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Boiler Oversized for home - ideas?

jfice13 Member Posts: 20
This is my first post here, please be gentle!

Relevant info

1800 sq. ft. two story colonial construction built 1986 in Connecticut hills.
Windows were upgraded in 2010.


Burnham RS-111

D.O.E. heating capacity 120 MBH
I=B=R heating capacity 104.2 MBH
Firing rate 1.05 gpm

Triple Aquastat settings 180 HI, 160 LO, 10 LO DIFF

1) Someone deleted the draft regulator at some point (no idea why). I measured the draft and it is too high. At -0.07" WC over fire and -0.09" WC in the breech. Will need to get a draft damper and install and correct this issue.

2) Net stack temp approx 475 deg F. CO2 not yet tested. Clean flame.

3) Beckett burner config guide says factory spec is a 1.1 gpm 80 degree hollow nozzle

Hydronic setup:

3 zones w/ Taco 570 zone valves for control
Single Taco 007 circulator


Honeywell PRO1000 w/ 3 CPH config and set to 70 deg F constant temperature


The boiler seems very oversized for the house. My back of a napkin figuring gets me closer to a heat loss of 80,000-90,000 BTU/hr. I will perform an actual heat loss calculation at some point, but I feel I am in the ballpark. I'm not able to buy a new boiler anytime soon. So, I'm stuck with my oversized boiler.

What are your best suggestions for improving this system and saving on fuel costs? In particular, here were some of my ideas:

1) Reduce thermostats to 68 deg - duh no brainer savings here

2) Install draft regulator and adjust draft back into factory spec range

3) Replace Honeywell triple Aquastat with a Hydrostat 3250 which will provide high and low limit, LWCO, and outdoor reset control. If I don't replace the triple Aquastat, would settings 160 HI, 140 LO, 20 LO DIFF give me a fuel savings? Wouldn't this just require me to cycle the boiler longer to satisfy a heat call and thus burn more fuel? Can someone explain how an outdoor reset can actually save me money in a conventional boiler?

4) Downfire boiler - switch to a 0.9gpm 80 deg hollow ? This should give me an I=B=R rating of approx 90,000 BTU/hr which is much closer to my actual heat loss. Is 1.1 gpm to 0.9 gpm too large of a jump? Maybe a 1.0 gpm nozzle?

Any comments? Suggestions?

Thank you all in advance for taking the time to read my post and give me your ideas.


    EBEBRATT-Ed Member Posts: 13,433
    To down fire you need a good technician with combustion testing equipment. He needs to look at the burner components and the boiler to decide how much it can be down fired.

    You must keep the stack temperature above 300 to prevent condensation.

    nozzle size also depends on the burner oil pressure. Nozzles are rated at 100psi, some burners run 140psi so that has to be considered.

    Sme burner parts may need changing. Don't know what beckett you have. Probably an AFG.
  • jfice13
    jfice13 Member Posts: 20
    I believe it is an AF. It's a 100 psi unit. I have a draft meter, a stack temp gauge, a Fyrite CO2 meter and soon will have a smoke tester.

    I was going to do the Beckett recommend tune of adjust for trace smoke. Measure CO2. Then add excess air until CO2 drop by 1.5% from first measurement. Then re-check smoke and it should be a zero. This is the way they teach to tune the burner for the season.

    I read that you shouldn't go past 80% less than the factory spec nozzle. Hence the 0.9gpm that I chose.

    I'd be willing to do it - but - I don't know how small is too small for the chamber. Thanks for the reminder to keep the stack temps up above 300 ! I would have probably forgotten that to be honest.
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,131
    Honestly nothing will really help, without spending money. ODR will make it worse. When you downfire you generally are less efficient. If using the smaller nozzle made the unit more efficient, or close to the same efficiency, then Beckett would publish it as an alternate firing rate. Sometimes lowers the firing rate requires a different end cone, or a low fire baffle, or both for a stable flame. Usually your analyzer will show more excess air, lower CO2, and possibly higher CO when downfiring, and more excess air and lower CO2 (higher O2), means less efficient.
    For example, what uses more energy to heat up a pot of water on a stove, medium (for 40 minutes) or high (for 20 minutes)?

    Don’t go by the efficiency number alone. The other numbers are more important.

    A small buffer tank may be the best option
  • Jolly Bodger
    Jolly Bodger Member Posts: 209
    Has the boiler been scrubbed? Soot is an excellent insulator.

    Derate will help if you have short cycles. If you go too low it will soot up the boiler. They are not made to condense.

    Out door reset can help in a couple ways. Not only does it make heating the water more efficient. The lower the water temp the better the heat transfer from the flame to the water. (limits here on oil, most of this efficiency is achieved in the from condensing and recovering the latent heat.

    When the water temp is lower the loss through piping and insulation will be less.

    Another benefit is in distribution. When the radiators release heat in the air it will lose heat to the exterior surfaces like walls and windows. If the hot air coming off the radiators is not as hot, it won't lose heat through those surfaces as fast.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 5,626
    edited November 2019
    Does this boiler provide domestic hot water (DHW) thru a tankless coil? if not, why is the low limit set at 160°

    If Yes.. 140° may not be enough to maintain DHW.

