Various boiler efficiency questions

Discussion in 'Boiler Forum' started by john2129, Jul 31, 2010.

  1. john2129

    john2129 New Member

    Jul 31, 2010
    Westborough, MA
    This my heating system:

    1991 Burnham Series V1
    Model V-14A-T
    DOE Htg cap mbh: 124
    Net I=B=R Water mbh: 107.8
    Firing Rate: Lt. oil gph: 1.05
    Standard damper installed
    Tankless DHW is not used

    Power vent:
    Field Controls SWG-4 Power Vent replaced in 2010, stainess steel blower wheel.
    Flue pipe is galvanized steel, run: 4 feet vertical, 45 degree bends to 3 feet horizontal.
    WMO-1 blocked vent switch, replaced in 2010.
    PPC-4 post purge switch, installed in 1991.
    CK-62 vacuum switch , installed in 1991.

    Heating zones
    Two zones, with individual circulator pumps, no zone valves.

    1990 Beckett AFG
    Flue gas temperature: 550 degrees, no smoke
    Air shutter setting: 4
    Nozzle: .75 gph, 80 degree hollow
    Oil Pressure: 100 lbs

    Hi limit: 160
    Low limit: 140
    Diff : Off (tankless DHW is not used)

    Domestic Hot Water (DHW)
    1999 Amtrol Boiler Mate WH7P - 41 gallon indirect water heater
    with it's own circulator pump. DHW setting : 130 degrees.

    Burner, boiler, flue, power vent etc are all maintained in "new" condition.

    This is my home:
    Located 30 miles west of Boston, Massachusetts, 300 feet altitude, 2000 sq ft, two story, cathedral ceiling, moderately well insulated, new windows, fiberglass exterior doors, with new storm doors, two adults and two teenagers, two Miniture Pinchers.

    Fuel oil consumption:
    1200 gallons per year

    I want to lower my costs for heating, perhaps by improving the efficiency of my existing heating system.


    1. How can I lower the 550 degree flue temperature, or, as I have been told, is this acceptable? I am concerned that a lot of money is going out the power vent. Would a lower gph nozzle with, perhaps, higher oil pressure increase the efficiency and lower the flue gas temperature? The air shutter is on 4 ; lower settings produce smoke, higher settings increase flue temps.

    1a. Would a new burner pay for itself over ten years?

    2. Would an Automatic Flue Damper (AFD) increase efficiency? After my burner stops, the power vent continues to run for about 3 minutes until the flue temperature is about 200 degrees. This setting can be changed. This is a post purge process to vent latent combustion products. This process, though necessary, sends a lot of heat out the vent. Many new boilers come equipped with AFD's and I'm not sure how they deal with the post purge issue.

    3. My system has a conventional flue damper, do I need it? My system has a power vent and the damper never made sense to me.

    4. Would a Beckett Heat Manager ( or similar) or a Boiler Outside Ambient Temperature Reset Control increase the efficiency of my 20 year old system? Would it pay for itself?

    5. Thoughts on increasing the insulation on the Amtrol DHW heater? Does anyone safely add insulation to the outside of the boiler - I would never do it, just curious.

    6. My boiler has a tankless water heater that is not used. Would it increase the efficiency if I plumbed this into my heating system? At the moment it seems like a lost opportunity to heat up some more water for heating. I'd test it for leaks first.

    Thanks in advance for your help and any other suggestions you may have.

  2. MAoilTech

    MAoilTech New Member

    Nov 24, 2009
    Oil Tech,HVAC
    Before you go ahead and spend your hard earn cash my advice will be replace your boiler it's close to 20 yrs old i'll hate to see if you spend the $$$ then a years or 2 it dies I know how it feel's I live in MA.
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  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    The flue temperature depends on the burner design and how well the burner is set is, essentially, what it is. Try to lower it without proper engineering, and you risk problems. Retrofits probably aren't cost effective, even if they are available. The newest boilers can use pvc for exhaust, since they are designed to extract much of that heat. This requires something designed specifically for it. These are more common on NG burners than oil, which is harder to control.

