Oil Tank Replacement Part II Should I go Gas?

Discussion in 'HVAC Heating & Cooling' started by Massjim, Jul 17, 2009.

  1. Massjim

    Massjim New Member

    Jul 1, 2009
    See part one under oil tank replacement thread

    Now a recap 275 oil tank leak..estimate is 1600 for replacement tank
    slim line tank should fit?? well it did not ok go with 138 gallon tank...need to order that on..I say how about two tanks single vent and fill..He has to check with Fire department....

    ok wife says how about gas..ok we have gas in the house (good)...I call the guy, he calls back 1200-1400 for the burner and install....but warns me about how the hotter flame may damage furnace.
    He will do a chamber inspection before install.

    I ask him to price out a whole new furnace no price on that yet.

    Local gas company gives $250 credit for switch over..They do not install equipment.

    I have left messages with two other companies for estimates.

    Anyway Hear are my questions

    Should I go to gas?..pros no oil in house, no need to worry about if I run out (happened last year)..Gas is cleaner less cost for maintance

    Go with new furnace or just burner? Cost, will old furnace last with new burner.

    I am not sure if chimney is lined.

    Forced Hot air system presently

    Not sure how old current system is I am guessing from 70's

    Please put in your two cents....and what would you do if you where in my place.
  2. Dana

    Dana In the trades

    Jan 14, 2009
    If the oil-fired furnace is from the '70s, methinks it's service-life is pretty much had it (although I've seen folks patch, prod, & limp 40+ year old oil furnaces along forever- to their financial detriment.)

    It kinda depends on how much cash you have to invest, but if the ducts are in good condition and reasonably well balanced from a flow point of view (no overheated or too cold rooms) putting in a properly-sized condensing furnace just isn't all that expensive and you'll likely use half the source-BTUs a season as an aged oversized oil-fired beast. Most early '70s furnaces were in the ~70-75% combustion efficiency range when new, and after 35 years of benign-neglect are running ~60-65%. Then, if it's 200%+ oversized for the heat load, as is typical (especially if there has been any insulation upgrades), the as-used AFUE is then only 75% of it's raw combustion-efficiency, or around 45-50% of the BTUs in the oil showing up as warm air in your living space.

    eg: If you used ~1500 gallons of oil last year, that's ~2000 therms, and you'd use around ~1000 therms of NG with a 90%+ AFUE condensing system. If you just went with a retrofit gas burner on your antique furnace, with the burner sized properly for the load and tuned to ~80% combustion efficiency (no higher, or you'll condense in the unlined chimney) you'd likely use around 1500 therms (more, if oversized.)

    Do a Manual-J type heat loss calculation no matter what. If your heat load is under 100KBTU/hr on the coldest hours of the year (probably is, unless the house is huge or located at the artic circle), for more money you can go with a combined furnace/hot water heater, or use a tankless HW heater/tiny condensing boiler with a "hydroair" hydronic coil in an air handler (preferably with a super-efficiency ECM blower, since your old one probably uses 500-1000watts instead of 100-300W) to heat both the house and hot water with minimal standby losses, and a higher overall annual net-efficiency.

    Higher than ~83% efficiency systems won't be able to use the chimney, but 90%+ efficient burners can be side vented using cheap PVC vent pipes (cheaper than a stainless chimney liner by quite a bit!). The installed cost of a mid-efficiency system may be higher than a high-efficiency version in your situation.

    On any hot air delivery system, sealing all duct seams & joints with duct-mastic improves efficiency and lowers the noise. Tape doesn't quite cut it in most apps, but if your ducts are still shiny-clean, 2" aluminum FSK tape can work almost as well as mastic.

    See if you can't get some competing quotes (with a written heat-loss analysis, not a "35BTUs per square foot" or similar wild-assed-guesstimate method) for a condensing gas furnace at least. Quotes on combi-systems are more complicated, unless you luck out and find the right contractors, but most can handle condensing boiler + indirect hot-water + hydro-air coil systems, which will be more expensive, but more flexible if you think you'll ever want to retrofit something cushy-cozy like, say, radiant floors or low-temp hydronic panel radiators etc. You may be pleasantly surprised how good the ROI on a higher-efficiency system is.

