Supply and return sizing

[Dennis Leonard]

I am a licenced electrician and my neighbor is a licenced pipefitter and we are replacing my old Weil Mclain boiler.
Ordered new Weil Mclain CGa-4 boiler and Grundfos Alpha pump. Now I am in the process of getting the black pipe and fittings.
Existing 3 zones are 3/4" with 1" headers. The new boiler is tapped at 1 1/4" and the pump has a 1" opening.
Should we change the header to 1 1/4" or can we keep it at 1"?

Thanks,
Dennis in Dewitt, Michigan

 

[Dana]

Assuming it hasn't arrived yet (and there may be time to change it), why are you installing the CGa-4 rather than a smaller boiler? The 88,000 BTU/hr output of that boiler is way too high for about 90% of all homes in MI. Realistic heat load numbers for the average house in MI would likely come in under 50,000 BTU/hr even for older houses (pre-1975) and under 35,000 BTU/hr for many newer ones that might be coming up on boiler-replacement time. Most homes that currently have a CGa-4 installed would be better served by a CGa-25, at half the rated output.

Some ~3000' code-min new houses come in with heat loads in the mid-20,000s @ 0F. The 99% outside design temp in Lansing is +2F, so yours probably isn't sub-zero. But your load may be over 30,000 BTU. And there's a way to figure that out without performing a manual-J load calculation.

Assuming you have a heating history with the old Weil Mclain, you can use fuel use against heating degree-days to come up with the true heat load of the place using the old boiler as the measuring instrument. If it's as oversized as the CGa-4 for the likely load, it probably never delivered it's AFUE or steady state efficiency, but using the nameplate efficiency for the calculation provides adequate oversizing margin for a replacement boiler.

To run this type of calculation we'd need the ZIP code for both degree-day weather data and the outside design temps, and at least one (but preferably 2 or 3) mid-winter billing periods of fuel use, based on the EXACT meter reading days, to align fuel use with the appropriate heating degree-days.

For a random point of reference, my 2400' sub-code 2x4 framed antique comes in a bit under 40,000 BTU/hr @ 0F- that boiler would be more than 2x oversized for me, and would never make it's efficiency numbers.

The recommended oversizing factor by the ASHRAE is 1.25x, (which is PLENTY of cover for the Polar Vortex extremes) , and the AFUE test assumptions are 1.7x, which is just before the knee of the curve where efficiency starts to take a dive. The larger the oversizing factor, the bigger the efficiency hit. At e 2x oversizing it's measurable but not a total disaster. At 3 x oversizing the as-used AFUE will be ~10% less than the tested & labeled AFUE.

The amount and type of radiation on each of the 3 zones matters too. How much baseboard, radiator, hydro-air on each zone, by zone? The smaller the zone radiation (and the larger the number of zones) relative to the boiler output, the greater the overall cycling and distribution losses are incurred, which also favors down-sizing the boiler whenever you can. A boiler that's short cycling due to being oversized for the radiation can be operating 10-15% off it's labeled AFUE too, and that's even with reasonable oversizing factors for the whole-house load.

Boiler replacement time is a once in every 20-40 years opportunity to get it right. When you do get it right you'll be more comfortable, using less fuel.

 

[Sponsor]

 

[Dennis Leonard]

Assuming it hasn't arrived yet (and there may be time to change it), why are you installing the CGa-4 rather than a smaller boiler? ....Most homes that currently have a CGa-4 installed would be better served by a CGa-25, at half the rated output.

The amount and type of radiation on each of the 3 zones matters too. How much baseboard, radiator, hydro-air on each zone, by zone? .

