Help size a Boiler

Discussion in 'Boiler Forum' started by Reicherb, Oct 3, 2019.

  1. Reicherb

    Reicherb Member

    Joined:
    Nov 7, 2013
    Location:
    Linden, MI
    I used the formulas at https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new to determine my BTU load. My math is below. Can you please help me make sure I'm correct?

    Meter Read on 1/4/2019 7695
    Meter Read on 2/4/2019 7918
    Difference 223

    Energy Use 1/4/2019-2/4/2019 22.3 Mcf Natural Gas
    Energy Use 1/4/2019-2/4/2019 227.46 Therm
    Furnace efficency 80%
    99% outside design temperature 3
    Sum 65 Degree Days 1374.9
    Sum 60 Degree Days 1219.9

    Heat Delivered (227.46therm x 80%) in Therms 181.968
    Convert to BTU 18196800
    Convert to MMBTU 18.1968

    65 degree BTU/Degree Day (18.1968 MMBTU / 1374.9 * 1000000) 13234.99891
    65 degree BTU/Hr (divided by 24 hours) 551.4582879

    60 degree BTU/Degree Day (18.1968 MMBTU / 1374.9 * 1000000) 14916.63251
    60 degree BTU/Hr (divided by 24 hours) 621.5263546

    Heat Load at 65 in BTU/Hr ((65-3)*551)) 34162
    Heat Load at 60 in BTU/Hr ((60-3)*621)) 35397

    Sizing Factor 1.4

    Size Needed at 65 degrees 47826.8
    Size Needed at 60 degrees 49555.8

    I live in central Michigan. I have 3 zones that consist of:
    Zone 1:
    Semi finished basement
    836 square feet
    Family Area - 11' of baseboard registers
    Scrapbook Room - 5' of baseboard registers
    Laundry Room - 8' of baseboard registers

    Zone 2:
    1st Floor
    836 square feet over basement and 140 square foot on a slab
    Bathroom - 5' of baseboard registers
    Mud Room - 6' of baseboard registers
    Kitchen - 0' of baseboard registers
    Dining Room - 8' of baseboard registers
    Play Room - 21' of baseboard registers
    Living Room - 12' of baseboard registers
    Hallway - 6' of baseboard registers

    Zone 3:
    2nd Floor
    836 square feet
    Bathroom - 5' of baseboard registers
    Master bedroom - 25' of baseboard registers
    Bedroom 2 - 20' of baseboard registers
    Bedroom 3 - 20' of baseboard registers
    Office - 10' of baseboard registers

    I'm looking to go with a combi unit for heat and domestic hot water. Is 50,000BTU the right size?

    Thanks!
     
  2. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    Since the meter readings didn't occur at 12:01AM on 1/4 and 11:59 PM on 2/4 there's an extra day's worth of HDD being applied to that fuel use, which for a period that short could introduce a relevant amount of error. One way of reducing that error is to average the HDD from 1/4 through 2/3, and the HDD from 1/5 through 2/4.

    After making those corrections the load numbers are going to be about 3% higher (possibly more, depending on the averege temp on 1/4 and the average temp on 2/4), so let's call it ~35,000 BTU/hr using base 65F , and ~36,500 BTU/hr using base 60F.

    But (without punching in the numbers myself) the rest of it looks right, assuming the place was heated to 68-70F for part of the day, and there weren't any ultra-deep setbacks in use.

    Also, "...Heat Load at 65 ..." etc. is conceptually incorrect. When using base 65F as the HDD base it means the heat load at 65F is ZERO. If using 60F as the base temperature the heat load at 60F is ZERO. The base temperature is selected to be the temperature closest to the average heating/cooling balance point, to establish the zero-load temperature to work from when doing the linear approximation model based on BTU/hr per degree-F. The load numbers derived using those two base temperatures are both the heat load at +3F, approximated from a presumed 60F or 65F balance point.

