Replacement boiler sizing in SE MA

Discussion in 'Boiler Forum' started by Peter Noble, May 7, 2019.

  1. Peter Noble

    Peter Noble New Member

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    May 7, 2019
    Location:
    Duxbury MA
    I live in a 3,223 square foot house we had built in 1995 in Duxbury, MA. We currently have the original Weil-McClain 151K BTU boiler with baseboard heat and a 40 gallon boilermate for hot water. I'm considering replacing the boiler before it fails to avoid an emergency replacement and benefit from the Mass Save rebate.

    I've completed Dana Dorsett's heating size calculation based on this past winter's fuel usage. In summary, we used 453 gallons of oil from 2 Jan-12 Mar, which was 2,387 degree days in Weymouth, the closet measurement station. 99% dry bulb temp is 11, and I used the boiler's stated 86% efficiency to calculate 54.03M BTU (I think I have the decimal in the right place!) for that period. At a 54 degree rise our BTU/DH is 943, giving us a balance point of 50922 BTU/hour. Sized at 1.4 that indicates a 71, 291 boiler size.

    Couple of factors affecting the heating demand, some add, some subtract. Just me and my elderly mom live in the house. She's in a 732 square foot space we added over the garage about 12 years ago. I'm not home during the day, but keep the house mostly at 66 degrees during the day, 58 at night. My mom keeps her space at 76(!) during the day, 74 at night. So I figure my cooler temps and less hot water use than typical for a house this size is compensated for by her high temps. Her space is well insulated.

    About 6 years ago we added solar panels to the house and Mass Save conducted an energy audit at that time. They subsequently sealed the foundation and bulkhead, gasketed the doors, we added storm doors, and they added 12" of cellulose insulation to the attic. We have single pane windows with combination storms.

    Since we moved in our boiler has always short cycled. We have 5 zones, the smallest being an entry hallway/laundry room/bathroom with about 15' of baseboard. However, that zone rarely runs on its own. We have a playroom in the basement that is currently not used, and I've not included that zone in the square foot calculation. It stays at about 62 degrees in the winter just from the boiler waste heat in the adjacent basement.

    We don't have natural gas on our street. So I'm thinking our best option is a Burnham MPO-IQ84 is a good option. The DOE output of 74K BTU is 1.42 of my calculated demand, assuming I did it correctly. If anything, it may be bigger than what we need, as our current boiler may be falling well short of its original 86% efficiency due to age and short cycling. The MPO-IQ84 has a .60 nozzle, which I hope will resist clogging.

    Does this all make sense? I want to use my same heating contractor for the replacement. It's a small, family run operation and they take good care of me. They're partial to Burnham boilers and Beckett burners. However, they designed the current system and I expect substantial push back on my boiler size selection, so I want to make sure my numbers are close to accurate. That the current boiler is in excess of 2x demand with our energy saving improvements seems typical of contractor boiler sizing. And I remind myself that this was new construction, so they didn't have much data to go with when specifying the original system 25 years ago.

    If the boiler size seems appropriate, should I include hot water priority controls? Our hot water demands aren't excessive, although they do increase when family visits. And should I combine the small hallway zone with the first floor zone (50 feet or thereabouts of baseboard and a kick panel) so it doesn't run on its own and short cycle the boiler?

    I welcome your feedback and questions. Although it's a little silly, I love this house and would like to stay in it for a while. But improving our heating costs is important to me.
     
  2. Dana

    Dana In the trades

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    Location:
    01609
    The load numbers and boiler sizing sound about right.

    A crude rule of thumb (that works for most, with exceptions on both sides) would be 12-15 BTU/hr per square foot of conditioned space. Your ratio would be 50922/3223= 15.8 BTU/hr per square foot, a bit higher than the anticipated range, but not insanely high, and it could simply be a reflection of the oversizing & age efficiency derating.