Thank you for your reply. We have not received the boiler yet so it's not too late to change size.
Perhaps the CGa-4 is too large. House built in 1972. New windows 2013, 12" additional insulation in attic, basement concrete blocks insulated with 1.5" Owens Corning Foamular, lots of other upgrades.
Sq. ft. of heated space - 2750
Existing boiler: Weil McLain P-CG-5 (gross output 96K, net I-B-R 83.5K)
Heat loss calculation (Slant Fin app) is 50K, total Slant Fin baseboard is 91 feet, and pipe length is ~225'. Fuel type = Propane
Per zone: Z1 = 47 (Living and dining room), Z2 = 27 (bedrooms, baths), Z3 = 17 (basement)
Would the CGa-3 be better? Please advise.

 

[Dana]

Slantfin's I=B=R calculator typically overshoots reality by 25-30% or more, with excessive infiltration loss assumptions, and no compensation for the 24/7 plug loads and mammalian occupant output, etc. If Slantfin's tool comes up with 50K, reality is probably 40K, but could be lower. As dumb none-too accurate rule of thumb sanity check, 50K/2750' of conditioned space is 18 BTU/hr per square foot of conditioned space. Most 2x4/R13 homes with replacement windows, an R50+ attic and an insulated foundation will come in around 15 BTU/hr @ 0F, which would be about 41K for a 2750' house. This can be verified if you have a fuel-use history on the place.

But that won't much affect the boiler choice, since the pickings are a bit slim for output below 50K.

Looking at your radiation, the shortest zone being 25' of fin tube, even at 200F average water temp it's not putting out more than 750 BTU/hr per running foot, or (25' x 750=) 18,750 BTU/hr, which isn't even half the output of the CGa25 let alone the 59K of the CGa-3. The other zones aren't much better, at 30' (22,500BTU/hr) or 35' (26,500 BTU/hr) respectively. At 180F AWT you'd be looking at only 575-600 BTU/hr. Even running the system at a smoking-hot 210-215F, and 800 BTU/ft-hr, with 90' of baseboard the most you'd ever get out of the radiation is 72,000 BTU/hr, even with all zones calling for heat, which rules out the CGa-4 completely. The CGa-4 won't heat the place any faster than the CGa-3, since you are radiation constrained.

But while the radiation can balance with the ~59K output of the CGa-3 with ALL zones simultaneously calling for heat, it'll be cycling on/off even during continuous simultaneous calls for heat from any pair of zones, and short-cycle significantly with only one zone calling for heat, since any one zone can only emit about 1/3 of the boiler output.

Odds are the calls for heat in the basement zone are short & rate, so primarily it has to balance with the two main zones, which would hopefully have overlapping calls for heat, which makes the CGa-3 far less desirable. The CGa-25 would be able to balance with 2 zones simultaneously calling for heat, and with the radiation capable of emitting roughly half the boiler output the burn cycles may be long enough to not put a lot of wear & tear on the boiler or short-cycle it into much lower as-used AFUE efficiency.

At the GGa-25's 44,000 BTU/hr of output with 65' of baseboard on the main zones that's 44,000/65'= 678 BTU/hr per foot of radiation, which would balance at something 190F-200F AWT, which is fine. You're in no danger of the return water coming back from radiation at damaging condensing temperatures. Even with the full 90' active that'a 489 BTU/hr per foot, which balances with 170F water. Only when the return water could be below 130F do you have to take measures with the near-boiler plumbing to protect it from condensation.

Looks to me like the GGa-25 is the best choice in that line-up. It's output is below the I=B=R calculated heat load so it would be good to verify the heat load with a fuel use against heating degree-day calculation, but you would probably have at least a 1.1x sizing factor (10% of margin). If (as I suspect is the) you've tightened the place up enough that the heat load is more like 36-38K you'd have over 20% of margin.

BTW: If the pink foam in the basement isn't covered with a thermal barrier against ignition it's a code violation due to the fire ignition/spread hazard. A non-structural 2x4 wall with unfaced or kraft faced R13s and wallboard (painted or not) on the interior side of the foam would bring up to IRC 2015 thermal performance for your US climate zone 5 location, as well as the thermal barrier against ignition. Depending on how much above-grade foundation you have that could be another 2-5K of heat load reduction.

