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Direct vent vs. indirect domestic hot water heat?

Marinko Member Posts: 10
I am replacing an old boiler. I have to decide how best to heat my domestic hot water. Is it better to separate the two functions, or is it better to combine them? Does heating domestic hot water with a boiler needlessly complicate my DHW system? Does linking the two functions reduce the life span of the condensing boiler?


  • Paul S_3
    Paul S_3 Member Posts: 1,261
    I like heating dhw with an indirect if i have room in yhe mechanical room ....mainly because of the great warranties compared to tankless and direct fired direct vent which has somewhere around a 12 yr warranty. The indirects i use have a limited lifetime warranty and ive had very little problems with them over the yeats.... and no linking the 2 does not reduce the lifespan....you need to take a look at the installer more than anything and make sure they are competent in your boiler replacement, an incompetent installer can reduce the lifespan of your equipment
    ASM Mechanical Company
    Located in Staten Island NY
    Servicing all 5 boroughs of NYC.
  • njtommy
    njtommy Member Posts: 1,105
    I've had both tankless and indirect for hot water. I like tankless, but find they have a bit of fluctuation in water temps during showers.
  • Marinko
    Marinko Member Posts: 10

    It is far more efficient to combine them. Heating with another fuel consuming device, typically not very efficient, is not desirable.

    It most definitely does not reduce the lifespan of the condensing boiler.

    I can understand how it is more efficient to combine the two functions. What I find difficult to understand is the notion that this does not affect the overall longevity of the boiler. If a boiler is used to heat a house only, it would fire less often than if it is also used to heat domestic water. It seems to me that this would be the case during the heating season, and especially during the non-heating season. Why is that the extra work load associated with heating DHW does not reduce the lifespan of a boiler?
  • Marinko
    Marinko Member Posts: 10
    edited March 2016
    It seems that indirect DHW heating is more efficient. I do have a couple of other issues that make me a bit uneasy about combining the two functions. If the boiler malfunctions, then I have no hot water. I assume that if the power goes out, I lose both heat and DHW. If on the other hand I install a direct venting high efficiency water heater, there are models that work even when the power goes out. I have received a couple of replies that seem to say that the lifespan of a boiler is not affected by the additional work load associated with heating DHW. How can this be? A boiler that heats DHW will need to run year round, rather than only during the heating season. During the heating season the boiler would have to fire more often to heat DHW. Why would more intensive use of the boiler not lead to a shortened lifespan? I am a homeowner, what am I missing here?
  • Marinko
    Marinko Member Posts: 10
    Hatterasguy, your response is helpful. If people are saying that using the boiler to heat DHM has no impact on the service life of a boiler, then the implication is that "duty cycle" has no impact on the durability. This may in fact be true. I simply don't know, that is why I am asking. Does anyone know?
  • modconwannabe
    modconwannabe Member Posts: 49
    Homeowner, not heating pro, but here's my 2 cents from experience. Depending on the indirect, they are so well insulated and designed that they heat extremely quickly, and retain that heat for a very long time. So in our 4 person home with a 60 gallon tank (which is oversized--we plan to have a renter) I don't think I'm getting calls more than a few times a day--basically after running dishwasher or shower, but that's it. In non-heating months we cook most days of the week and even then the gas usage between hot water and cooking is 20 therms or less, so there's neglible usage.

    We'll see in a few years when something breaks but any extrea wear and tear seems extremely modest to the point of being meaningless. Again, this is a 2 family home, so your situation may be different.
  • Brewbeer
    Brewbeer Member Posts: 616
    edited March 2016
    Here's a data point: in the last two months, our boiler (Lochinvar WHN55 mod con) has 1,000 hours in space heating mode, and 60 hours in water heating mode (family of 5, 40 gallon indirect).
    Hydronics inspired homeowner with self-designed high efficiency low temperature baseboard system and professionally installed mod-con boiler with indirect DHW. My system design thread: http://forum.heatinghelp.com/discussion/154385
    System Photo: https://us.v-cdn.net/5021738/uploads/FileUpload/79/451e1f19a1e5b345e0951fbe1ff6ca.jpg
  • arborus14
    arborus14 Member Posts: 8
    Have you seen the Turbomax tanks. They store boiler water instead of DHW. http://www.thermo2000.com/content/en-US/s2_produits/optimizer.aspx
  • John Mills_5
    John Mills_5 Member Posts: 952
    What direct vent high efficiency WH doesn't need electricity?
  • kcopp
    kcopp Member Posts: 4,443
    One of the reasons I am not a fan of two units doing separate jobs is that I don't think is great for a boiler to sit around for 5+ months doing absolutely nothing... That is not great for boiler life either.
    Its a kin to you sitting around idle for weeks and then waking up and immediately sprint a quarter mile w/o being in shape or warmed up.
    Let the boiler operate 1-2 times a day regardless what the temp is outside.
  • Gordy
    Gordy Member Posts: 9,546
    I agree why have two btu producing appliances when the job can be done easily with one.

    However if cost is a factor then a direct vent water heater does the job at much lower efficiencies.

    In the end I think an indirect makes the most economical sense. An indirect cost is comparable to a tankless cost, and will produce endless hot water if sized correctly, and storage temps are elevated.

  • NY_Rob
    NY_Rob Member Posts: 1,370
    edited April 2016
    OP- Have you done a heat loss analysis?
    How are you sizing the new boiler?
    Is there a combi unit that will fit your smallest zone requirements without short cycling?

    I was looking into a combi mod-con for my 1960's boiler replacement, but none of them modulated down low enough for my smallest space heating loop of 43' at under 110f return water temps. I have to go with a 10:1 turndown 80k BTU mod-con boiler (HTP UTF80) and an indirect tank.

    A heat loss analysis and zone specifications are critical before choosing a new boiler. Once you complete the analysis- you may not have a choice of combi or indirect due to firing rates.