oil boiler conversion to gas

Discussion in 'Boiler Forum' started by ctreefer, Oct 26, 2012.

  1. ctreefer

    ctreefer New Member

    Sep 11, 2012
    Looking for a little advice on this one. I need to break this question into a couple parts. I hope it doesn't make this confusing but we want to convert our current house to gas heat and our old house that we converted to a rental we'd like to do the same hence the two different scenarios.

    Here goes.

    First off, our current house is about 600' away from the end of the gas main. I'm working with the neighbors and the gas company to see how low I can get the price down for extending the line but that's a different story (although it factors into my ROI).(return on investment)

    Current system is hydronic baseboard with 185000 WM boiler and indirect HW heater on 2400 sf split level brick house. We typically use about 850 gallons per year and supplement with about 1.5 cords of wood in a stove on the main floor. I'm in the process of adding insulation to the attic (current 22 will add 30 plus cover/seal top plates)(no insulation in the walls). Also hope to do rigid foam wrap on basement walls over the winter but worse case this will happen next year. I've also sealed as many things as I can in the house (around light fixtures, can lights, wires running through walls and outlet covers.) Windows are mostly double pane with a few exceptions and doors aren't too bad but will all eventually be replaced.

    The guy initially just took down all the information and said he'd get me a quote going in the next couple of days. I asked him how to determine the proper boiler size and he said we can measure the length of the baseboard to get an estimate (worries me he didn't do this prior to my question). Is this even enough to determine proper size?

    He offered several options: 84-85% efficient boiler with indirect hot water tank
    84-85% boiler with separate water tank
    90+% boiler with separate water tank (wasn't promoting saying it would take 15+ years to get ROI.) I do need a new chimney liner which wouldn't be needed with this option.
    He wasn't big on offering on demand water saying people were complaining about the hot cold change in the supply because of hot water in the lines from the previous demand having a gap of cold before it heats up again. (I don't see the issue. It sounds like something you just get used to.)

    He mentioned if we go with a separate hot water tank we can bring down the BTU's even more for the boiler.

    Second house:
    1500 sf house single level with hydronic baseboard 105000BTU with inner coil hot water. All new Anderson windows, insulated walls (15),ceiling(38). They use about 600 gallons per year.
    Pretty much the same options he offered. He said they don't do gas powered boilers with the inner coils any more so that's not an option.

    Overall I'm not too impressed with this guy. Waiting for the next contractor for next week (If Sandy doesn't delay that...)

    Any thoughts or advice at this point?

  2. Dana

    Dana In the trades

    Jan 14, 2009
    First, measuring the baseboard has nothing to do with sizing the boiler, and the boiler should be sized for the heat load of the house. With zip code we can calculate the heat load at the 99% outside design temperature for your location based on your fuel use against degree day data and the boiler & woodstoves' approximate efficiencies (see example below).

    Anybody who uses rules like "lessee, 130 feet of baseboard times 600 BTU a foot comes ta 78,000BTU/hr", or " OK, 2400 square feet times 30 BTU/foot is 72,000 BTU" then rounds up to the next boiler size "just to be sure" is a hack. The boiler they install is reliably oversized often by 2x or more- it'll never leave you cold, but it'll take a hit in efficiency, maintenance and often comfort to boot. Those methods are common, but should be banned from the industry, IMHO.

    Every good system design starts with a real heat load calculation. If not ACCA Manual-J, at least use I=B=R methods, or when there's a good fuel use history, calculate from there.

    Most oil boilers in 2400' houses in CT are at least 2x oversized for the actual design-condition heat load, so don't be surprised if we come up with a much smaller number. But being more than 1.6x oversized on a cast iron boiler means it probably won't hit it's AFUE efficiency, and at 3x oversizing it's slipping down the steep part of the efficiency regression curve. The length of the baseboard might give a sense of whether or not you go with a condensing boiler though. If you have enough baseboard so that it can deliver enough heat at the 99% design condition with only 140F water you should be able to hit the mid-90s for average efficiency.