    If your duct design is all unbalanced & drafty with leaky rusting ducts, you may want to opt for a zoned hydronic system to even things out. Designing for low temp (under 130F water) on the radiation (baseboard, radiant floor, or radiators) you can meet/beat condensing furnace efficiency and achieve a higher comfort level. (This is LOTS mo' money, in many cases, but if you have the wall-length for going with low-temp cheap fin-tube baseboard it's not as bad as you might think. It can sometimes be done for under $10K in smaller houses with simple layouts.)

    Hot air delivery systems are inherently less efficient than hydronic (pumped hot water)- they rely on air pressure differences to achieve the air flow, and this drives air-infiltration in/out of the walls of the house displacing heated air with outdoor air. And ducts have orders of magnitude higher surface area than pipes, losing more heat to basements/crawlspaces/attics when run in un-conditioned spaces. This typically adds up to 10-20% higher fuel use than with equivalent-efficiency furnaces/boilers, depending on the air-tightness of the house. Air handlers also use an order of magnitude more electricity than pumps. If you're on the edge of biting the bullet for a whole new system go hydronic.

    Too much to think about? :)
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  4. Massjim

    Massjim New Member

    Jul 1, 2009
    Thanks for your response. This is right now a summer home that we use on weekends. We leave the heat on low in the winter and I think used less than 500 gallons of oil.

    At some point in time we may live in the home full time 2-5 years from now.

    I think I got out of your reply that I will not have to line the chimmney if eff is less than 80%.
  5. Dana

    Dana In the trades

    Jan 14, 2009
    83% is something of an absolute limit where flue condensation is pretty much guaranteed, but depending all factors 80% is safe for retrofit into an older furnace.

    Given the level of use & all, it sounds like the right thing is to drop in the smallest reasonable retrofit gas burner throttled down to it's lowest setting to nurse it along for the next few years. It just has to keep it above freezing with some margin, not a guaranteed toasty 72F at 5AM on the coldest February morning. If anything, undersize the output 25% for the estimated coldest-hours heat load, and it'll still be oversized for keeping it at 50F, and you'll never be cold.

    Something like the Carlin G3B one-size-fits most (~$1000, + installation) adjusts down to ~55KBTU out, which may be 2x your actual worst-case heat load at a 50F indoor temp if its a small weekender place, but that's lower than most older oil burners jetted to their smallest. (It can be adusted up to about 150KBU out too, probably WAY over your actual heat load.) Riello probably has something similar. Just be sure the burner tech adjusts it so that it's 80-81% combustion-efficient 'cuz they're capable of ~85% (which would be desirable in some systems, a disaster in others.)

    Then when you decide it's time to move in full time you'll likely be upgrading all sorts of things (including insulation & air tightness) at which point retiring the old system for something more efficient might be in order. I'd expect the retrofit burner turned way down to pay for itself in fuel cost in under 5 years even at your weekender level of use, and the kickback from the utility should cover a large share of the installation cost. If you needed/wanted to nurse the system along for awhile, most retrofit burners are good for at least a coupla decades if not a half-century. The as-used AFUE of a system a retrofit burner running at 80% combustion efficiency is still probably no better than 70-75% unless you've done a lot of modifications to the system already.
  6. Massjim

    Massjim New Member

    Jul 1, 2009

    I have a qoute from a guy that will put in an 80% eff. which includes ne pie from meter to unit with two tees for dryer, which is now elec, and tee for fireplace which I plan on putting gas in someday. Also will put pie down chimmey for vent. Cost 3,200

    90% unit was like 3800 would have to drill thur sill to vent.

    I am waitng on the contract that did the oil tank removal for his qoute and I have a guy coming on Friday for another qoute.

    How much should the guy chrage me that removed the oil tank. 1/2 day work removed tank and I have about 100 gallons of oil???
  7. Dana

    Dana In the trades

    Jan 14, 2009
    If it's only $600 more to go with the condensing unit vs. an 80% AFUE version it's probably worth it. (Gas has gotten cheaper recently, but it won't always be.) If it's sealed-combustion and the 80% unit is atmospheric-draft, the efficiency difference will be better than the simple AFUE numbers might imply.

    I'm not sure how to guesstimate the tank removal charges- different places have different handling & disposal costs. If it's over a grand I might squawk, but it's a one-time-ever type of charge, don't be losing sleep over it. I'm not sure anybody will credit you anything for the oil in quite the same way they might have LAST year around this time...
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