Understanding of a Combi Boiler

Discussion in 'Boiler Forum' started by Kengie, Mar 1, 2021.

  1. Kengie

    Kengie New Member

    Joined:
    Dec 12, 2020
    Location:
    New England
    Hi there,

    I'm looking into converting my iron-cast boiler to a combi boiler but I would like get an understand of how the combi boiler works.

    If starting from 13C to 21C will take a long time to heat up a room. Would the combi boiler be constantly on until the desired temperature is reach and the thermostat stop calling for heat?

    What if on a very cold night and the desired temperature is never reach? Will the combi boiler be on all night?

    My main concern here is that the gas bill due to be combi boiler is constantly on, thus a very high gas bill.

    Thanks in advance.

    Kengie
     
  2. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    Unlike your cast iron beast, the burner on a combi boiler will modulate the firing rate up and down. A typical combi boiler big enough to serve a 2 bathroom house in new England has a high firing rate of nearly 200,000 BTU/hr (which is a LOT) but will throttle down to 20,000 BTU/hr at it's minimum firing rate. Being constantly on isn't a problem if it takes ten hours of firing at the minimum firing rate to use as much gas as it would use in just ONE hour firing at it's maximum rate, assuming you the burner is correctly sized for the space heating load. (Most combis are crazy-oversized for the load at max-fire.)

    The most important thing to get right when contemplating a combi boiler is having enough radiation (on each zone, if zoned) to not cause the boiler to short-cycle the burner at water temperatures low enough to get condensing efficiency out of it. The napkin math version of how that analysis is done can be found here.

    With condensing equipment it's usually less efficient to use deep overnight setbacks as low as 13C, since that requires a higher temperature water and a higher firing rate during the recovery ramp back to 21C. Both the higher temperature and higher firing rate cuts into the raw combustion efficiency, erasing the fuel savings gained by letting the house cool off:

    [​IMG]

    While you're at it, use some wintertime fuel bills and a bit of napkin math to figure out what your 99% design heat load is. If the design heat load is less than 2x the minimum firing rate it will usually be less efficient and less comfortable than going with a right-sized modulating condensing boiler with a much smaller minimum rate.

    Condensing boiler nirvana is a boiler that burns almost continuously through the heating season, at the lowest possible temperatures & firing rates that still keeps up with the heating needs as the weather changes, maximizing both comfort and efficiency. This is done using and fine-tuning the "outdoor reset" function and outdoor temperature sensor that comes with nearly all condensing equipment.
     
    Last edited: Mar 2, 2021
  3. Sponsor

    Sponsor Paid Advertisement

     
  4. fitter30

    fitter30 Well-Known Member

    Joined:
    Feb 2, 2020
    Occupation:
    Retired service tech
    Location:
    Peace valley missouri
    Combi boilers are only 95% efficient with return water temperature 130*f - 54.44*c or below. Above that temp 85% efficient. They pick up their efficiency by condensing.
     
  5. Kengie

    Kengie New Member

    Joined:
    Dec 12, 2020
    Location:
    New England
    Thank you Dana for the great explanation and fitter30 for the information.

    So to take into example last night in New England, the over night outside temperatures were 13f (-10c) and the zone thermostat was set to 75f (23c). The zone temperature never made it to 75f and only reach 70f, thus call for heat was all night (let's say 8 hours).

    With an iron cast boiler, I understand that the boiler heats an iron cast inside the boiler and the water reserve get heated from the iron cast metal. So the boiler only turn on to heat the iron cast when the water reaches a certain temperature (180f) and then turns off.

    From my understanding combi boiler, there will not be an iron cast object and the water is heated through a heat exchanger which will heat water was it flows through. In the example of the temperature last night, so if the thermostat calls for heat for all 8 hours as mentioned above because the desired temperature was never reached. would that mean that in a combi boiler, the call for heat would have the combi boiler on for all 8 hours until the desired temperature is reached, which would in turn means high gas bill.

    Sorry in advanced for all the newbie questions.

    Thanks.
     
    Last edited: Mar 2, 2021
  6. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    The call for heat was all night, but....

    ... what was the boiler's output temperature?

    Was the burner cycling on/off, or burning continuously?

    The heat exchangers in a cast iron boiler are made of cast iron, which has more thermal mass than the heat exchangers in a combi boiler, but in that regard the principles are the same. But a cast iron boiler has a fixed burn rate, whereas a combi boiler's burn rate changes, and can be adjusted to where it's only firing at the minimum rate needed to keep the boiler at the programmed output (or with some combi's, return water) temperature.

    The rate at which heat is emitted from the radiation changes with water temperature. with an entering water of 130F the radiation only delivers about half as much heat into the rooms that it would with an entering water temp of 180F. So if the water temp is set to something too low for the radiation's heat rate to keep up with the heat loss from the house the boiler will run all the time (or cycle) never satisfying the thermostat, but it will also be firing at a low rate. Bumping up the programmed temperature to find the minimum amount necessary to replace the home's heat loss using your existing radiation takes a bit of experimenting, but it is unlikely that even the smallest combi boilers out there can't actually fire high enough to cover the real load.

    Using the "outdoor reset" function of the combi boiler allows it to automatically adjust the boiler temp to compensate for the heat loss under varying weather conditions. That takes even more tweaking and adjusting, but once adjusted to near perfection the firing rate will always low, with the highest possible combustion efficiency.
     