    Using an embedded coil for domestic hot water requires significant oversizing of the boiler to deliver acceptable hot water performance, and has abominable summertime efficiency (for almost all cases.) It's worth installing an indirect operated as a separate heating zone and sizing the boiler correctly for the heat load.

    It's possible to get 86-88% 2 & 3 plate cast iron boilers that are sealed combustion/direct vent, and that's probably the right thing to do if you're not going for a condensing unit. Anything over 83% can't be vented into a terra cotta lined chimney without running into flue condensation issues, and the chimney is probably oversized for a boiler that is right-sized for your heat load. The additional cost for the boiler upgrade will probably be lower than the cost of installing a flue liner, but either way, by going sealed combustion/direct-vent you have zero chance of backdrafting, and don't have yet another flue driving outdoor air infiltration 24/365. (All condensing boilers are sealed combustion direct-vent.)

    For yuks, using the fuel use method of measuring the heat load, let's assume you're near Hartford (design temp = +6F ) which averages about 6000 heating degree-days (base 65F). Let's assume your boiler has a steady state efficiency of 85%, and your wood stove is 70%.

    At 138,000BTU/gallon your 85% efficient boiler delivers (0.85 x 138,000=) 117,300 BTU to the heating system sends the rest up the flue.

    For 850 gallons that delivers 99,705,000 BTU to the system over the course of a year.

    Assuming you're burning something local like red-maple you have a source fuel heat content of 18,700,000 BTU per chord, but in a 70% burner that's delivering (18,700,000 x 0.70=) 13,090,000 BTU per chord to the house.

    For 1.5 chords, that's (1.5 x 13,090,000=)19,635,000 BTU of heat delivered to the house in a season.

    Combined, that's (99,705,000 + 19,635,000= ) 119,340,000 BTU for a ~6000HDD year.

    So per HDD that's (119,340,000 / 6000 )=19,890 BTU/HDD

    With 24 hours in a day, that's (19,890 / 24=) 829 BTU per degree-hour.

    With a heating/cooling balance point of 65F and a 99% outside design temp, you have (65 - 6 =) 59 heating degree,

    So the design heat load (at MOST) would be (59 x 829 =) 48,911 BTU/hr, call it 49K.

    That's a realistic number for a house that size in CT with no basement insulation. It's a bit on the high side but not outrageously so. Since you're heating hot water with the boiler you can knock about 15% or more off that number- call it 42K, and if you insulate & air seal the basement knock another 15% or so off that, make it 36,000 BTU/hr, which (believe it or not) would be in the right ball park. After insulating and doing some air sealing I'd be surprised if your true heat load were over 40KBTU/hr @ +6F, but it probably won't be as low as 30K.

    The smallest oil boilers out there have north of 70K of output, and if yours is 185K (input or output) it's operating efficiency is going to be well under the 85%, which is also contributing to the estimate hitting to the high side. But cast iron gas boilers come as small as ~30K, and there are a number of ~82% efficiency cheap 2-plate atmospheric drafted versions that run 50K-in/41K-out that would work. At your stated fuel use, resist the urge to install anything more than ~50-55KBTU/hr output, if cast-iron.

    With a modulating & condensing boiler you'd be OK to be a bit over that, but not a lot- it needs to have a minimum-fire output of half your estimated 36-40K heat load to reap much efficiency from modulating, and if it's broken up into zones you may have short-cycling issues at low temp operation. The smallest boiler that actually meets the heat load at design condition is what you're looking for, since it will have the highest efficiency and fewest system design considerations to worry about. (The amount of baseboard on your smallest zone and the minimum fire output of the boiler will determine your lowest operating temp, and lower water temps are the only way to break in to the 90%+ efficiency range.)

    On-demand tankless hot water heaters are as complicated as modulating condensing boilers, but have a range of issues to be aware of. From a cost/benefit point of view there's zero payback, but if you have a monster tub to fill or like to take 2 hour showers you might want one. With a boiler + indirect, size the tank for the biggest tub you'll ever need to fill, and if the boiler output is less than 100KBTU/hr (which it surely will be), use a zone controller that gives priority to the hot water, inhibiting other zones whenever the indirect is calling for heat.