    If you're on a regular oil fill-up service that stamps a "K-factor" on the billing slips, what are some of the wintertime fill-up K-factors? A K-factor is HDD/gallon, so it's simple & quick arithmetic to verify the load.

    With a right-sized boiler you'd definitely want to make the indirect the priority zone. The space heating zone calls would be held off while the water heater recovered, but with 74 KBTU/hr of boiler output behind it the water heater will recover and the heat will be back on by the time you are dried off and dressed after a shower, not long enough for the house to lose ground temperature-wise.

    What are all the baseboard lengths currently, zone by zone? That would be needed to estimate the short cycling risk, and whether it's worth combining zones, and whether you would need bypass plumbing to protect the boiler from too-cool return water when all zones were calling for heat. A 15' zone is going to short cycle if it's the ONLY zone running, but with the smaller burner size and higher duty cycle the odds of that zone's call overlapping others will go up. The "-IQ" in "MPO-IQ" refers to their on board heat purging boiler controls which are capable of taming some amount of short cycling (within limits).

    Don't be surprised if a contractor totals up how many feet of baseboard there is total in the house, multiplies the baseboard length by 500 or 600 and proposes a boiler slightly higher than that. (I'm going to guess with 151K beastie it's something like 250-300' of baseboard total- if not either the original installer was an idiot, or it was intentionally oversized that much to be able to serve domestic hot water from an embedded tankless coil.) Don't buy into that methodology- sizing it that way will keep the house warm, but it's more prone to short cycling, and buys you less comfort than a smaller boiler. It is always more comfortable in cold weather when the duty cycles on the calls for heat are very high, rather than the hot-flash followed by the chill. (Your existing boiler won't run a 50% duty cycle even when it drops below zero.) It takes longer to warm up the house from a deep setback, but you'll notice the difference in how much steadier the room temps are with a right-sized boiler than it was with the 2x+ oversized beast.
     
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  4. Peter Noble

    Peter Noble New Member

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    Duxbury MA
    I don't save the fuel slips, but I'm sure I could get degree day data per fill period. However, I'm not confident that the tank is filled consistently to the top, so there would be some variation. That's why I calculated based on several (four) fills, not just one. I feel my fuel use may be a bit on the high side because of the temperatures my mom maintains in her space. She's not reluctant to push the thermostat to 78 for a while most afternoons. She also tends to practice what my brother calls a "wood stove mentality" where she overheats the space then opens her sliding door to cool it off. Except she's using oil, not wood. Keeping the peace is more important to me than saving fuel, at least for now. That space also has 12 foot ceilings and she won't run the ceiling fans to circulate the heat.

    Baseboards by zone are:
    • First floor living: 40' plus under sink kick panel with blower
    • Hallway: 14'
    • Second floor: 46'
    • In-law over garage: 36'
    • Finished basement (not currently in use): 27'
    I'm only using one of the four upstairs bedrooms, and have the covers on the baseboards closed on the unused rooms and the room doors closed. I feel I have more baseboard than needed upstairs: the rooms stay pretty warm even with the baseboard covers closed.

    What's interesting is that if I add up the baseboards for the original house (without the in-law space) it's 127 feet of baseboard, x 600 BTU/hr = 76,200. If our contractor doubled that for boiler size we're at 152,400, just about the size of the current boiler. Perhaps we know how they calculated boiler size originally.

    With the in-law space I now have 136' of baseboard with the in-law space but not the basement area, 163' for the whole house.

    Thanks for the advice on the indirect priority zone. I'll do that.
     
    Last edited: May 7, 2019
  5. Dana

    Dana In the trades

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    It's a bit insane to install a boiler with twice the output that the radiation can emit, but I see that all the time. (sigh)

    So, you have a total 163' of baseboard plus a toe-kick. At an average water temp of 180F (boiler output= 190F +) that's 98,000 BTU/hr + what another 5K for the toe kick (?) call it 103K of heat emitter at a temp you never really want to use. Take off ~16K for the basement zone that will never ever be used and it's 87K at high temp, still WAY more heat than you'll ever need, even during record low cold snaps.