    Zone 1 has 24' of baseboard, which at condensing temperatures emits about (200 BTU/ft-hr x 24'= ) 4800 BTU/hr.

    Zone 2 has 52' or baseboard, which at condensing temps emits about 10,400 BTU/hr.

    Zone 3 has 80' of baseboard, which at condensing temps emits about 16,000 BTU/hr.

    It looks like you have enough baseboard to run in at mid-90s efficiency condensing mode more than 90% of the time, since all together it can deliver 31,000 BTU/hr at an average water temp of about 120F, but the outdoor reset curve would need to be set to bump it up to a temp delivering about 92% efficiency when it's +3F, and lower still when it's in sub-zero territory.

    A single low-flow 2 gpm shower in the middle of winter when the incoming water is 40F takes about 65,000 BTU/hr out of the combi boiler. To have any margin at all for other simultaneous hot water use you'd want to at least double that to 130,000 BTU/hr (/95%= 137K-in) assuming its a one-bathroom house. If you need to support two showers while also running a load of laundry you'll need a 199K ( input) burner if it's a low-mass wall hung combi boiler. The only way around that is to use a tank-type combi, so that the peak BTU/hr draws are pulling heat out of the tank rather than just the burner.

    The other argument against a low-mass combi in your case is the limited amount of radiation on the zones, in particular zone 1, but also zone 2. Most 199K combis have a minimum output of about 18-20,000 BTU/hr, or about 4x the amount of heat that zone 1 can emit when operated in condensing mode (in fact even at 180F temps out of the boiler zone 1 could only emit about 12,000 BTU/hr). Even the smallest low mass combi boilers only throttle back to about 11,000 BTU/hr which would be fine for zones 2 & 3, but those smaller burners would have issues supporting the domestic hot water loads. For the napkin math and further explanation of the problem see this.

    Bottom line, low-mass combi boilers are only a good fit for homes with low to moderate hot water needs, and HIGH heat loads, and the radiation to deliver the higher heat rates. Your house is in the low to moderate range for both hot water and heat, with radiation to suit, making it a lousy fit for a low mass combi boiler. There are several modulating condensing boilers that can throttle down to under 8000 BTU/hr that can still deliver more than the ~50,000 BTU/hr needed to cover the load during Polar Vortex disturbance cold snaps, but they aren't combi-boilers.

    A 50-80K mod-con boiler + indirect water heater is usually the "right" solution. In your case you have enough total radiation to emit about 40,000 BTU/hr at a typical water heater storage temp of 140F, and you could use a condensing water heater with a ~75K burner using it for both heat and hot water, bumping the storage temp up to 150-160F (cutting into condensing efficiency) when the forecast is for outdoor temps in negative double-digits.
     
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  4. Reicherb

    Reicherb Member

    Joined:
    Nov 7, 2013
    Location:
    Linden, MI
    I'm struggling to fully understand. I'm wondering if I'm better off to keep the boiler and water heater separate units.

    We are a family of 4 with 3 full bathrooms and 2 soon to be teenagers.

    Thanks.
     
  5. Reicherb

    Reicherb Member

    Joined:
    Nov 7, 2013
    Location:
    Linden, MI
    I used the Weil McLain calculation tool and came up with 55,000 BTU. To be save and o ensure I can install it successfully I'm leaning toward the CGA-4. Is this so overkill that I'm going to lose efficiency?

    Thanks.
     
  6. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    With Weil-McLain tool is an EXTREMELY crude IBR type calculator. A fuel use method is a load MEASUREMENT.

    You real load is about 35K (including standby and distribution losses) , W-M says 55K, so if you installed a cast iron boiler that had exactly 55K (DOE) output you'd be at 55K/35K= 1.57 x oversizing. That's less than the 1.7x oversizing assumption behind AFUE testing parameters, so it would probably still meet it's efficiency numbers despite being bigger than ASHRAE's 1.4x oversizing recommendation.

