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High vs Low efficiency gas boiler?

tromboneman Member Posts: 4
I am about to convert my 2,200 s.f. house with HW baseboard heat from heating oil to natural gas. Most contractors recommend some variety of high efficiency condensing gas boiler. One contractor, however, said that high efficiency condensing boilers lose much of that high efficiency because the house has an older baseboard heating system designed for 180 degree water. He recommends a standard efficiency gas boiler which is significantly less expensive.

Is there merit to this thought?


  • SWEI
    SWEI Member Posts: 7,356
    As usual, it depends

    How many gallons of oil have you been burning in a typical year?
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    If efficiency and the environment are part of your religion,

    then clearly you would have to go with a modulating condensing boiler. But if you do that, you may wish to modify the existing baseboard. It may be that the system was not designed, or not designed for 180F baseboard. Though it probably was.

    I am just a homeowner. I think there is merit to your contractor's thought, but it depends on your short-term and long-term budget. The mod-con will cost you more in the short term, Especially if you increase the size of your baseboards. But it will lower your long-term heating costs. And only you can decide this. So figure it out both ways and decide. For example, if you expect to sell the house in 5 years, you may want the system with the lowest up-front cost, but if you are going to live there until you die, you probably want the one with the lowest long-term cost. Of course you actually want the one with the lowest total cost of ownership, but I do not know how to calculate that until it actually happens.

    So first, do (or have done) a room-by-room heat loss for the building. Then pick a maximum return temperature water you wish to supply to your boiler (130F or less, preferably less). Then see how much baseboard it will take to deliver that much, In my case, I had 3 feet of baseboard in each of two rooms, and I increased it to 14 feet in each. Do that for all the rooms. Then answer the question: am I willing to do this? For me, increasing the baseboard size was just a small part of my boiler replacement exercise (like 10% of the total job cost). But I had room for the extra baseboard.

    Consider the total heat loss to size the boiler. Do not oversize it. Put outdoor reset on it if it does not come with that.

    See what all that will cost. If it is way too much, change the efficiency part of your religion.

    Notice that condensing is just part of the deal. I think the modulating and outdoor reset do most of the efficiency improvement, and the condensing just the icing on the cake.
  • HomeOwner1
    HomeOwner1 Member Posts: 134

    I am a homeowner as well that went through the same decision process.

    We had a way oversized oil boiler that did hot water as well. So we needed both hot water and a boiler.

    The guys on this site helped out a lot in figuring out what we needed.

    We did the heat loss and then evaluated our options.

    We had a few choices:

    1) Lowest price option was an 110,000 KBTU 80% gas boiler with hot water coils built in for hot water supply as well.

    2) A 92% 240,000 KBTU high efficiency combi mod con boiler which does both hot water and heat all in one. This was the same price as option 1.

    3) The next bracket up in price was an 80% gas boiler with an external 50 gallon hot water heater.

    4) The next bracket up was a High Efficiency 90 to 98% mod con boiler set of options with an external 50 gallon hot water heater.

    5) The next cost bracket up was a High Efficiency 90 to 98% mod con boiler set of options with an indirect hot water heater. This option was now about 3 to 4 times the cost of option one and two.

    6) The next cost bracket up was a High Efficiency 90 to 98% mod con boiler set of options with an external on-demand tankless hot water heater. This option was at least 4 times the cost of options one and two.

    We chose option 2 based on budget and bang for the buck. The factors to consider in option two would be how many bathrooms you need to service. The combis are sized based upon hot water, not the heating load, hence the modulation is very important to get efficiency on those. Also, some brands don't yet have the same proven track records as others.

    We had a greatly oversized boiler that ran oil previously. So, simply doing a gas conversion would have saved us 50% off alone. With the mod con in the equation now, we are seeing about an 85% reduction in utility bills. I would imagine we are an extreme case give our so poorly oversized previous boiler though and the fact it ran 24x7x365 to provide hot water as well. So far, we are very happy with our decision. It heats our 3700 sq foot home with 3 bathrooms perfectly. Time will tell on reliability for this new technology, but for the large cost savings, we took the plunge.