    With the remaining 136' of baseboard + toe kick and a 160F average water temp (170F out, 150F back) the baseboard would emit ~61-62K and the toe kick a few more, which is also plenty, and probably where the system will run.

    Worst casing it for return water temp, 74,000 BTU/hr DOE output divided by the full 163' of baseboard is 453 BTU/hr per foot, so even with the toe-kick it's going to balance at a high enough temperature that the return water won't drop below 140F even with all zones calling for heat, but it might drop a bit below 150F, which is fine. When fewer zones are calling the return water temps are higher- you won't get destructive condensation on the boiler plates, even in the worst case scenario. Thus, no bypass plumbing needs to be added to protect the boiler.

    The duty cycle at design conditions will be about 70%, so when it's really cold out the hallway zone will definitely be overlapping with other calls most of the time. But at your average winter temp of ~30F the overall duty cycle will be less than 50%, so it's probably worth combining that stub in the hall with another zone. The even though no zones are big enough to emit the full 74K (/600= 123' minimum to balance at high temp) the overall duty cycle is high enough that the -IQ controls will keep it under control by utilizing the thermal mass of the boiler to best advantage.
     
  6. Peter Noble

    Peter Noble New Member

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    Location:
    Duxbury MA
    Sounds like the MPO-IQ84 is a good fit. I'm wondering if I'll see much in fuel savings, but I think there will be some from less short-cycling (sometimes the current boiler will run for 3-4 min when a zone calls for heat), the outside temperature controls, and the better insulation and less standby heat loss. When the system is working in really cold weather the basement can get to 70 degrees. And I'll see about combining the hallway into the first floor zone. My builder liked the separate zone because his thinking was, outside door opens, hallway cools, that zone only kicks on to bring the heat back up. Appealing, but not practical.

    I'll post up what the heating company says. The owner thinks I have some crazy ideas, but he knows I won't blame him if the choice doesn't work out. So he'll probably be fine with it.
     
  7. Dana

    Dana In the trades

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    Expect the standby & distribution losses to the basement to go down, and a somewhat cooler basement on average, since the average temperature of the boiler will be lower. A cooler basement also translates in to less heat loss from the house. That alone is probably going to be good for more than a 5% reduction.

    With the heat purging controls it's possible that much of the time the brief door-opening load to the hall would be served up much of the time by the residual heat in the boiler without firing. You might leave it for now, and run some experiments by turning down all the thermostats except the hallway zone, then timing and counting the burns when leaving it at the normal setpoint then opening up the door until it trips the thermostat.
     
  8. Peter Noble

    Peter Noble New Member

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    Duxbury MA
    Stopped by my heating company today and talked about replacing the boiler. I was preparing for an argument about what size replacement I needed, but when I said I wanted to go smaller, the technician (who's been at this for at least 40 years) actually suggested the MPO-IQ84. That was good news. Mass Save is offering an $800 rebate for the boiler, and another $100 for the outside reset. We'll probably replace in July.

    Two additional questions came up. First, my contractor said I don't need to put a smaller lining in my center chimney for the smaller boiler. Hope that's correct. And second, he's a proponent of bringing in outside combustion air, but wasn't really clear on what size air delivery I'd need. The boiler room has no windows or easy access to windows. It is in the same space as the bulkhead, but that's about 20 feet from the boiler. I could drill through the sill for air delivery, but for obvious reasons don't want to make too big a hole.
     
    Last edited: May 21, 2019
  9. Dana

    Dana In the trades

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    The MPO-IQ boilers have the ability to set the up as a "direct vent", where outdoor air is ducted directly to the burner, but I don't know if they have a kit to do that with the -IQ84. It's been around awhile for the -IQ147 and larger. Maybe they've come up with that for the smaller boilers too. The 6" diameter FDVS-56 direct vent kit for the -IQ147 does not fit on to the -IQ84, which has a 5" diameter vent collar.