    But the CGa-4's DOE output is 88K which would be (88K/35K=) 2.5x oversizing, which would be starting to slip over the knee in the efficiency curve. If you're insisting on a CGa series cast iron, the CGa-3 at 59K-0ut would be (59K/35K= ) ~1.7x oversized, perfectly aligned with the AFUE test, but the CGa-2.5 at (44K/35K=) 1.26x out would be more comfortable.

    All total you have 156' of baseboard, which at 170F AWT (180F-out, 160F-return) delivers about (500BTU/ft-hr x 156'=) 78,000 BTU/hr, which means the CGa-4 is even slightly oversized for the radiation. Don't do it! It will almost certainly short cycle on zone calls if you go that big.

    With all zones calling for heat simultaneously for an extended period of time the CGa-3's 59,000/156= 378 BTU/hr per foot would result in an AWT of about 145F-150F, which would be well above condensing temperature danger, thus no bypass branch protection would need to be designed in. With the CGa-25's (44,000/156=) 282 BTU/hr per foot it would stabilize at about 135F AWT, which if pumped slowly could result in the entering water temp at the boiler being on the threshold of condensing, so it would be prudent to install a system bypass near the boiler to mix a small amount of direct boiler output into the return water entering the boiler. As long as the EWT at the boiler is 130F there's zero chance.

    Simpler still, for about the same money as the CGa-4 you could install an inexpensive all stainless fire tube modulating condensing boiler, which in your system could be pumped direct (no primary/secondary needed, no bypass plumbing needed.) The Westinghouse WBRUNG080W (= HTP UFT-080W) can be had for about $1600-1800 at internet pricing. That boiler can modulate down to about 7600 BTU/hr out , which means zones 2 & 3 would have no cycling issues whatsoever, and zone 1' would only have (7600-4800=) 2800 BTU/hr, or (/60=) 47 BTU/minute of excess heat going into the system when only zone 1 is calling for heat.

    The heat exchanger in that boiler has about 26lbs of water in it, and there's probably at least another 10lbs of water in the zone-1 radiation + return plumbing, call it 35lbs. With 47 BTU/minute of excess going into the 35lbs of water the temp is rising by 47/35= 1.34F per minute. Even if programmed for a 5F overshoot before quelling the burner the minimum burn time would be 5F/1.34F= ~3.75 minutes long. If set up for an 8F window it would be 8F/1.34F= ~6 minutes long, neither of which is a short cycle for a mod-con.

    With the outdoor reset curve dialed in the odds of calls for heat from zone 1 overlapping in time with calls for heat from one or both of the other zones is high, so even with the stubby zone radiation on zone 1 there should be minimal on-off cycling if you take the time to optimize the curve for comfort + efficiency.

    At max fire at non-condensing temperatures the WBRUNG/UFT-080W boiler puts out about 70,000 BTU/hr, which is more than the CGa-3.

    Another aspect of that particular boiler is it comes pre-plumbed with a second port and associated controls for running an indirect water heater. It's EASIER TO INSTALL than a CGa boiler (!), with or without an indirect.

    There are similar fire tube mod-con boilers in that size range that could work too, eg Lochinvar KHB/WHB-085 (which has less thermal mass to work with than the UFT), as well as some water tube mod-con boilers that could be made to work such as the Navien NHB-80.

    With three full bathrooms you'll probably want to have a bigger indirect water heater than a cheap 30-40 gallon version. A 60 gallon would pretty much cover it unless you have a big soaker tub to fill. With a WHB-085 or UFT-080W (or CGa-3) behind it the recovery rate would still be faster than a 50 gallon standalone if set up at the priority zone.

    If yours is a showering family rather than tub-bathers it can be useful to install a drainwater heat exchanger, if you have at least 5' of vertical drain down stream of the main shower(s). The current best-in-class is EcoDrain's V1000 series. A 4" x 54" version will return nearly 60% of the heat going down the drain back into the incoming water stream, adding "apparent capacity" to the water heater, and saving substantially on the water heating fuel use. With one of those and a UFT-080W sized boiler it would support two simultaneous showers 24/7, independently of the size of the indirect water heater tank.