    The 1500' house with decent windows & insulation levels is going to come in at or under 30KBTU/hr, and even the tiniest cast iron gas boilers out there would heat it. With enough radiation you might do well with a "combi" heating system based on a tank-type condensing hot water heater, which would be less expensive than a modulating-condensing boiler, but run about as efficiently.

    With most gas cast-iron boilers if there is so much baseboard out there that a right-sized boiler will never get hot enough that the returning water is under 130F there needs to be a short "system bypass" loop or similar plumbing configurations near the boiler to mix in boiler output with the returning water to keep the gas exhaust from condensing on the heat exchanger plates and rusting them out in short years. But cooler return water is exactly what you'd want with a condensing boiler, so "too much" baseboard would be push you toward the mod-con solution, or even a condensing tank HW heater (like the AO Smith Vertex, or Polaris etc.) with a plate-type heat exchanger isolating the heating system water from the potable.
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  4. Smithtb

    Smithtb New Member

    Oct 29, 2012
    Adding up the amount of baseboard in a house is beneficial to determine the maximum size of boiler needed if you don't want to go adding baseboard. If your house heats to your satisfaction now, no need to have a boiler that puts out more heat than you baseboard will emit.

    As far as heat load calculations - talk to a down to earth contractor and they will have a btu per sqare foot calculation for your area that will get you close. People like to use a lot of math and formulas, but at the end of the day, given local building codes, it takes a fairly constant amount of btu's to heat a square foot of real estate. In Southcentral Alaska, we use 25btus per square foot in standard 2x6 residential, and 35 btus per square foot in commercial or high infiltration areas (shop with big doors or view home with lots of windows.). If your construction is marginal, 30btus per square ft is good. This is output (input btu/efficiency). Our design temp is somewhere around 30 below, so you could probably figure a little less. Your boilers are way oversized.

    And indirect is the way to go. Lochinvar squire. Stainless steel welded tank. Only one appliance to service. No moving parts on the wh. Put a honeywell amx mixing valve on the top and crank that puppy up.
    Last edited: Oct 31, 2012
  5. Dana

    Dana In the trades

    Jan 14, 2009
    "...given local building codes, it takes a fairly constant amount of btu's to heat a square foot of real estate."

    Gotta call BS on this one. If all houses were the same size & shape and built the same, with the same size & type of windows, & doors, and tested at the same amount of air leakage that might be true.

    But fact is, they're not, not even close. In CT there are houses 250+ years old with varying amounts of retrofit insulation & windows, as well as better-than code Net Zero Energy houses, and everything in between.

    Heat loss is a function of exterior surface area of the house and the U-factors of the exterior surface assemblies/sub-assemblies, and NOT floor area. Any " down to earth contractor" who sizes BTU x floor area should be avoided like the plague!

    Rules of thumb based on a floor area multiplier reliably oversize, usually nearly a factor of two and often more. Using 25BTU/foot or 30BTU is definitely NOT good- it's the OPPOSITE of good (and readily disprovable by analyzing fuel use against degree-day data, which isn't very burdensome.) Using fuel use data is better than a Manual-J- it's a measurement, using the boiler as the measuring instrument.

    Oversizing a cast iron boiler results in lower operating efficiency.

    Oversizing a modulating-condensing boiler leads to short-cycling on zone calls when the outdoor reset is calling for lower temps. Knowing the amount of baseboard on each zone is useful for determining the lowest temp at which it won't short-cycle, but it's important to get the mod-con with the lowest minimum-fire that actually meets the design-condition load to keep burn times up even at low temp when heat loads are low.
  6. Smithtb

    Smithtb New Member

    Oct 29, 2012
    My apologies, I'm new to this forum.

    I must've misread somewhere, I though this was a DIY forum for people trying to do things themself and understand how to solve their own problems.

    I was speaking in generalities for simplicity - I thought that was understood. Obviously if you live in an underground hut or a castle built in the dark ages, your btu needs will vary. But, it just so happens that most of us live in a house with fairly similar construction - timber framed with 8-9' ceilings.