    With a stainless liner there is a bit more freedom regarding sizing, but I'd be reluctant to vent an -IQ84 into an oversized clay-lined flue. The vent collar on the boiler is 5" in diameter, and at a minimum flue height of needs a minimum of 6" diameter or 8" x 8" clay lined flue, (see page 7) but more flue cross section is definitely not better. A right-sized stainless flue liner is how it's usually done. How big is the existing flue?
     
  10. Peter Noble

    Peter Noble New Member

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    Duxbury MA
    I'll check around and see if there's a vent kit for the IQ84. My oil dealer wasn't aware of one, but he didn't research it either, yet.

    From what I can see my flue is 8x8. And it's an interior chimney, so hopefully that means flue temperatures will be higher and more stable. Chimney is probably about 28-30' total.
     
  11. Dana

    Dana In the trades

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    An 8 x 8 with 25'+ of height will be fine. A 6" stainless liner would be insurance against flue condensation damage, but may or may not be "worth it" in your case. Inspecting the flue with a camera could help make the decision one way or the other.
     
  12. Peter Noble

    Peter Noble New Member

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    Called my heating contractor last week and told them I wanted to move ahead with the boiler replacement. Received the proposal/contract last night, but they wrote it for an MBO-IQ 115 instead of the 84 we'd agreed on.

    Stopped by this morning and they told me the 84 was "too small." I showed them the spreadsheet I created with actual energy used this past winter and the contractor agreed the 84 should be OK. He said they usually use a 70 degree target temperature instead of the 65 I used, but his bigger concern was that someone inspecting the house (should I sell) would object to the boiler size. He acknowledged, however, that the difference between the 84 and the 115 was the burner nozzle size, and agreed to revise the proposal to specify the 84, which I originally requested. I think he was more comfortable because a nozzle swap in the future could accommodate any heating shortfall (which I'm confident will not happen).

    We also discussed outside air: the Beckett burners do have a kit. He said I probably don't need it, but I said I want it. I think it will help even if my my basement isn't tight enough to require it.
     
  13. Dana

    Dana In the trades

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    The upsize in design heat load between a 70F indoor temperature and 65F at a nominal 10F outdoor temperature is approximately (70F-10F)/(65F-10F)= 1.09x, a 9% boost. With your estimated ~50K load based on fuel use (that includes all distribution and standby losses too) you still have plenty margin with the -IQ84.

    Itappears to be true that the difference between the IQ84 and IQ115 is simply a nozzle size issue. According to the short sheet spec the shipping weight and water volume is identical.

    The whole argument that you need to oversize the boiler to overcome the potential anxieties of some unknown future owner is something I hear from time to time. Those making the arguments seem to have no idea just how LUDICROUS that argument is!

    I once even heard it from a ground source heat pump installer that it needed to be upsized just in case some future owner might be an older person who needed to keep it 80F indoors all the time. As if they should spend another 8 grand up front to save some random potential future owner who might eventually be living there from having to use more auxiliary resistant heat more often than the person paying to have it installed would!?! :confused:

    Yeah, that's something I'd be willing to pay for (not! :)), 'cuz ya never know...
     
  14. Peter Noble

    Peter Noble New Member

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    Duxbury MA
    Boiler is in! Electrician will come tomorrow to wire up the thermostats and the outside temperature gauge, but it's up and running. I'm amazed by how much quieter it is than the old system. I was never happy with how loud the original boiler was. The installer made a point of telling me I "made a mistake" by not going with the next size up boiler, because if it gets really cold and I need to heat the house up this may not be able to keep up. Kind of what you predicted, Dana. I'm not worried.

    I asked him where he typically sets the max temperature, and he said 190, which is what it will heat to when the hot water heater calls or when the outside temp is below 32F. I though you suggested that the max should be set at 170. Should I ask him to change it to 170?
     