    If No... A different control that does not maintain low limit will save a good amount on fuel consumption... OR... You can cut a wire on the existing control to disable the low limit that maintains a minimum temperature. No need to maintain a minimum temp if you have a separate water heater.

    Outdoor reset on older boiler does save on operating cost since the off cycle loss is less Because the 180° or 190° temperature in the boiler transfers more heat up the vent system at a higher rate then a boiler that is limited at 140° boiler temp. That said, After derating the firing rate and getting a lower stack temperature as a result, you may be damaging the steel or cast iron in the flue. This is from condensation of flue gasses in the boiler flue. (Not the flue pipe... In the actual boiler where the flue pipe is connected) That is why @EBEBRATT-Ed mentioned minimum 300° stack temp. 130°minimum return water temp is important too!

    If you are going to replace the boiler soon anyway, maybe not a bad idea for a year, But if you are looking at a permanent fix to keep this boiler... maybe you should rethink outdoor reset.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 5,626

    Sometimes a boiler is oversized if it also supplies DHW. @jfice13
    you are lucky the boiler is only 22% over sized. That may be right size because the DHW needs, or that is the smallest available with a tankless coil. Cant find RS111 document online. Will need to look in archives in my old office files.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • CMadatMe
    CMadatMe Member Posts: 3,086
    Investing in an old steel boiler is like dumping money into a 20yr old car. Basically money in the wind..Yes you can invest in system (boiler is what it is, can't upgrade it) efficiency but better off saving your pennies and moving on to a new heat source.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 5,626
    edited November 2019
    Off topic but I have an old car that I dump money into.

    On topic:
    what is the actual cost to operate the beast? (don't answer on the forum, we don't talk price here. it one of the rules)
    What is the cost of new system?
    What will the new cost to operate be?

    Take that our 5 years and see what happens, 10 years, 20 years.

    If it cost $100.00 to operate per year
    and a new one cost $1000.00 and will cut cost of operation 25%
    in 10-years the do nothing cost is $1000.00
    The replace cost is $1000.00 + $750.00 (25% less) = 1750.00

    Not really sound financial advise.

    If however the cost to operate is $500.00 per year, that changes the picture

    10 year do nothing cost = $5000.00
    10 yr. replace cost = $1000.00 + $3750.00 = $4750.00

    Same job for 2 different clients will have different results. @Chris_110 Advise is sound 9 times out of 10. But not always!

    @jfice13 Are you experiencing very high cost of operation?.. or is it affordable?
    Use common sense and don't put lots of $$$ into the old Burnham if you have high cost to operate. If it is affordable and you are keeping for several years then tune it up the the best efficiency possible. Forget outdoor reset if it makes DHW.

    If you have a separate water heater, cut the wire that maintains the DHW. Lower the high limit to 160° until you get extreme cold weather. If the 160° limit does not keep up when the outdoor temp is extreme... raise the limit to 170° or 175° or more, as needed. You may find the radiators in the home are insufficient and require that higher limit temperature. Experimentation and experience will give you your final answers to the right Limit Settings.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • jfice13
    jfice13 Member Posts: 20
    This unit is not producing DHW anymore. Would the low limit on the Aquastat set to 140 still be advisable during the shoulder seasons to keep condensation problems at bay? With a manual shutdown during late spring through early fall?

    Best advice set HI LIMIT to 160 and LO LIMIT to 140 with a 20 deg LO DIFF or something?

    I don't quite understand the yearly cost to operate? Is that just maintenance? Fuel costs are really all it costs to operate. I do the maintenance on the system myself now.

    My plan is to give it a good scrub and vacuuming, add back in the deleted draft regulator (still no idea why someone did that), put a new nozzle in (probably a 1gpm instead of 1.1gpm), set the draft, and then tune for max CO2 with no smoke.

    Then will set the HI to 160 and LO to 140 w/ a 20 deg LO DIFF.

    Sound reasonable?

    I set all my Honeywell thermostats to 3 CPH. They were set at factory default of 5 CPH. Probably wasn't helping!

  • jfice13
    jfice13 Member Posts: 20

    My plan is to give it a good scrub and vacuuming, add back in the deleted draft regulator (still no idea why someone did that) and re-pipe the chimney connector to code, put a new nozzle in (probably a 1 gph instead of 1.1 gph), set the draft, and then tune for max CO2 with trace smoke, check CO2 level, then open air shutter just enough to drop max CO2 by 1% as per Beckett's burner tuning procedure.

    Then will set the HI to 160 and LO to 140 w/ a 10? 15? 20? 25? deg LO DIFF. The DHW coil was deleted. I realize I could modify the controller so there is no LO LIMIT setting, but then I would fear running below 140 in the shoulder seasons and having condensation issues...

    Or maybe I will just get a Hydrolevel 3250 and let it automagically handle everything for me.
  • icy78
    icy78 Member Posts: 404
    Since you don't actually know the building heat loss, I think I would start there. Get a firm figure to go on first.
  • Jolly Bodger
    Jolly Bodger Member Posts: 209
    it's a boiler, knowing your building heat loss is only 1/2
    the puzzle, distribution capacity is the other 1/2.