    I warn you though, this forum is very anti-combi.
  • Zman
    Zman Member Posts: 6,992

    As usual the answer is "it depends". Although your baseboard heaters were designed for 180 degree water, they may not need anywhere near that much to heat your home. This is particularly true on warmer days. A heat loss calculation of the house should be done on a room by room basis. This calc should be compared to the amount of radiation you have installed. If this study indicates that the system water temp can be reduced to less than 140 for the majority of you heating system, a condensing boiler would be a could call. The temp would be reduced automatically by the ODR controls in the boiler.

    Here is a great presentation‎

    I do not think this forum is against combi systems.

    Their good brands and not so good. Combi's work better in some applications than others.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • SWEI
    SWEI Member Posts: 7,356
    This forum is very anti-combi?

    As a professional, I take umbrage with that assertion.  I see nothing inherently wrong with combi boilers. though I do see a disturbing number of mis-applications and/or bad installations of combi boilers.  Most have been subjected to one or more forms of  knuckleheading, but there are some real challenges that prevent their widespread adoption.  The most important of these is the disparity between space heating demand and domestic hot water demand in a modern American house.  Build or remodel to current energy codes (the legal minimum) and in most locations you will end up with a dwelling which requires several times its worst case space heating demand in order to satisfy its daily hot water needs.

    A high efficiency heat source with  with high turndown (say 8:1 or better)  would be ideally suited to the task.  Unfortunately, most currently available boilers and tankless water heaters with >5:1 turndown seem to suffer from low efficiencies when operating at high turndowns.  A properly applied heating boiler spends the overwhelming majority of its operating hours at moderate to high turndowns.  A typical condensing tankless water heater (regardless of whether it is tagged as a boiler or not) hits maximum efficiency at or near full fire.  I'm thinking the ideal boiler would have very high efficiencies at lower firing rates, yet perhaps even veer into noncondensing territory at full fire.  I see no problem firing at ~83% efficiency a few hours per year if I get 95+% the majority of the operating hours.
  • HomeOwner1
    HomeOwner1 Member Posts: 134
    There are 10 to 1 units out there

    I am not going to mention the brand, else you will get a lot of hate comments on this board.

    But there is a combi offering a 10 to 1 turn down ration that I wish I had more brand choices on when we selected our unit.

    It was very frustrating dealing with local contractors who told us the lower end unit combis would work when clearly the hot water demands were insufficient in almost all the combis out there to supply a typical three bathroom suburban home these days in a colder climate.

    We really took our time and did research to make sure it fit our needs and asked lots of questions. Then thoroughly read the installation manual to ensure ours was installed to the manufacturers guidelines, which paid off as well with our contractor.

    You seem to be more open minded then others then. You absolutely correct on installing the right way. The primary and secondary loop is a very clever trick to get more BTUs delivered in recent years. Most installers we had out did not know what that was when we inquired. We also found that installers originally from Europe seemed to know their stuff much better, which was interesting.

    I would hope that the other brands out there follow that same path to make similar units. You are right again, I am certainly willing to take a hit on inefficiency for just occasional hot water and get the savings on heat with the condensing and modulation technologies these days. Our energy bills are very low now which is very satisfying.

    It came down to the choice of a 80% cast iron boiler with hot water coils or a 92% average rating 240kbtu combi boiler for the same exact budget.

    I don't understand why contractors don't offer them more often. It is a very attractive option. Not one contractor ever even mentioned one, we had to ask.

    Quick question, a family member is also facing a new heater and hot water system decision soon. They saw our unit and were impressed but they have forced hot air.

    Have you ever seen an application of using a boiler to feed a forced hot air system? If so, hot did it work out? Certainly sounds like an interesting concept otherwise.
  • Zman
    Zman Member Posts: 6,992


    I am truly happy that you love your boiler and have had no issues with it.

    I am not sure why you take it so personally when the pros on this site discourage people from buying one due to it's overall poor track record. Do you think they are making it up? It is very expensive for a contractor when they install a lemon. Many of the navien units were lemons. This is not the contractors fault, however they lost money trying to warrantee a faulty product.

    As for the 10 to 1 turndown ratio. Going from 5 to 1 to 10 to 1 is an easy thing to claim, it is a difficult thing to do. Anyone who has either researched the subject or used a combustion analyzer to verify these claims will tell you that navien is stretching the truth at best. The excess air goes to pot and the efficiency suffers. This is a fact. Standout companies like Triangle Tube will tell you that this is why they do not offer 10 to 1.