 

[Dana]

In case you're worried that the 44,000 BTU/hr output of the CGa-25 might leave you cold, consider this:

With only 65' of baseboard on the main zones, at the water temps most systems are being run at (170-180F AWT), the baseboard can only deliver ~35-40,000 BTU/hr. If the system was able to keep up with the Polar Vortex events of the past couple of winters, the CGa-25 would also be able to keep up with the living space no matter what. During extreme cold events the worst that would happen is that you may need to lower the temp in the basement to something less than 68F to keep the other two zones at 70F so that the boiler is only serving the upstairs zones.

Odds are that even that won't be necessary, but it's much better for both efficiency and wear and tear on the boiler to not oversize it for the radiation. The CGa-3 would be ~3x oversized for any one zone, but the CGa-25 would be only(?) 2x oversized for any single zone, and would see far fewer and somewhat longer burn cycles, and would likely hit it's AFUE numbers.

Once it's up and running, as part of the commissioning process it's worth measuring the minimum burn time and burns per hour for when serving a single zone. Setting up the boiler controls for the maximum possible differential (and at a fairly high temp) would help on both burn-time and numbers of burns. Burn lengths shorter than 5 minutes deliver a measurably lower efficiency, and more than 5 burns/hour puts wear & tear on the ignition system, but depending on the actual thermal mass on the zone plumbing you may still do OK despite 2x oversizing for the radiation , but it's probably not possible not at 3x. If the measured numbers indicate that it'll be doing a lot of short-cycling, it will be worth installing a heat-purging economizer control (a ~$200 part, easily DIY-able for a licensed electrician) to maximally utilize the available thermal mass. This is roughly how it pencils out based on best-guesstimates.

The boiler itself has 1.5 gallons, or abou 12 lbs of water in it plus about 17-18 lbs "water equivalent" of thermal mass in the cast iron. Assuming it's all 3/4" copper, and the main zone radiation + plumbing adds up to about 100', you have another ~20 lbs of water in the zone plumbing, for a total of about 50 lbs of thermal mass.

If the differential from low limit to high limit is set to 20F, it takes 50 lbs x 20F= 1000 BTU of excess boiler output to make that swing. At 44,000 BTU/hr of boiler output, with the zone emitting about 20,000 BTU/hr, that's 24,000 BTU/hr of excess, and it only takes 1000/24,000 of an hour to slew the system temp 20F, which is 2.5 minutes. With a heat purging control you can get much bigger low-to-high swings than with most aquastat type boiler control setups, so if you program the low-limit to 130F (so that it won't condense inside the boiler) and leave the high limit at 180F or higher, you should be able to get 5+ minute burns out of the CGa-25 with the economizer control under all load conditions.

But with the 59,000 BTU/hr output of the CGa-3 you would have ~40,000 BTU/hr of "extra" heat being dumped into the zone plumbing in stead of 24,000 , and would end up with (60 x 1000/40,000=) 1.5 minute burn times at a 20F differential, cutting severely into efficiency. An economizer control would still cut the number of burns, but would not be able to lengthen the minimum burn times to more than something like 3-4 minutes. While that's a heluva lot better than 1.5 minutes, it isn't long enough to hit it's efficiency numbers. To make the CGa-3 hit it's numbers you'd either have to increase the radiation, or increase the thermal mass.

This is just the napkin math version of the basic problem but it's close enough to reality to know that with your radiation you'll be lot better off with the smaller boiler.

 

[Dennis Leonard]

Wow! Thank you so much for the terrific evaluation. However, I called in the order with my guy at the supply store and the CGa-4 is sitting on his dock. If I tell them I don't want it, I'll get slapped with a 25% ($450) restock fee.
I'm struggling with this decision. What would you do?

PS-Pink Foamular is covered with drywall.