  7. Kengie

    Kengie New Member

    Joined:
    Dec 12, 2020
    Location:
    New England
    The burner was cycling on and off, not burning continuously. The boiler's output temperature is set to 180F at which the HydroStat would cut it off and the burner on temperature is at 150F.

    So because of the larger thermal mass, my understanding of an iron boiler is that the burner doesn't need to be on continuously as the cast iron already heated and has a larger thermal mass. In turn in a combi boiler, the thermal mass of the heat exchanger is much smaller and holds less water capacity (according to the Rheem combi boiler the water holding capacity is 2 gallons). Does that mean burner of a combi boiler will need to be on continuously to supply heating to a zone that never reaches the set thermostat temperature?

    So another example with yesterday outside temperature was 23F and the zone thermostat was set to 75F. When I woke to look at the thermostat current temperature it was 72F. Looking at the graph of the thermostat to call for heat was basically from 12AM to 8AM. In this situation, would the combi boiler burner be on continuously to supply heat (even at a lower output) or will it be operating same like a cast iron boiler that burner will be cycling on and off?

    I guess it was my experience with a tankless water heater for DHW at my sister house, was that as long as I am showering with the hot water on. The burner will be be on for the duration

    Reason for all these question is that I don't want to do an conversion only to find out in my situation the combi boiler will rack up a high gas bill and then I'm stuck with a combi boiler after the conversion.

    Thank you Dana for your time and explaination. They are super help and very insightful too.
     
  8. automaton

    automaton I just like reading manuals

    Joined:
    Mar 7, 2021
    Location:
    New England
    Hi, long time lurker here - Dana is the absolute expert on this and touched upon this but there are a couple of things I wanted to add, feel free to correct me if I'm wrong.

    1. It may be more helpful to think about this in terms of heat dissipation vs. heat capacity. I think it's somewhat moot to be talking about the boiler constantly firing or not, whether or not it's cast iron or tankless. If your zone does not reach the set temperature, that sounds like it is losing more heat than is being delivered, i.e. the BTU heat load on that zone is actually higher than the zone can deliver via the emitter.
    What happens at the boiler is going to depend on the temperature of the water returned. If it's cold enough, the tankless boiler will continue firing. If it's too warm, the boiler will modulate down to use less fuel but at a certain minimum it will turn off... then eventually cycle on again, which is not something you want. Your cast iron fires at 100% then shuts off. If you get the supply/return temps right, you should see reasonably long burn times; although like Dana said, running all the time at a very low rate would theoretically be most efficient since you're not dealing with things like combustion purge, coil cooldown, etc.
    Your cast iron fires at 100% all the time hence the cycling. But it might be burning 120,000BTUs worth of gas while the tankless will modulate down to 20,000BTU and fire longer. It's not just about how long it runs, you have to calculate how much fuel it's using overall during that call for heat. That's what's going to determine how much fuel you end up using.

    2. I wish I had radiant floors but I have forced air hydronic, so running 24/7 is not efficient for me. My air handler blower specs at 800W. I'm at around ~6hr/day average runtime, which is ~$30/month at current electric rates just for the blower. But that's not a lot in the context of a gas heating bill.

    3. Hot water demand is actually much more taxing on the heater than closed-loop heating. My tankless sits there and purrs during a call for heat but when my wife is taking a shower or even worse, is filling the tub, you can tell it's firing on many more cylinders. You're dealing with a much larger temperature rise at a possibly higher flow rate (55F to 120F). delta temp * flow in GPM * 500 = BTUs needed
     
  9. fitter30

    fitter30 Well-Known Member

    Joined:
    Feb 2, 2020
    Occupation:
    Retired service tech
    Location:
    Peace valley missouri
    3 gpm @65* rise = 1624 btu's min 97,461 hr, cast boiler or combi 85% efficient 112080 btu,s big shower double that flow
     
  10. Kengie

    Kengie New Member

    Joined:
    Dec 12, 2020
    Location:
    New England
    I totally agree with you. I read all the links he gave to me and his blog is an excellent source of information.

    If I'm understanding this correctly, a combi boiler will still cycling on and off depends on the water return temperature and the need for heat. And even if it is running all the time when there is a need for heat. It would modulate and fire at the right output for the needed amount of BTU.


    This actually makes a lot of sense since in CH, the close loop would maintain some temperature thus wouldn't fully fire up all the way high and DHW actually heats the water from the water main temperature which is a lot less than the CH return temperature.

    Thank you Dana, fitter30 and automaton for all your input, information and your time to read my post. You guys are all wonderful and great people.
     
  11. automaton

    automaton I just like reading manuals

    Joined:
    Mar 7, 2021
    Location:
    New England
     
Similar Threads: Understanding Combi
Forum Title Date
Boiler Forum Understanding hydronic/boiler setup Feb 23, 2018
Boiler Forum Need help understanding and adjusting temp on boiler system pics included Feb 14, 2017
Boiler Forum Trouble with one zone and general understanding Oct 14, 2009
Boiler Forum Combining condensate drains? Apr 6, 2021
Boiler Forum Mod Con. or Combi? Feb 17, 2021

Share This Page