    It is great that you have had success with your boiler. Your opinion is based on your experience with your boiler and the marketing hype from Navien. And yes it has saved money over your old system.

    The people you claim "hate" combi's on this site, have a significantly different  skillset and level of experience.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • SWEI
    SWEI Member Posts: 7,356
    Boiler feeding forced air

    is often referred to as hydro-air.  It works quite well when properly designed and implemented.  Sizing the coil(s) for 140 or 150F water will keep the boiler in condensing mode.  Outdoor reset delivers far better comfort than a conventional or two-stage FAU.

    I assume your "92% average" is an AFUE rating.  Do you know what its efficiency is at various turndown ratios?
  • CMadatMe
    CMadatMe Member Posts: 3,085
    To the Original Poster

    The anwser to your question is. It depends. Was a heat loss of your home done? Did anyone measure the existing emitters and calculate the actual needed water te o at design conditions. Inhave 25 years of experience that says your probably 35% over radiated and a condesning boiler is probably the way to go but without the heat loss and emitter measurement it's just one mans opinion.

    I with Kurt on his response to HO1..What HO1 leaves out is that if it wasn't for the professionals here, he'd probably be sitting home right now not as happy as he is.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • heatpro02920
    heatpro02920 Member Posts: 991
    Smott people here..

    This board has some of the most advanced heating minds in the country, being in the trade I have spoken with professionals all over the country, from contractors, to designers, and reps of all kinds. Heating help has some of the smartest and most experienced minds I have ever seen in one place.

    The smart consumer will take the information from these guys and do very well letting it assist with their decisions.. BUT, this is a free board that people volunteer to post on, so you can get home owners and non professionals, or even manufacturer reps, giving biased recommendations... HO1 seems to love his Navian unit and that is great, but I and many other contractors have had more bad than good units pushed through our invoices, customer support has gotten better, but the unit is a tankless water heater modified for space heating, and for the money a consumer would be much better off, with a cast iron conventional boiler {one that can withstand an ODR} and an ODR control. Do not get caught up on the printed percent numbers...

    Now to help with your question, I would have a heatloss done and have the emitters measured and see where you stand, in other words at what temperature can you run your boiler and still get the needed BTU's into the space.... In my experience most houses are grossly oversized with an abundance of baseboard... This will work to your advantage when going to a mod con.... Good luck and please post any more questions or concerns you may have...
  • tromboneman
    tromboneman Member Posts: 4
    5:15 PM, 6/23

    Used 1,100 gallons of heating oil 6/1/12-5/31/13, including domestic hot water off the boiler.

    In case you are interested, 158' of standard baseboard and a 9,400 BTU kick space heater serve about 2,000 s.f. of heated space. In addition there is a 225 s.f. sun room with lots of windows and radiant floor heat also coming off the boiler.

    The existing boiler is a Burnham 160,000 BTU, rated at 139.1 BTU with a 1.35 nozzle installed.

    What do you think now?
  • R Mannino
    R Mannino Member Posts: 434
    Your Post Should Read

    Big boiler vs. small boiler. Smallest you can get should do it, with the appropriate sized indirect of course. Just my two cents.
  • tromboneman
    tromboneman Member Posts: 4
    9:50 PM 6/23

    Thanks for the advice. Certainly makes sense to me.
  • tromboneman
    tromboneman Member Posts: 4
    9:50 PM 6/23

    I appreciate your advice. I have 2 1/2 baths and there are just two of us in the house, so the 3 gallons of hot water I think we get our of our old cast iron boiler seems generally adequate. What do you think the house will need if we sell it to a family with 2 teenagers?
  • SWEI
    SWEI Member Posts: 7,356
    1,100 gallons of #2 oil

    Is equivalent to 1,529 Therms of NG, assuming equal system efficiencies (which may or may not apply here.)  Compare your fuel costs, figure the savings, and multiply by how many years you're willing to wait for payback.  Leaving out future fuel price increases and interest, that will tell you roughly how much this upgrade is worth to you.

    DHW demands are entirely dependent on use patterns and fixtures.  Even a cursory examination of the math will rather quickly endear you to low-flow fixtures.
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