 

[Dana]

The CGa-4 would have ultra-short burn times on zone calls, with sub-minute minimum burn times. The -4 has about 5lbs more of thermal mass but that's not nearly enough to help the fundamental problem. If you installed it now to save the $450 you'd be paying more than $100/year in extra fuel use & maintenance forever. Even if you combined the two main zones it would be 2x oversized for that zone, and would need more thermal mass to make it all happy.

Or, you can keep it, spend a bunch of money replacing all the baseboard with sufficient flat-panel radiator to handle the 88K of boiler output without short cycling, and live in the lap of higher-mass lower temp radiation luxury. (But we'd be talking several thousand dollars to get there.)

Personally, I'd put something much smaller in there. If the original boiler had that much output into that small amount of baseboard it's as-used efficiency had to fall well short of it's DOE nameplate numbers, but that's a shockingly common thing to see.

Before you make any other decisions on the system, how are you heating your domestic hot water?

Do you have propane fill-up dates & quantities so that we can run fuel use against heating degree-days to get to the actual heat load?

Is the drywall tight to the foam, or is it on a studwall? (Retrofitting blown fiberglass into a studwall can be done without ripping it all apart, using a box-store rental blower.)

 

[Dennis Leonard]

Ok. I will eat the restock fee to get the right size. My mistake.
Wife is home all day and likes her feet warm so all 3 zones will be open all the time.
Existing boiler: Weil McLain P-CG-5 (gross output 96K, net I-B-R 83.5K)
Hot water is Rinnai tankless installed 5 years ago. Works great.
Will try to find the propane delivery dates for last year.
Drywall is on furring strips over Foamular along 90% of the walls. Other wall coverage is framed AND insulated.
Thank you very much.

 

[Dennis Leonard]

Dana,
I went back and rechecked my baseboard lengths. Gave you the wrong ones earlier. I apologize.
Per zone: Z1 = 47' (Living and dining room), Z2 = 27' (bedrooms, baths), Z3 = 17' (basement) TOTAL = 91'

Propane usage:
9/23/14 - 12/23/14 = 180 gallons (used quartz heaters until boiler start up on 10/22)
12/24/14 - 2/23/15 = 350 gallons
2/24/15 to present = 250 gallons

(paraphrasing)...the shortest zone being 17' of fin tube, even at 200F average water temp it's not putting out more than 750 BTU/hr per running foot, or (17' x 750=) 12,750 BTU/hr, which isn't even one third the output of the CGa25 let alone the 59K of the CGa-3. The other zones aren't much better, at 27' (20,250BTU/hr) and 41' (30,750 BTU/hr) respectively. At 180F AWT you'd be looking at only 575-600 BTU/hr. Even running the system at a smoking-hot 210-215F, and 800 BTU/ft-hr, with 90' of baseboard the most you'd ever get out of the radiation is 72,000 BTU/hr, even with all zones calling for heat, which rules out the CGa-4 completely. The CGa-4 won't heat the place any faster than the CGa-3, since you are radiation constrained.

Will the corrected zone lengths and propane usage have any bearing on choosing the CGa-25 or CGa-3?

Thank you for your help.
Dennis

 

[Dana]

Almost no systems are actually running 200F AWT (210F out, 190F back) but you'd have to run there or a bit hotter to actually get the 750 BTU/ft-hr out of the baseboard. Most systems are set up to run ~180F-190 AWT with no more than 600 BTU/ft-hr from the baseboard.

At 59,000 BTU/hr output and 180-190F AWT (= ~600 BTU/ft-hr out of the baseboard) the CGa-3 almost 4x oversized for the 27' ( x 600 BTU/ft-hr = 16,200 BTU/hr) bedroom zone radiation, and almost 6x oversized for the 17' ( x 600 BTU/ft-hr = 10,200 BTU/hr) basement zone radiation system operation. If you haven't already, take a peek at a spec for Slantfin baseboard output. Be mindful that those numbers are for unobstructed clean baseboard, not the baseboard behind the couch full of dust-kittens. Many (most?) systems running baseboard at 180F AWT are only delivering ~550 BTU/ft-hr.