    [​IMG]

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    Last edited by a moderator: Feb 5, 2021
  7. Reicherb

    Reicherb Member

    Joined:
    Nov 7, 2013
    Location:
    Linden, MI
    My concern is that I simply don't fully understand everything you are saying. Primary/secondary/bypass loop, condensing concerns, programming the unit, etc. I'm not sure I'm going to get it right.

    I have a 1966 Weil Mclain PD7 boiler that/was outputting 80,000 BTU. I have no programming to do, no bypass/secondary loop, and I assume there is no condensing issue.

    The CGA series is so similar that I feel like I can't get it wrong other than choosing the wrong size. I do understand that I'm going to lose efficiency but how much cost will there be to that compared to the risk of specing/installing a more advanced unit wrong.
     
  8. Reicherb

    Reicherb Member

    Joined:
    Nov 7, 2013
    Location:
    Linden, MI
    What is this? just a manifold?

    Should I have a pressure/temp gauge with the new boiler?
     

    Attached Files:

  9. Reicherb

    Reicherb Member

    Joined:
    Nov 7, 2013
    Location:
    Linden, MI
    I've got 3 wire zone valves and 3 wire programmable thermostats. I don't understand the difference between 2 wire and 3 wire. Why should I stick with 3 or move to 2?
     
  10. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609

    The PD-7 boiler takes 150K-in, and delivers 120K out (80% efficiency). Ignore net-IBR ratings which are an attempt to estimate the distribution losses when the boiler is outside of conditioned space. Those losses are already accounted for in the fuel-use load calculations (since there is no way to separate them out. Within the CGa series the closest model would be the CGa-5, but it's so oversized for your radiation that would be a mistake on several fronts.

    Condensation on the heat exchangers is only an issue when down-sizing to the point that there is excess radiation relative to the burner size. With your current setup you have the opposite problem. An oversized PD-7 with 120KBTU of output driving only 156' of baseboard condensation in the boiler is never an issue, but since the baseboard can't emit (120,000/156'=) 770 BTU/hr per foot, excessive cycling (particularly on zone calls) becomes the issue. But with the low duty cycle that happens when the burner output is so oversized relative to the load there is a somewhat increased risk of flue condensation during cold weather, especially if the flue is is also oversized for the burner. Those issues are an even bigger risk when moving to the CGa series. Down sizing to a right-sized the boiler (for either the radiation, or for the design heat load) can increase the duty cycle, but the flue also needs to be right-sized.

    Mid efficiency gas boilers such as the CGa series (83-84%) have cooler exhaust than the 80% efficiency boilers, which increases flue condensation risk, which is why no matter which one gets installed a right-sized flue liner should be installed. When the boiler right-sized for the heat load (rather than merely for the radiation) it runs a much higher duty cycle, which keeps the flue warmer, and less prone to flue condensation.

    Unless you're changing up the whole control scheme let's not steal a defeat from the jaws of victory here. If the zone controls are working as-is there is no point to changing from 3-wire to 2-wire zone valves.

    If it's "CGa or Bust" and you don't want to get hung up on the condensation risk mitigation that comes with a CGa-25, go with the CGa-3, and the appropriately sized flue liner. As stated previously, it's about the same oversize factor as the AFUE test- it will hit it's efficiency numbers just fine, and there isn't enough baseboard on the system to put it at risk of condensation on the flue plates. The CGa-4 would short cycle more on zone calls, and wouldn't really buy you much more heating margin on colder outdoor temperatures- it would only make sense if there was a risk of -75F cold snaps or something. The CGa-3 has the load covered all the way down to about -50F- how often does that even happen (if ever)?
     
  11. Reicherb

    Reicherb Member

    Joined:
    Nov 7, 2013
    Location:
    Linden, MI
    Thanks. CGA-3 with a flu liner and 3 wire zone valves is the plan.
     
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