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Combi boiler idea that needs feedback...

CrispyCrispy Posts: 11Member
I'm building out a house that has a couple personalities. It's small at 1200 sq. ft. but has three bedrooms and two full baths. While the house will be my wife and my retirement home, it's in a resorty area with good summer rental opportunities. I have an IBC DC 20-125. I am concerned that the tankless hot water at 3.2 gpm might not keep up with three couples getting ready to go out and maybe some action in the kitchen. Of course the obvious solution is an indirect hot water maker. What I'm thinking though is piping the tankless to all the domestic hot water EXCEPT the two showers and run the showers from the indirect tank. It's my house and I'm doing all the work so I'm not concerned about crunching all the monetary numbers around buying and installing the indirect tank and paying extra for the combi (I already have it). My rational that needs checked is looking at all the hot water that gets used daily that's not showers. You have kitchen use with dinner prep, cleaning and some manual dish washing. You have clothes washing, shaving, bathroom use other than showers like shaving ect. Is the tankless more efficient for these loads than the indirect? In between showers, is the indirect an efficient storage unit? If you all think the tankless is good for these loads one benefit of this scenario is keeping the shower load off the tankless to reduce maintenance due to the rather hard city water (10 grains) we have. I own the combi and would like to use its HW feature rather than simply not piping it for use. Comments please.... Thanks in advance for any help.


  • EBEBRATT-EdEBEBRATT-Ed Posts: 6,500Member
    my vote is a decent sized indirect. to me tankless heaters can be problematic. Indirects are insulated pretty good. I have an 80 gallon electric (no gas or oil here) I can shut the circuit breaker off, go on vacation for a week and come back and the water is still pretty warm. a 50 gallon indirect would be fine
  • GroundUpGroundUp Posts: 768Member
    There's only 2 showers right? 3.2 GPM is at a 70 degree rise, do you know what the incoming cold water temperature is? Let's say, for fun, in summer it may be 60 degrees. Shower temps seldom exceed 110 but let's call it a 60 degree rise needed to meet in the middle- which would extend your availability to roughly 3.7 GPM. Perhaps installing low flow showerheads for rental season would alleviate the potential shower issue and after the first fist fight, the tenants won't be playing in the kitchen anymore while there are 2 showers running. Personally I'm not a combi fan, but if I were in your shoes I'd give it a try and leave tees for an indirect zone if it becomes necessary down the road.
  • CrispyCrispy Posts: 11Member
    Thanks GroundUp, that is the plan I always had in the back of my mind. I like the trick of giving the renters the lo-flow. The only hitch is going through that trial period with my wife. Given some of my back history with her and boilers, I don't have much leeway.
  • CrispyCrispy Posts: 11Member
    Ebebrat, I'm a bit worried about the problematic. I've had these old school, rock solid Utica boilers that run for 20 years without much. I'm jumping in the deep end. Thanks.
  • CrispyCrispy Posts: 11Member
    Can someone speak to basic efficiency of each? I get that the indirect tank has to be kept hot 24/7 but they're pretty good at holding hot water right? What about when each one is cranking? Is the tankless better at heating water? Once down on hot water, my present indirect in a different house has trouble keeping up.
  • Jamie HallJamie Hall Posts: 12,104Member
    Crispy said:

    Can someone speak to basic efficiency of each? I get that the indirect tank has to be kept hot 24/7 but they're pretty good at holding hot water right? What about when each one is cranking? Is the tankless better at heating water? Once down on hot water, my present indirect in a different house has trouble keeping up.

    Do not confuse efficiency with recovery or steady flow rate! Very different beasts. In principle -- but only in principle -- if a tankless is sized for the maximum flow, its efficiency at that flow would be very good indeed. At partial flow rates... not so good, and if it doesn't modulate -- or doesn't modulate down far enough -- you'll have possible problems with varying hot water temperatures (which can be cured, up to a point, by using a thermostatic mixing or tempering valve -- more maintenance). On the other hand, an indirect or a storage type water heater will be able to deliver much more hot water initially, until it starts to run out. After that, it will only deliver as much hot water as the burner or electric elements can provide. Averaged over time, the efficiency (measured as the amount of fuel or electricity used in relation to the amount of hot water) will be slightly less, depending on how good the insulation on the tank is.

    On the whole, if your maximum hot water demand is pretty constant then a tankless will have slightly higher efficiency. On the other hand, if it varies all over the place, an indirect or direct fired storage tank may give better end results for the users.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • bob eckbob eck Posts: 904Member
    If you have 2 showers take a look at installing 1.5 GPM shower heads.
    What is incoming water temp in the winter?
    Let’s say 40F going to 120F going out to the showers. That is a 80F temp rise. Check with the Combi boiler manufacturer see how much domestic hot water this combi boiler will be deliver at the 80F temp rise.
    With two 1.5 GPM shower heads running at the same time I believe you would need about 1.3 GPM of 120F hot water going to each shower head. That is about 2.6 GPM
    Can the combi boiler you are looking at deliver 2.6 GPM?
    Using a good indirect water heater storing the water temp at 140F or a little higher and then using a good mixing valve to send out 120F water temp to your faucets is a great option.
    There are some indirect water heater that also have electric elements in them so if the boiler goes down the tank could still make hot water with the elements until you get the boiler fixed. There are many options. Find yourself a good professional heating contractor and they will help you get what you need and want.
  • hot_rodhot_rod Posts: 12,537Member
    Need to know more about your incoming water temperature, summer and winter, and expected temperature rise and flow rate.

    I have 16 years on a 130 Laars/ Baxi combi without a single problem. During holidays it easily keeps up with two showers and dishwasher all running. Incoming around 50- 55° from well.

    I guess it could come down to the amount of DHW you supply and guests or tenant lives with that.

    Or try and plan for an unknown DHW load.

    I too would try the combi as is, add indirect if needed. Keeping in mind the boiler needs to supply heat while also covering a DHW load either via the combis function or indirect and combi calling.

    It's not uncommon in a small rental property or vacation home to have a DHW load larger than the heating load.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • mattmia2mattmia2 Posts: 816Member
    The indirect gives you more options to adjust things to make more dhw. hotels with low flow shower heads are a pet peeve of mine. i have considered going out to my car to go get a wrench to remove the flow restrictor so i could rinse the soap off and shampoo out in a timely manner more than once.
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