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terminology help - combi vs tankless

ron Member Posts: 203
edited May 11 in THE MAIN WALL
What is meant by combi boiler? And is such a thing available in oil, or is it just gas?

What's the difference between a combi boiler and a tankless?

I have a WGO boiler, from their product catalog I also see WTGO... as in tankless? which is just a second heat exchanger within the same boiler to heat domestic hot water (which is also known as a chimney heater) correct?

Asking around, about replacing an existing oil boiler, responses have been u should get a combi... but give me zero details about it it's just better more efficient.


  • mattmia2
    mattmia2 Member Posts: 5,279
    edited May 11
    Normally a combi is a boiler that produces domestic heating and domestic hot water.

    A Tankless is short of tanlkless hot water heater. It produces domestic hot water only.(sometimes it is used for domestic heating but it isn't designed for that and the controls don't work well for that application as well as it isn't designed for the high return water temps that domestic heating systems normally have.)

    A domestic heating only boiler with an indirect water heater is usually a better option than a combi but it depends on the particular application and your preferences to some extent. With a combi the boiler frequently ends up much larger than it needs to be for the heating load to cover the domestic hot water load. There also is a bit of cold water before the burner fires and starts heating hot water in a combi. A tankless hot water heater can be sized to store hot water for a big draw so the boiler doesn't necessarily have to be sized to cover the instantaneous hot water load. A combi could potentially be a bit more efficient because of lower standby losses and sometimes is less expensive than a separate indirect and the controls for it.
  • GGross
    GGross Member Posts: 67
    Generally the terms "combi" and "tankless" are referring to a method of DHW production that does not involve a storage tank of hot water. In my world when someone says "tankless" they are referring to stand alone wall mounted tankless on-demand water heaters. "Combi" generally (again in my area) refers to a high efficient boiler that also produces hot water on-demand without a storage tank. In your world with oil fired and cast iron equipment "combi" is probably the same thing, but "tankless" may also refer to an accessory for your boiler that will allow on-demand DHW production through an added coil that is installed in your boiler.

    To answer your question, yes I do believe that oil fired boiler manufacturers offer a tankless coil as an accessory, I do not sell oil fired equipment so I can't say specifically which models offer that option (none in my region, nothing against oil)
  • mattmia2
    mattmia2 Member Posts: 5,279
    I think viessman makes some mod con oil fired boilers, perhaps one is a combi.
  • Jamie Hall
    Jamie Hall Member Posts: 18,699
    Part of the problem here is history. A long time ago, many boilers had what was called a tankless water heater coil installed. It was a substantial improvement on the hot water coil in the kitchen stove, but that isn't saying much. One can still get tankless coils in some models of stand alone boilers, but the efficiency is poor. Now a modern tankless is a stand alone unit which has enough power to heat the incoming water to hot as it is running through, but they don't run at all unless hot water is being used, so there are no standby losses (the bad part of the old in-boiler tankless coil: the boiler had to be kept hot). They work well, but one has to be rather careful not to exceed the maximum flow ratings if you don't like cold showers. A combi is a unit which incorporates a tankless -- in the modern sense -- water heater with a heating boiler in the same unit. Again, in some applications they work well. Very often the domestic hot water demand is much greater (while the demand is there) than the heating load, resulting in a size mismatch. If the sizing works, they're fine.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 16,118
    The biggest difference between a combi and indirect is the boiler recovering an indirect can take 1/2- an hour or more depending on incoming water temperature, tank size, etc.

    Whereas the combi or tankless is only firing while DHW is flowing. So a combi rarely leaves you without heat for long periods. Unless you take 1/2 hour or longer showers.

    Bottom line, always, is how your DHW lifestyle matches the expected performance of the DHW device.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • In_New_England
    In_New_England Member Posts: 49
    edited May 13
    Sizing a combi is a balancing act:
    1. The high fire of the boiler limits the maximum hot water output.
    2. The low fire of the boiler determines if the boiler is oversized for your heating
    If the boiler doesn't modulate, you will very likely be over-sized for your heat. If you can find a combi that modulates in the 1:15 ratio you might be ok, but you have to check your heat loss against the boiler specs.

    My 1400 sq ft house has a heating load of 30 MBH on design day and 20 MBH on a typical winter day. I have one bathroom, a dishwasher and a washing machine.

    The initial proposal was for a NKC199 with a 4.8 GPM for hot water and a low fire of 18.9 MBH.

    I debated and decided on an NKC150 which has 3.6 GPM and a low fire of 13.9 MBH.

    I decided that 3.6 GPM was enough for one bathroom and the low fire would give me enough wiggle room for the shoulder months.