This steers you even harder toward the 44,000 BTU/hr output CGa-25. Assuming 600 BTU/ft-hr it takes 73' of baseboard to balance with the output of the CGa-25 and ideally you'd have at least half that amount on any individual zone. If there's room to reasonably add more baseboard to the stubby basement zone it's probably worthwhile, even with the CGa-25. Adding proportionally (to maintain room-by-room temperature balance) to the bedrooms may be worth it too, depending on the system behavior once it's up and running.

It would take double the total amount of radiation (all zones) to have a condensing hazard at 44K of output. Adding length reduces the number of burn cycles and increase the lengths of the burns, and it also heats up the zone more quickly.

Whoever hooked up a 5 plate boiler with 96K of output to only 91' of baseboard (cut up into 3 zones, no less!) wasn't particularly interested in hydronic heating design. That's pretty typical of homes where the potable hot water was sourced by an embedded coil though- sizing the boiler big enough to deliver 3-4 gpm of hot water AND the the full space heating load simultaneously.

If the domestic hot water wasn't being served by the tankless it may have been worthwhile to install a reverse-indirect HW heater/buffer tank between the boiler and the zones, in which case you'd then have 200lbs+ of additional thermal mass to work with in the buffer tank, even with the smaller ones. That would be enough to make the CGa-3 work efficiently on your system without touching the radiation, but it would take a bigger buffer or custom controls to make it work well with the CGa-4. While the $450 restock fee buys a decent fraction of a 25-30 gallon reverse-indirect/buffer tank's cost, a 60-80 gallon unit would be pretty steep, and your hot water is already taken care of.

 

[Dennis Leonard]

Thank you very much for your fantastic replies. We will be working on the project in three weeks.

 

[Dana]

I should have brought this up before, but it occurs to me that the monster 5-plate 80% combustion efficiency boiler is probably vented into a terra-cotta lined flue. At efficiencies over 83% the flue will need a properly sized metal flue-liner to limit and manage flue condensation.

The size of the flue liner for the lower-BTU output boiler is much smaller than what would have been appropriate for an 80% efficiency 120KBTU/hr-in boiler like the P-CG-5.

Even the 84% efficiency CGa-4 would have needed a flue liner, even if the terra-cotta flue was sized correctly, and the CGa-25 would DEFINITELY need a narrower liner.

It may be better/cheaper overall to go with a powered side-vented unit. I'm not sure all of what's out there in cast-iron that would be both power-vented and reasonably sized for your radiation. The 3-plate Burnham ESC series is ~60K-in/52K out, and already comes outfitted with smart controls which will automatically heat purge and limit the number of burn cycles, but it would be better if you had sufficient radiation on each zone to deliver more than half the output (26K) into each of the zones. That's something like 40-45' per zone. If it's a bit shy of that or if the calls for heat always overlap it wouldn't be a disaster.

The ESC3 is something like $500-700 more expensive than the CGa-25, but you also get more. You avoid the cost of the liner, and you wouldn't have to bother retrofitting smarter controls if the system is mis-behaving. If your true heat load is on the order of 40K, it's fairly right-sized per the ASHRAE oversizing recommendations, and if it turns out that 50K is the real load (highly unlikely) you'll still be covered. (I should have mentioned this unit up before.)

With a sealed combustion power vented unit your actual load drops a bit, since you can seal up the old flue, which reduces your overall infiltration rate.

 

[Dennis Leonard]

We have decided to go with the CGa-3 and will be adding more Slant Fin to the basement zone.
Will be installing a chimney liner too.

I have figured out our propane use and the Heating Degree Days for Jan/Feb 2015: gas 350 gal, HDD 2,295. Also know the heat loss is 45,200 (Slant Finn app, water 180* inside 65* outside 6*), baseboard length is 91 feet. What can I do with this info?

Thank you for your help Dana.

 

[Dana]

The SlantFin load calculation app is an oversizing piece of IBR junk, provable by the fuel use:

The source fuel BTU of propane is 91,600 BTU/gallon. Burned at 80% efficiency (the nameplate efficiency of the old boiler) it delivers 0.8 x 91600 BTU/gallon= 73,280 BTU/gallon to the heating system.

So 350 gallons over 2295 HDD, is 0.1525 gallons per HDD

73,280 BTU/gallon x 0.1525 gallons/HDD = 11,175 BTU/HDD

In a 24 hour day, that implies an hourly load of 11,175/ 24= 466 BTU per degree-hour.

The presumptive heating cooling balance point for base-65F is the base temp of 65F- heat load is nil at 65F , and grows linearly below that.

At outdoor design temp of 0F, that's (65F-0F=) 65F heating-degrees, so the implied heat load is then:

65F x 466 BTU/F-hr= 30,290 BTU/hr.

And that's at an outdoor temp of 0F, not +6F.

For the SlantFin's app to be correct that the 45K at an outdoor temp of +6F (only 59F heating-degrees) that ancient hunk of scrap iron would have to be running an efficiency well north of 100%. Or maybe the propane your supplier delivers has as many BTU/gallon as #2 heating oil?

Either of those explanations would violate some fundamental laws of physics- I'm gonna run with the "SlantFin's tool oversizes by at least 25%, sometimes more." explanation.

The 30K number a bit on the low side for most 2x4 framed houses, but for a tight 2x4 framed house with a simple footprint shape (pretty much a rectangle or L shape?) pretty-good windows and an insulated basement it's not out of the question. Most hit closer to 15 BTU/ft^2, you are in the 11s.

If you used overnight setbacks regularly you may want to run that arithmetic using base 60F HDD, which will yield a somewhat higher number, but it'll still be well under 40K, which means you've done a pretty good job with your retrofits!

There's no way that the heat load of the house is anywhere near 45K, unless you skipped out to Belize for a week during that period of time and set all the thermostats to 50F ? Maybe you were heating the first floor with a wood stove or something and just forgot to mention it? If propane was your only space heating fuel, your load is under 40K.

You used some propane for hot water, and receive some free heat from solar gains, but in reality the old boiler probably wasn't hitting the full 80% efficiency either, especially since it was probably short cycling on zone calls. If you were setting the T-stats back to 60F every night and while you were away, your real heat load is no more than 35-36,000 BTU/hr, and if you kept it at 68-70F, it's no more than 31K.

If the real heat load is ~30K, it means the CGa-3 is 2x oversized for the fuel-use implied heat load.

That also means the CGa-25 oversizing factor is 1.47, which is nearly ideal.

If recalculating using base 60F HDD (probably closer to the real balance point for tighter 2 x 4 houses at 68F interior temp, and crummier 2x6 houses), it might be as high as 36,000BTU/hr, but do the exercise.

With a 36K heat load the oversizing factor for the CGa-3 is about 1.66x, which is approximately the AFUE testing oversizing presumption. The oversizing factor for the CGa-25 would be 1.22x which is still sufficient cover for Polar Vortex coolth.

Bottom line- CGa-25 is still the right boiler in that series (especially given your radiation & zoning constraints!) Energy-use math doesn't lie- it doesn't care how your house was built, or what your insulation levels are. It's a measurement, however crude the instrument. It takes a much more refined load calculation tool than the one you used to be more accurate.

 

[Dennis Leonard]

.....Maybe you were heating the first floor with a wood stove or something and just forgot to mention it? If propane was your only space heating fuel, your load is under 40K.
.....and if you kept it at 68-70F, it's no more than 31K.

During the 2014 heating season:

• on the warmer days we used a 1500W quartz heater
• turned the t'stat down to 64* at night
• kept at 68* during the day
• the basement and bedroom zones valves were locked open
• zone valve for the living room controlled the boiler

Thank you for the great analysis.

 

[Dana]

In thinking about your heat load and heat load ratio over the weekend it occurs to me that a large fraction of that 2750' of conditioned space is below grade, which will have a much lower BTU/ft^2-hr ratio than the above-grade part of the house, bringing the average down.

For yuks I tweaked the heat load calc for my house that I keep on file to see what the load & ratio of my ~2400' sub-code 2x4 framed 1.5 story antique would be at 0F if I fully heated the 1500' basement to 68F rather than letting the basement temps float. If I fully heat the whole 3900' of space it comes in at at 41,616 BTU/hr, which is 10.7 BTU/ft^2- hr.

If I let the basement temps drop and only actively heat the 2400' that's above grade it comes in at 32,627 BTU/hr @ 0F, or 13.6 BTU/ft^2- hr, which is in the typical range for tight 2x4 construction, of ~15 BTU/ft^2- hr , +/- 2 or so. There are exceptions outside that band (to both the high and low side) to prove the rule of thumb, which is why relying on a load per-square-foot rule of thumb is always a mistake.

If 800-1000' or more of that 2750' of conditioned space is basement, that ~11 BTU/ft^2-hr ratio and ~30K heat load is right. If literally half the conditioned space is below-grade basement it's a bit on the high side of expectations, possibly due to higher-than average above-grade foundation exposure.

If your setbacks are 64F and daytime temp is 68F using base 60F HDD for the fuel use calculation will be a bit closer to reality than using 65F. But any way you cut it, your heat load is probably going to come in under 35K.

When you use base 60F you have fewer HDD, but a significantly larger BTU/degree-hour constant, and it grows linearly with temperature below 60F instead of 65F.

 

[Dennis Leonard]

Thank you Dana for the great analysis.
I went to supplyhouse.com and asked a question about the Weil Mclain CGa-3...

Q: Just wondering if the Weil Mclain CGa-3 boiler is too big for my Lansing, Michigan propane powered house. I did the Slant Fin heat loss calculation (water 180* inside 65* outside 6). Came out to 45,148 Kbtu.
Have 91 feet of 3/4" Slant Fin 30 in three zones.
Z1 = 47'(living, dining, garage)
Z2 = 27'(bedrooms, baths)
Z3 = 17' (basement)
We leave Z2 & Z3 manually blocked open so boiler works the entire 90' of baseboard.
Built 1970, 3 br ranch ~ 3000 sq.ft (this includes 500 for attached garage)
ground floor (1215sf) all new windows, below grade (1215sf), new windows, 3/4 finished basement with drywall over 1 1/2" foam on block walls, 500 sq. ft attached 2 car garage that has 8' of baseboard (because water softner is against heated wall), 14" fiberglass inslation in entire attic (includes above garage)
Old boiler - Weil Mclain P-CG-5 (gross output 96K, net I-B-R 83.5K)
Heating Degree Days for Jan/Feb 2015 = 2295, propane usage 350 gal

Their answer.
.... it appears this unit will be too small for 3000 square foot area, rough estimate would be a requirement of 132,000 BTU. Therefore the CGI-7-145000-BTU would be a more suitable suggestion for a boiler for your home.

Wow.

 

[Dana]

WOW indeed! (Da's fugnutz mon! !)

It doesn't even take arithmetic to figure out that a house that has been successfully heated with a 96,000 BTU/hr boiler for four decades does not need a 132,000 BTU/hr boiler.

It's also not hard math to know that 91 feet of fin-tube can't even deliver the full 96,000 BTU/hr of output the old boiler was putting out, let alone 132K.

132,000 BTU/hr for 3000 square feet of conditioned space is a ratio of 44 BTU/ft^2 hr.

Houses in central Alaska with outside design temps in the -50s F don't have heat load ratios more than 30 BTU/ft^2- hr, unless they sleep with the windows open ! (Maybe they were figuring you sleep with the windows open when it's 0F outside?)

The CGa-25 is the right boiler, for both the design heat load and the radiation.