  15. Dana

    Dana In the trades

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    163' of baseboard with 170F entering water temp (EWT) or about 160F average water temp (AWT) would emit ~73,500 BTU/hr, approximately the full boiler output.

    At 190F EWT / 180F AWT it emits about 95K, which is more than the boiler's output. In other words it won't manage to stay at 190F when all zones are calling for heat.

    The default setting for the -IQ boiler controller is 180F, which would deliver ~170F AWT for about 82,000 BTU/hr out of 163' of baseboard, which is more than enough.

    With a multi-zoned system the more important features for efficiency will be the heat purge timing and differential setting parameters in the -IQ control settings. See page 5 of the manual. The default pre-purge is 2 minutes, but if you open that up to 10 minutes some calls for heat from smaller zones may be satisifed by the remaining heat in the boiler, not firing the burner at all. The max setting is 20 minutes, but that's long enough that under colder conditions it might start to get cold. The default differential is 15F, which means it re-fires when the temp drops 15F below the high limit. You'll get better efficiency and fewer burns out of it when cycling if you open that up to the max 30F differential, so that it won't re-fire until the boiler temp hits 30F below the high limit. So with the high limit set to 190F and it's serving a smaller zone the off times between burns will be more than twice as long, and the average temperature of the boiler will be lower, for lower standby loss.

    There is also a post-purge overrun time parameter that would increase efficiency too, but a long post-purge could end up delivering significant temperature overshoots in the zones/rooms, lowering comfort.

    Rather than bugging the installer about every fine-tuning tweak, it's worth spending a bit of time dialing it in yourself.

    By "...which is what it will heat to when the hot water heater calls or when the outside temp is below 32F" it sounds as if the outdoor reset module option was installed? If yes, there are other parameters to tweak, explained starting on page 26 of the manual.
     
  16. Peter Noble

    Peter Noble New Member

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    Those adjustments make sense. And it seems like it'll be pretty simple for me to do the fine tuning myself. I'd prefer to set the max temp to 170F unless it'll hinder hot water heating.

    And it does have the outdoor reset option card installed. What adjustments different from the defaults would you recommend?
     
  17. Ron Beck

    Ron Beck New Member

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    If you installed ODR card the boiler water temp will be 190f at "0" degrees outside not 32f unless they changed that parameter.
     
  18. Dana

    Dana In the trades

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    For one end of the reset curve, use the fuel use load calculation and the radiation sizing figure out roughly what the average water temp (AWT) needs to be to cover the load at your design temp , and set the ODR curve to deliver a water temp ~10F above the estimated AWT when the outdoor temp is at your design temp. (It's unlikely to ever need 190F water, even at 0F or below. )

    Then figure out at at what outdoor temp your heat load hits the amount of heat that's emitted at an average water temp of 150F and make that the other end of the curve. If a system bypass or boiler bypass was installed you may be able to drop that a bit, but the goal is to keep the entering water temp at the boiler to never be chronically below 140F.

    If the boiler doesn't short cycle at the lower-load higher outdoor temp end of curve you can fine tune it a bit from there, perhaps dropping it a bit lower. When colder weather arrives if it's not keeping up, adjust the water temp of the high-load /lower outdoor temp end of the curve up by 5F and see how it does.
     
  19. Peter Noble

    Peter Noble New Member

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    Thanks, Dana, but I'm not sure I understand. My design temp (99% dry bulb temp) is 11F. Sounds like the low outdoor temp should be set to 11F, and the boiler set point should be 180 (170+10). Is that correct? Not sure how I calculate the heat for 150F water temp. And is the "other end of the curve" the High Outdoor Temp? Hope you can shed some light on translating your instructions to the descriptions on in the manual. Here's the instructions for the IQ Reset Option Card.
     