    An outside reset might help the overall efficiency, but may not be much with what you have. The efficiency depends on how well the burner assembly can utilize the heat it does produce and the overall system's efficiency also depends on how well that heat is moved throughout the house. There's less losses with lower temperatures, but you may find (depending on the type of heat exchangers you have - big iron radiators, finned radiators, etc.) that feeding them lower temperatures doesn't provide the level of comfort you need. What a proper outside reset does, when working, adjusts the boiler water temp to match the needs of the house and keeps the burner running longer. This works much better with a modulating boiler like is common in a condensing boiler. One of the big things you don't want is to short-cycle the burner. A non-modulating burner would have more trouble keeping the water temp lower while matching that to the house needs than a fixed burner design. You lose some heat gained each time it turns off, the fewer turn-offs there are, the better, but that's hard with a fixed burner while maintaining comfort in the house.

    If you lower the flue temperature too far on a boiler not designed for it, it will rust out to nothing in short order...they just aren't designed for the potential of condensation getting into the boiler and rusting it out.

    There's a few people here that make efficiency their goal in'll get some thoughts. While you may be able to make some changes to improve efficiecy, I'm not confident keeping what you have would end up the best long-term choice. If you have some other fuels available, the choices improve. If you are 'stuck' with oil, I don't know.
  5. Dana

    Dana In the trades

    Jan 14, 2009
    First off, you shouldn't be monkeying with the air-mixture control without testing the combustion efficiency (trust me, you don't own the right equipment for this, and it's not worth buying for a 1-off test, nor is it rentable). It's producing some stuff a lot more toxic than mere soot when it's smoking. Properly set up this burner would be able to run ~85-86% combustion efficiency. You'd normally only be ajusting the air when re-jetting it or tuning it up (which is probably worth doing.)

    Based on your fuel consumption and your location, your design-day heat load can be estimated:

    1200 gal x 138000btu/gal x 0.85 efficiency=140.76Mbtus/year

    Your climate is ~6000 heating degree days (base 65F), so per heating degree-day you use 140.76M/6000= 23460btu/hdd. Divided by 24hrs/day you have 978btu per degree-hour.

    Your outdoor design temp is probably 5F, but for margin let's figure it's 0F (similar to what it is in Worcester). That means your peak heat load is (65F-0F) x 978= 63570BTU/hr, or 63.57MBH which is about the output of the very smallest jetted oil burner, which means as-installed you're currently ~2x oversized, and will have some loss of efficiency related to excessive cycling & standby losses. You can think of this as a upper-bound on your true heat load anything your true heat load is 10-15% lower than that, because your summer hot-water heating is adding to the fuel consumption (but at least you have the indirect tank, and aren't keeping the boiler hot for massive standby loss). There's also a high likelihood that your combustion efficiency is significantly lower than 85%. The narrow hysteresis on the high/low limits of the aquastats can also lead to lowered efficiency if it causes the burner to cycle on & off during calls for heat. 140 is about as low as you can let it get without suffering flue condensation or condensation within the boiler, which would ruin it, but you can safely raise the high limit to lengthen the burns. Anything shorter than 5minutes per burn is going to cut into efficiency, and below 2 minutes you're well off the efficiency cliff (!).

    If the boiler is in otherwise pretty good shape an you plan to keep it, there are some options. If the boiler plates on this one can handle it, you could de-rate the burner by re-jetting it to something smaller and retuning the air intake. There may be other reasons not to do this though- if the flue is oversized for the lower heat output you're setting yourself up for flue condensation or backdrafting problems. Re-lining the flue with a narrower liner could handle that but it's expensive. Take this measure ONLY with a professional assessment.

