Need Advice Switching to Gas

Discussion in 'Boiler Forum' started by Phillip Allen, May 13, 2013.

  1. Phillip Allen

    Phillip Allen New Member

    May 13, 2013
    I am looking to switch to natural gas. I currently have a 20 year old oil burner (also does hot water). Brick House built in 1901, replacement windows upstairs, attic has insulation (and is a living space), down stairs has original windows with storm windows.

    I just got a quote to put in a slant fin Victory VSPH or Lynx Combi. The VSPH (86%) is for $5,080 (with hot water heater installed) and the Lynx (92%) is for $6,575 (tankless hotwater heater).

    The lower efficient one has a 25 year warranty and the high efficient one has a 15 year warranty.

    What should I chose? I probably wont be in this house forever and I would like to stop paying 2,800 a year in oil.

    I am also worried about the life of the boiler. Aluminium verses Cast Iron etc.

    Here are the links to the boilers:
  2. BadgerBoilerMN

    BadgerBoilerMN Hydronic Heating Designer

    Mar 4, 2011
    hydronic heating designer/contractor
    25 years--probably 30--of guaranteed low performance or 15 years of stellar performance if sized, installed and maintained properly. One with with an open chimney and the other eliminating the chimney and it's energy draining perpetual draft, the other with an ecm modulating blower, burner fully condensing with built-in outdoor reset.

    Boilers warranties have no real value and are in no way a indicating factor relative to reliability, durability or serviceability. Things you really want.

    We install Bosch combi boilers with aluminum heat exchangers and a 5 year parts & labor warranty. We also flush, clean and treat all of our residential and commercial boiler systems ensuring long heat exchanger life and better system thermal and system efficiency.

    If you have cast iron radiators, a condensing boiler is a perfect match as the radiators were designed for relatively low operating temperatures. We do not use the SlantFin condensing boilers on single-zone gravity conversion cast iron heating systems in old homes, since they have a warranty enforced mandate to add a redundant pump (primary/secondary) where it clearly isn't needed. Baxi, Bosch, Burnham, Crown, HTP, IBC, Lochinvar, Navien, Triangle Tube and Viessmannn have no such requirement.

    As for as not living there forever, you can sell your low fuel bills with the house.
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  4. Dana

    Dana In the trades

    Jan 14, 2009
    The issue may come down to "... if sized, installed and maintained properly", which may be hard to do, since the smallest Lynx has a min-fire of 30KBTU/hr, which is almost CERTAINLY above the design condition heat load of a house in MD with an oversized boiler burning "only" $2800 in $4/gallon oil in a year (~700 gallons) that's also doing hot water.

    According to the NORA FSA calculator's boiler model (based on Thomas Butcher's Brookhaven National Labs test data) the heat load at +10F outdoor temps will be somewhere between 15-25,000BTU/hr, depending on the particulars of the boiler & coil. (With the BTU ratings of the burner we'd be able to zoom in on that, but any way you cut it even the smaller Lynx is at least 4x oversized for the load.

    If the system has big cast iron rads and is running as a single there may be sufficient thermal mass to make it work, but it's still probably not a great match. There are plenty of boilers with half the output ratings that would be far more suitable. The smallest Victory may be a better match size-wise, but still more than 2x oversized.

    All successful & efficient heating system designs start with a careful heat load calculation, not some WAG rule of thumb based on the size of the house, the type amount of radiation, or the rated BTU output of the old boiler. The amount & type & amount of radiation matters in terms of system design, but is not a factor in sizing the boiler. Only when the heat load is known is it possible to narrow in on the "right" solution, but neither of those SlantFins look like great candidates based on your guesstimate of fuel use.

    If you have a mid or late winter oil bill with a "K-factor" stamped on it (as is commonly done in New England, not sure about MD), that number can be plugged into the FSA calculator, or can be used to put a firm stake in the ground as an upper bound of what the heat load might be. A K-factor is the ratio of base-65F heating degree days per gallon of oil use, which is easy to convert to BTUs per degree-hour based on the input and output BTUs on the boiler's nameplate, and 138,000 BTU /gallon for heating oil. Then calculate the difference between the 99% outside design temp for your area and 65F (between +10F and +20F in MD) to come up with a max possible heat load.

    For example, let's say your K-factor from a February bill is 6.8, your boiler's ratings are 120BTU/hr in 102BTU/hr out, and your outside design temp is +10F.

    For every gallon in the boiler delivers 138,000BTU x (102,000/120,000)= 117,300 BTU to the heating system. With a K-factor of 6.8 HDD per gallon that's 6.8 HDD x 24 hours/day= 163.2 degree-hours for every 117,300 BTU.

    So your BTU per degree-hour is then 117,300 / 163.2= 719 BTU/degree-hour.

    At the 99% outside design temp of +10F you are at 65F - 10F= 55F heating degrees, and your max possible heat load is going to be:

    55F x 719BTU/degree-hr = 39,545 BTU/hr.

    You can discount that by at least 15% for the combined hot water use & standby losses (unless you're setting back to 50F or something every night, and it's actually dropping into the 50s every night), for a heat load of 33,613 BTU/hr.

    Use your real K-factor and real outside design temp then see what you come up with. I'm betting it'll probably be about 25-30K before adjustments, maybe between 20-25K.

    When you have that peak number in hand, if going with a modulating condensing boiler, the MINIMUM fire input or output of the modulating boiler would ideally be less than half your load number, in which case you'd be able to set it up for TREMENDOUS efficiency and nearly continuous burns during mid-winter conditions. Go with the SMALLEST mod-con that actually meets the load- as long as the max-fire output number for the boiler equals or exceeds your unadjusted fuel use calculated load you will have margin for when it's colder, but the lowest min-fire is the key to maximizing the efficiency.

    If going with a cast iron boiler, ideally it's D.O.E. input rating would ideally be not more than 2x the unadjusted fuel-use calculated number, (<1.5x would be better) in which case it would probably come close to meeting it's AFUE rating. Anything over 2x oversizing pushes it ever so slowly over the cycling-loss efficiency cliff. Since all gas-fired boilers can be cold-started without damage, use an indirect-fired hot water heater, nothing with an embedded coil- that way your standby losses will fall dramatically, since you don't have to keep the boiler at 150-160F all the time just to get reasonable hot water performance.
  5. BadgerBoilerMN

    BadgerBoilerMN Hydronic Heating Designer

    Mar 4, 2011
    hydronic heating designer/contractor
    Which is why we usually use manufacturers with a deeper commitment to condensing technology i.e. a bigger selection of boilers to choose from.

    For small loads we use smallest (lowest output at low-fire) including Bosch, Burnham, HTP, IBC, Lochinvar and Triangle Tube. The combi boilers work well on the odd job where heat and DHW loads line up, but most often we find combi-boilers misapplied and grossly over-sized for the heating loads--the bulk of the duty load in our cold climate--and just a little short for the DHW load especially when their are tubs to be filled.

    The winners are likely the HTP Contender and the Lochinvar Knight and just over 10,000buth output coupled with a 40 gallon SS indirect and all done.

    For the purest, an HTP Hydroversa is a fully modulating water heater with built-in HX with plenty of horsepower and standing water for thermal mass and micro-zone versatility, pun intended. We love them and but for the capital investment seem to be nearly fits all appliance for residential applications.

    ACCA Manual 'J' heat loads and experienced DHW design.
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