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  20. Dana

    Dana In the trades

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    At an AWT of 170F just the 163' of baseboard emits 73,500 BTU/hr @ 11F. Your fuel-use estimated heat load was ~51K@ 11F, or 943 BTU/degree-hour. So you you probably don't need or want it to hit 170F AWT until 65F- ( 73,500/943)= -13F or so. In the default settings that would be:

    SP ............ 180F


    Lo ............. -13F (the default is 0F)

    Hb ............180F

    Either that, or set the water temp at +11F to where the 163' of baseboard is emitting 51,000 BTU/hr, which would be about (51,000/163'=) ~ 315 BTU/hr per foot. For typical fin-tube baseboard that happens at an AWT of about 140F (145F out, 135F back, or thereabouts.) According to the manual:

    "The MPO-IQ is designed to withstand thermal shock from return water temperatures as low as 100°F, but prolonged return temperatures of below 135°F can cause excessive flue gas condensation and damage the boiler and/or venting system.''

    Since you don't want the entering water temp to run below 135F chronically, the lowest output temp should probably be ~145F to see how it behaves. If there is a system bypass or boiler bypass branch in the near-boiler plumbing that may be tweaked to still do the right thing as the load falls with higher temps, still keeping the return water temps in a reasonable range (TBD.)

    The lowest High Outdoor Temp the reset controller allows is 35F so that's what you should set it to:

    Ho ............. 35F

    It's not clear to me whether it's the Low Boiler Temp or the Minimum Boiler Temp that controls the output temperature at the High Outdoor Temp (it's probably described in some other part of the manual) so for now leave it at the defaults and see how the system behaves early in the season:

    Lb .......... 110F (default)

    Lt .......... 130F (default)

    Since the outdoor reset control is integrated with the IQ controller it may still do the right thing under light load, which would be to not fire until it reaches the Lb temperature when it's not very cold outside when the thermostat first calls for heat, but run the temp up to only 10-15F above the Lt temperature during a continuous call for heat, but refiring only when it drops back to the Lt temperature as it's cycling. So with an Lt of 130F the entering water temp would drop below 135F when cycling under light load, it would always average above that temp. (This may or may not be described in a manual somewhere, but we can figure it out by observing the behavior.)

    Your load at the Ho temp of +35F is about 943 BTU/hr x (65F -35F)= ~28,000 BTU/hr which is less than half the output of the boiler, so it's going to cycle on/off no matter what, but as long as it's not short cycling that's fine. If the average output temp during cycling is 140F the 163' of baseboard would be emitting about 51K (as previously estimated), so when it's +35F out with a load of barely more than half that it should satisfy the thermostats fairly quickly, but not the hot-flash/chill scenario you'd get with an output temp of 180F. As the outdoor temp drops below 35F it would start bumping up the boiler's output temp, but not so rapidly that it creates room temperature overshoots. When temps are above 35F it'll still be running with the Lb & Lt numbers, which will satisfy the thermostats even more rapidly at warmer temps, and probably won't run as many cycles (as long as it's not short cycling.)

    It's too warm to be running experiments on it now, but try punching in those numbers, and we'll revisit it during the first chilly week in October to see what it does during light loads.
     
  21. Peter Noble

    Peter Noble New Member

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    This is much clearer, thanks. I will make these adjustments soon and report on how it works when it gets cooler out. Couple questions: In an earlier post you suggested I change the pre-purge from the 2 min default to 10 min, and the high limit differential from the default 15 degrees to 30F. However, if I set the output temp to 145, is that 30 degree differential too much? Or perhaps there are other controls that keep the entering temperature from getting too low.

    I'm looking forward to finding out if the house is more comfortable with 145F water circulating for longer periods than the 190F water (probably where the old boiler was set) cycling frequently.

    One more question: You mention that the boiler will cycle, but that's OK as long as it doesn't short cycle. I'm not sure what constitutes a short cycle. Can you clarify? Thanks,
     
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