    On ANY boiler system, insulating all of the near-boiler plumbing to at least R8, and all of the distribution plumbing in semi-conditioned space to R6 will be cost-effective over time. Insulating the boiler itself is possible to do safely in many/most cases, but with a 20y.o. beastie-girl like yours she may not live long enough to make it worth the effort especially if it's in an insulated basement. Most of the heat-loss from the Amtrol isn't out the sides, it's typically from uninsulated plumbing to the boiler, and uninsulated domesting plumbing. It's well worth insulating the DHW plumbing with the 3/4" wall closed cell stuff (you may have to find an online source- most box stores & hardware stores only carry the 3/8" goods, which is better than nothing I s'pose, but more is still cost-effective.) Even the cold feed and pressure/temperature valve & plumbing should get the full treatment.

    Think about installing an Intellicon HW+ economizer (an improvement over it's cousin, the Beckett Heat Manager IMHO). The installation & setup instructions are online look 'em up. If you're handy this is a DIY job, but if you'd rather, a pro could install it in an afternoon for something on the order of $500-800. (The Intellicon unit costs ~$200-250 from internet stores and some distributors, but they can be had for wholesale or less on that major web-auction site.) On a 2x oversized boiler you can expect true double-digit percentage savings with these when set up right (don't program the low-limit to less than 140F though. You can get away with lower on a ng-fired unit, but not with oil.) These units are microprocessor controlled and "learn" the characteristics of your system, and adjusts as the heat load changes over the course of a day or season, but their fundamental approach is to optimize the burn-times & temperatures in reaction to how the system is behaving, while guaranteeing a minimum burn time. In practice you'll get fewer burn cycles, but overall less burner on-time to serve the same load as well.

    Outdoor reset control can work well in some systems, but without knowing your temperature requirements we can't say for sure if it would buy you anything. It could be that your radiation is 2x oversized as well, in which case even your design-day temp requirement could be even lower than your boilers low-end return-water tolerance, which might mean you'd have to re-plumb the whole shebang as a primary/secondary, or even add mass to the system to really get much advantage out of ODR. In an oversized system with oversized radiation the temps are low making for lower distribution losses, but the burn times can get ridiculously short at light-load, erasing any benefit with short-cycling losses. The total thermal mass of the system will either make it or break it. The Beckett/Intellicon approach uses the thermal mass of the boiler & radiation for optimal-effect for the actual load no matter what the mass or load is. Done right they often beat ODR on high-mass boilers.

    With a gas main on your street there are options worth considering before putting any real money into optimizing what you have. A DIY Intellicon will pay for itself in under a single season on an oil-burner, but re-plumbing for better ODR could take quite awhile to pay back in full. Scrapping the ol' girl for a right-sized gas-fired modulating/condensing boiler might pay back sooner. ODR has HUGE benefits for mod-cons (it's usually built-in), since it'll spend most of it's burn time well into the high-efficiency condensing range, it lowers the flame to minimize cycling loss. Between the lower source-fuel BTU costs and the ease of getting it to run at highest efficiency you'd likely see an operating cost about half what you're paying with oil heat:

    Oil in MA is running ~$2.50 & up for a 138kbtu gallon, or $1.80 /therm (1 therm==100,000btu). Gas (in central MA anyway) is ~$1-1.10/therm. Your aging 2x oversized unit is likely to be only getting ~82% AFUE even if it's raw combustion efficiency is 86%, so it's costing you more like ~$2.17 per therm delivered to the house. With a mod-con you get at LEAST 90% AFUE, for a delivered cost of ~$1.22/delivered-therm.

    There may be better places to spend $10K though, and if you don't have the $10K to spend there are smaller mid-budget boosts to the thermal envelope that can have big payback such as:

    Insulate & seal the band joists & foundation sill to at least R12 with closed-cell foam.

    Have an air-sealing contractor cut your air leaks down to under 2 air exchanges/hour @ 50 pascals (if you can- with many home getting it under 5 can be tough.)

    Insulate basement walls with foam board or spray foam to at least R10 (careful what you use- there are some gotchas here.)

