. The outdoor reset is independent of the combi boiler right? We are most likely using LAARS with a 20 gallon buffet tank.

    I couldn't find LAARS outdoor reset but are there any good reliable ones you recommend that are on the premium side and will be okay in outdoor NYC weather with snow etc.

    1. Expert Member
      Akos | | #9

      The outdoor sensor comes with the unit, there is nothing extra to buy. It just needs to be hooked up and the reset curve configured.

      I've have seen modcons where all this was hooked up but the installer never bothered to configure anything. Not much point to a modcon if it is not condensing because the installer left it set at 170f.

      P.S. If these units are in the basement, you'll have some very long runs for the domestic hot water. It will take a long time for hot water to make it all the way up the 4th floor. It might be worth it to install a demand based recirc at least for the units on the upper floors. Water is expensive here, the water savings pay for the recirc setup in a couple of years.

      1. cbut8995 | | #10

        Is the outdoor sensor wireless. I saw the contents included in the combi boiler but it doesnt look like theres a wire to connect to the boiler. Is there like an optimal place for the sensor. I was thinking of placing it in the front of the building towards the floor so it can be accessed without going through the building and through the front brick facade.

        The demand based recirculator is something separate for each combi boiler I assume? We have a water pump to increase water pressue already but am not sure if this is the same or we also need the recirculator. Is it small enough to fit underneath the combi unit as we dont have much space now given we are adding an expansion tank infront but below the wall hung combi boiler.

        1. Expert Member
          Akos | | #11

          The sensor is wired, last thing you want is a wireless connection on a heating device.

          Best spot is the north side of the building. You want to put it somewhere where it won't get any sun. A well shaded spot also works.

          The recirc would be seperate for each combi. You want something like this:

          With a combi you only want to run the recirc when you need to as, so some time of demand learning system or a timer is a must, running it continually will cause it to short cycle.

          These are very small, about the size of a pop can. It can also be installed in the apartment under the sink.

          This is different from the pressure booster pump, that is for the main water feed.

  4. cbut8995 | | #12

    When you mean to install it in each apt sink. Does this mean I would need it for every application for water?

    For each apt on the 3rd floor and above, we need it in the kitchen sink, bathtub/shower, bathroom sink. If so I still dont quite understand the use or what the comfort system does.

    Are the circulators similar to what you are talking about (red things in the link below) or that is separate:

    1. Expert Member
      Akos | | #13

      You generally have a 3/4" trunk feeding each apartment. In the apartmet there will be a manifold which feeds all fixtures.

      The recerc pump need to connect to this manifold somehow. You can connect it under the sink in the bathrooms, right at the manifold or in the basement. There should be a recirc line (1/2 or 3/8 pex) running back down to the cold water feed of the combi.

      There are some systems that use the cold water feed as the recirc return avoiding this extra pipe. This keeps the cold water slightly luke warm, not a fan of this as now you are running the cold water tap for a couple of minutes to get a cold glass of water.

      Each apart needs only one pump, you only need to keep the water in that 3/4" trunk up to the manifold hot. The local lines don't have much water volume and can be flushed out quickly just by opening the tap.

      One way to reduce the number of pumps is to share one between apartments close by. You need an inline check valve and a balancing valve on each return to avoid having one apartment feed hot water to the other one. In this case, it is probably best to have a motion sensor in each bathroom in both apartments that runs the recirc pump for a pre-set period. You would have to adjust the balancing valve to make sure both loops get get purged when the recirc pump runs.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |