Sizing a boiler - does feet of baseboard matter at all?

[Jeff_G]

It's time to replace my 35 year-old natural gas boiler. I've had two contractors come over so far and both wanted to know how many feet of baseboard I have. Neither cared at all about the heat load calculations I spent hours on. They said it doesn't matter. Feet of baseboard is what matters.

Even if my heat load calculations are correct, is there any reason why feet of baseboard might matter? Does X feet of baseboard require a minimum boiler size of Y to run properly?

My house has 200 feet of baseboard with 5 zones (plus a 6th zone for the basement, which we never heat, that has 45 feet of baseboard). Based on this the contractor wants to install a 140K BTU (input) boiler.

I did six heat load calculations and most were between 70K and 80K BTU (DOE output).

So if I buy an 83% efficient cast-iron boiler, it seems like a 100K BTU input boiler should be enough???

Or is that too small for 245 feet of baseboard or a house that is 3700 sqft?

* We had the attic sealed and insulated in 2013, which reduced nat gas use by 20%. Heat load before this was about 100K BTU. The house was built in 1982. Location is NYC metro area. I used a 99% outdoor design temperature of 12 degrees F.

Should I listen to the contractors and go bigger? Or should I insist on a boiler size based on heat load calculations???

 

[Dana]

Who did the heat load calculations? Those are about twice the typical numbers for an older 2500' 2x4 framed house with clear glass (no low-E) double panes or storms over single-panes. If it's tight 2x6/R19 construction with at least R30 in the attic it would be twice the typical 3000' house.

If you have a heating history on this place, using wintertime fuel use only , run a fuel-use based load calculation as a sanity check (or maybe one of those load calculations was already based on fuel use?)

The 99% outside design temp in New City is about +5F (yes, I know it gets colder than that sometimes, but only for ~1% of the hours over the past 25 years). For a cast iron boiler ASHRAE recommends no more than 1.4x oversizing. This is both a comfort and efficiency issue. For the sake of argument and to make the math easy, let's assume your heat load really IS about 70K @ 0F. That's a 70F temperate difference, 1000K of additional load for every degree cooler. A 140K-in/115K-out boiler would be able to support a 115F temperature difference, which means it would be good down to 70F- 115F= -45F, a temperature not seen in New Falls since the last ice age. Using ASHRAE's 1.4x multiplier the biggest boiler to install would be 70K x 1.4= 98K, or a 115-120K, input boiler.

A heat load of 70K/200' of baseboard is 350 BTU/hr per running foot, which typical baseboard would deliver at an average water temperature ( AWT ) of 140-145F, which would be returning water to the boiler just above the condensing temperature. Since you only need water that hot or hotter 1% of the time that makes your system & house an excellent candidate for a modulating condensing boiler, even if it IS broken up into five (or six) zones. A 120K condensing boiler with a 10:1 turn down would have a minimum modulated output of about 11,000 BTU/hr at condensing tmpes, and wouldn't s cycle at condensing temperatures on zones with 50' or more. A 100K condensing boiler with a 10:1 turn-down would still cover the design load with 87-88K of output at non-condensing temps, and would be able to throttle back to ~9500BTU/hr out at condensing temps, which would cycle a bit but won't short-cycle on 35-40' zones. The napkin math on sizing mod-cons lives here.

But if you'r totally stuck on cast-iron...

With a ~100K out cast iron boiler you're looking at 100K/200'= 500 BTU/hr per foot, which would have return water WELL above the condensing point, no need to protect the boiler from too-cool return water. Add another 50' for the basement zone and it's 400BTU/hr per foot, still above the condensing zone. If the boiler output is sized EXACTLY to 80K there is a small risk of condensing when all zones are calling for heat, including the basement, but a simple system bypass branch at the boiler mixing a small amount of boiler output with the return water would be all that it takes to protect it. The 4-plate Burnham ES2-4 has a DOE output of ~90K, and is internally protected from return water as cool as 110F to make life easy.

Run the fuel-use load numbers and check back.

 

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[Jeff_G]

Thank you, Dana!

I did the heat load calculations myself and came up with 70K-80K BTU. Contractor came up with 140K BTU input based only on feet of baseboard.

I followed the instructions in the article Out With The Old, In With The New
I used my actual heating bills from last winter and the previous winter.
I used a 99% outdoor design temperature of 12 degrees, which is for the closest city to me - White Plains, Westchester, NY.
House is 3700 sqft, built in 1982.

It sounds like I don't have to worry about the feet of baseboard as long as I get a boiler internally protected from cool return water or use a system bypass at the boiler.

Thanks!

 

[Dana]

Contractors will often add up the baseboard length then multiply by 500-600, which is the most the baseboard can actually deliver. But that's a crappy way to size a boiler, especially when it's all broken up into zones.

Any 100K-in cast iron boiler (not just a self-protecting Burnham) would really be fine- a system bypass and ball valve for tweaking it adds maybe $100 to the cost of installation, but it's often cheaper to install a ~100K mod con. A new cast iron would require a right-sized flue liner if replacing an 80% efficiency oversized beast vented into an even more oversized terra-cotta lined flue.

A heat load ratio of 80K/3700' = 22 BTU/hr per square foot, which is on the high side even for a 2x4/R11 kind of house unless there is no foundation insulation. I suspect there is some low-hanging fruit such as air-sealing to be done. If the boiler is in the basement and there is no foundation insulation, even if you're not actively heating the basement the basement losses could be responsible for 15-25% of the total fuel use, and that would drop to less than 5% if insulated to the current IRC code min of R15 continuous insulation (recommended.) The fuel use in my own house dropped by more than 15% after putting up 3" of reclaimed roofing polyiso on the interior a decade ago, air sealing & insulating the band joist & foundation sill at the same time. The wintertime temperatures down there increased by 5F to a fairly stable 65F (no matter how cold it is outside), even without actively heating.

 

[Jeff_G]

Improving the basement insulation is a great idea. Band joist insulation is something I can work on myself to keep costs down.

Can you explain why it could be cheaper to install a 100K mod con than pay $100 to install a system bypass? I thought mod con boilers cost thousands more to install? Is it because of the cost to replace the flue liner?

Can I avoid the problem with the flue liner by switching to a direct-vent boiler?

Thanks!

 

[Dana]

Mod-cons used to be a significant upcharge, but that's not the case anymore. In my area (MA) the HTP UFT series mod-cons (also sold under the Westinghouse nameplate) are cheaper up-front than cast iron, and can use plastic venting, that's as-cheap or cheaper to install than a flue liner or B-vent. HTP's pricing through distribution isn't too different from Westinghouse's, online pricing, but may be cheaper for installers than to the general public. But the support may be better for one than the other in your area.

Either the 100K WBRUNG100W or 120K WBRUNG120W only run about $2KUSD, shipped to your door from online retailers. A 105K cast iron minimum-legal 82% efficiency cast iron beast from the same webstore is only $2oo less, and has more expensive venting.

Insulating the foundation wall with rigid foam is a fairly straightforward DIY if it's poured concrete, concrete block or brick. Using reclaimed foam at less than 1/3 the cost of retail it's cheaper than batts. If insulating with batts & studwall you'd need at least R7.5 foam between the studwall& foundation anyway.

[edited to add] Direct venting exhaust components for non-condensing equipment are usually stainless steel, and though short, not necessarily cheaper than a full height stack of B-vent or a flue liner.