    A full energy audit complete with blower-door testing and thermal imaging runs about $500-600 in this area, and if you can find & fix most of the major air leaks and spot-insulate where there are demonstrable gaps with that information it's usually VERY cost effective. Most of the cheaply-rectifiable heat loss in a moderately well insulated house is from unseen imperfections- mostly air leaks & insulation voids, and trust me-they're there unless it's already been gone over thoroughly at least twice.

    Last, maybe least, but still worth considering since you have 4 people showering there daily, if you have at least 4 feet of 4" vertical drain downstream of your shower(s), there's a strong financial rationale for installing a drainwater heat exchanger if you're heating water with the oil boiler. They're not cheap, but they return more than half the heat used in a shower back to the incoming water stream, and will last about 40-50years before performance has fallen off to the point you'd want to replace it. They're hard to find in the US, but EFI out in Westboro is the US distributor for PowerPipe. They typically only sell to installers, but you can buy 'em onesie-twosie by opening a commercial account with them (which is literally a phone call with credit card in hand) and it ends up being about what it costs on-sale in Canada (where they're much more widely sold). See: Bigger is always better- both in length and diameter, but EFI's biggest 4-incher is the 48" long one. Natural Resources Canaday pays to have them 3rd party tested to a standard temp & flow rate for apples-to-apples performance between models & within a line. See:

    With a 2-teenager family the drainwater heat recovery alone may run a 50 gallon/year kind of deal on fuel savings. They don't do squat for tub-fills though- the drain and domestic water has to run at the same time. To fully max out performance it need to feed both the Amtrol and the cold-water feed to the shower (I set mine up to feed all cold water distribution to the house. In the summer the cold water ends up on the tepid-side if drawn while someone's in the shower. In winter when incoming water is ~40F it's not much warmer than room temp, but it's way warmer than 40F.)
  6. john2129

    john2129 New Member

    Jul 31, 2010
    Westborough, MA
    Many thanks - I'm overwhelmed!

    It’s going to take me a while to digest all of suggestions and data provided by Jim and Dana. Many, many thanks.

    Unfortunately we do not have NG on our street and as far as I can tell, NG is not projected to be installed. I had no idea that NG therms were so much cheaper than oil.

    Regarding the low-limit of 140 degrees and condensation in the flue and boiler. I have a power vent with a 24 gauge, 6 inch galvanized steel flu running about 40 inches vertical and 40 inches horizontal before venting directly the outside. Does this set-up lend itself to a lower low-limit?

    I will get back to you soon after I thoroughly review your responses. I can’t thank you enough.

    Warmest regards,

    John Walsh
    Westborough, MA
  7. nhmaster3015

    nhmaster3015 Master Plumber

    Jul 30, 2008
    The granite state
    I agree with Dana, go with the Intellicon control or similar.
  8. Dana

    Dana In the trades

    Jan 14, 2009
    The stainless liner means the flue can take it but that does noththing for the heat exchanger plates on the boiler itself. If you don't have it kicking around the house somewhere, find it online, but even cold-start oil-burners are typically hard-limited for 140F return water temps. You might get away with 135F (violating the warranty terms), but never 130F, and neither is enough of a difference that it'll buy you much going with outdoor reset.

    Traditionally oil has been cheaper than gas per delivered BTU, but the view for now into the next decade doesn't appear very likely for that past norm to return. There are higher-efficiency oil boilers out there, and right-sizing it for your actual heat load would help (by quite a bit, actually), so when this one croaks give it a lot of analysis based on historical use and a Manual-J heat load calc, not just existing radiation & boiler output capacities.

    In answer to the damper question, whether power-drafted or not, dampers keep the flue from convecting heat out of your boiler & house 24/7- it DOES improve efficiency measureably. Post-purging the flue after the burn is necessary for safety reasons, but 3 minutes is unduly long unless you have a very tall flue indeed.
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