    If you want to get things down to the gnat's fanny, and if you want to see lots of complex calculations and fancy U-factor formulas - then hire an auditor or an engineer or a contractor and have them do a heat load calculation. I hope you know what kind of insulation you have in your walls, and saved the R-factor stickers from the windows and doors that you installed in 1984, cause if not, the guy doing the heat load calc will have to GUESS. If you think you want to maybe just get a clue of about how many BTU's it will take to heat your home then use the law of averages. If you have newspaper for insulation, or decided to go with the 'open air' feel of no windows, maybe that won't work. If you live in an AVERAGE home, it will get you close.

    I've replaced a LOT of boilers, usually cut at least 1/3, sometimes 1/2 off the size of the boilers when replacing them, and have been achieving around 30-40% savings in natural gas in customers' homes in the process. Never done a heat load calculation in my life. Oops.

    I have made many recommendations to customers based on my redneck math, only to find out that they had their home audited by an energy auditor for a state efficiency program we have. The auditors do heat load calculations, and charge accordingly. Guess what, their figures and my figures are usually darn close.

    I have warm, happy customers. It gets pretty cold here.

    Oh, this is a DIY forum. I'll shut up now.
  7. Dana

    Dana In the trades

    Jan 14, 2009
    A lot of amateurs can do a pretty good job of a heat-load calc, and homeowners tend to know their construction type better than a heating contractor making a WAG at it.

    But it takes less than 30 minutes to look up heating degree day data online and run the simple-math calculation when you have accurate fuel use data to work from, and that puts a reliable upper bound on it. No fancy formulas- you didn't even have to pass 8th grade algebra to run it, (as the example above demonstrates.)

    Using a rule of thumb approach that ends up oversizing by 2x doesn't have much downside when specifying a hot-air furnace, but it's a downright lousy way to size a hydronic boiler. I've yet to run into a contractor in this region using anything less than 25BTU/ft for rule-of thumb sizing, yet fuel use even on houses with NO studwall insulation rarely indicate heat loads anywhere near that high. Most come in between 10-15BTU/ft when measured by fuel-use, and typically 10-20% under a careful Manual-J. A lot depends on the window type and window/floor-area ratios, and there are many exceptions where the actual load are in fact north of 20BTU/ft.

    Oversizing by 2x won't leave your customers cold, for sure, but it isn't doing them any favors either. Reducing something that's 5x oversized to merely 2x is a huge step in efficiency, but at 2x it's still taking a hit. Manual-J isn't rocket science, nor does it take an inordinate amount of time to perform, but it doesn't trump fuel/HDD.

    There is no such thing as an average home in CT, even if such a thing might exist where most of the housing stock has been built since 1980 and houses fall within a narrow range of sizes & shapes. But a 1500' slab-on-grade simple rectangle single story built to current code min will have a substantially different BTU/ft number than a 4500 2.5 story McManse with lots of bump-outs & dormers and full basement built to the same code min. Both are pretty common in ctreefer's area. But the fact that he has fuel-use history means it doesn't really matter what the construction size or shape of the house is, since the heat load has already been measured by the previous heating plant- it's only a matter of applying some very simple arithmetic that any DIY-er who made it past the 6th grade can do (and should, to sanity-check any proposals from contractors using other estimation methods.)

    For most 2400' split levels homes in this area it's actually pretty difficult to undersize the boiler unless the place has 1950s single-panes, no storm windows, and no wall insulation. I'm sure it's much easier to undersize boilers in AK, but I'd hazard that most have more than a 1.6x oversizing factor (the factor used in AFUE testing). If the 99% design condition is -30F, and 1.6x oversizing means you'd be good down to -90F (I'm sure, it got that cold at least once- mayhaps during the last ice age), and at 2x oversizing you'd be good down to -130F, for no good reason. Most boiler replacements will have a fuel use history. Look up the relevant degree day data- it's not hard. A heat load calc that you run in your head isn't worth much.
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