What size boiler for New England Cape home

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ColoradoKid

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What size boiler do we need?

We live in a New England Cape house built in 1968 in southern NH (Nashua). House is a 4 bedroom, two full bath, two story structure and was, when we bought it, electric baseboard heated. It has a "mostly" finished, walkout basement which was heated by a wood burning stove when we bought the house.

After the first winter in NH in 1988, we learned the error of our ways and removed baseboard electric and installed baseboard HW fed by natural gas boiler in 1989. The contractor installed a cast iron Weil-McLain HE-6 (DOE rating 137,000 Btu/hr capacity) and it only fed two zones, the first and second floors. At the time of install, only about 1300 sqft were "conditioned" space or heated by the boiler. The boiler is installed in the unfinished laundry room section of the basement (sits on concrete floor next to uninsulated concrete wall) and is directly vented out the side of the house above the foundation (we had no flues or chimneys being an all electric home with exception of wood stove). The contractor also installed a traditional gas HW heater next to the boiler with a power vent exhaust out the side of the house.

We haven't had to "heat" our basement very often, I assume due to the residual heat flowing off the boiler and water heater permeating through the basement spaces. It gets cool but never really cold in the basement unless outside temps go very low. When we have had to heat the basement, we use a Vermont Casting Radiance natural gas stove which we installed in place of the wood burning stove. This has been useful in times when we have lost power and hence the boiler (once for 6 days last winter). We cranked up the Radiance stove, open the door down to the basement, and kept the house from freezing. The basement is insulated from the first floor via R-19 batting in the floor joists above fiberglass suspended ceiling in the finished sections.

Combustion air for both boiler and water heater are fed from the ambient basement air volume.

We add a third heating zone in 2003 when we added a well insulated 4 season room onto our deck and heated it via baseboard radiators as with the rest of the house. Its worked great. We replaced the heating system expansion tank and relief valves and some rusty piping and failing gaskets about 10 years ago. All zonal heat piping has foam insulation tubes installed wherever possible.

The boiler has kept our house warm for 27 years with minimal maintenance but is now showing its age. An inspection by a local HVAC contractor brought in for maintenance advised the boiler may soon fail and there is much rust/corrosion on piping again, iron rust flakes on the floor, some leakage stains, and expansion tank and valves need replacing again. So we are looking a new boiler before the major snows & cold of winter hits.

Reading this forum, and other internet pages, I'm convinced the original boiler was way oversized for our house. Our kids are now grown and gone and the second floor bedrooms kept normally around 55 degrees. Main floor with living room, master bedroom, office (former bedroom), kitchen, and 4 season room is usually kept around 68 degrees.

For the winter time frame of Oct 14-Apr 15 with the above situation, I burned 692 Therms of natural gas. In the same window, I consumed approx 2,244 KW of electricity (eqiv, I think, of 76.6 Therms). I looked up the local Home Degree Days (HDD) for the above timeframe and it came to 6056.9. I'm assuming a boiler efficiency of 75% (probably high) for WM HE-6. HDD were measured against a 65 Deg norm and I used a -3 Deg 99% temp. Using an equation I found on this forum below:

Formula (excel spreadsheet) was btu/hr = ((((ThermsGas+ThermsElec)*100000)/HDD)/24)*(65-(-3)).

I calculated the period consumption at 26,964 btu/hr (adding the electricity therms to the natural gas therms) during the above winter time window. That gas consumption includes, of course, the natural gas Hotwater Heater, clothes dryer, cooking stove, and occasionally the Radiant Gas stove for the basement. Have I done the calculation correctly? Is it appropriate to add in the electricity therm equivalents? From what I've read, at least 75% of electricity flowing into a house turns into heat subsequently deposited in said house. If the 2nd floor bedrooms were utilized more and kept at 68 degrees, the above consumption would go up.

I next tried to estimate the Heat Loss of my house. I used a WM Boiler Replacement Guide, circa 1997, and also an iOS iPad App from Slant-Fin. With the 4 season room in the mix, sqft heated is now around 1460 for my 4 BR, two full bath home. First floor has 841 sqft, second floor 463, 4 season room 156. The WM "model" (as best I could match the categories) indicated heat loss around 40,200 btu/hr BEFORE adding the 4 season room. 4 Season Room has 90 sqft of Harvey Classic double hung new construction vinyl windows (U=.47) and 36x80 steel door, cathedral celling, floor area is 11'8" x 13'4". French doors connect to main floor of the house. Room floor is over deck with 6" fiberglass batting in deck joists beneath the floor, celling is 2x6 with batting, 2x4 walls have 2" batting with 2 " of styrofoam insulation installed.

The Slant-Fin app comes up with a household loss of 34, 279 btu/hr including the 4 season room. The iPad app provides a lot more flexibility in options than the WM guide and I tend think it more accurate (opinion?). The Slant-Fin app indicated the 4 season room heat loss at 8885 btu/hr with indoor 68 deg, outdoor 1 deg, design water temp 180 deg which seems reasonable to me.

Bottomline is the HDD & gas consumption based formula as well as the house heat loss calculations are all WAY LOWER than the 137,000 btu the Weil-McLain HE-6 boiler is supposed to be capable of delivering by a factor of almost 3. So, from this site and others, I'm assuming I have some great heating system inefficiencies in play, short cycling on the boiler, etc are going on (What is "short cycling" anyway?) . How do I tell by boiler observation (boiler and heating system are now on given its 28 degrees outside) if this is the case? When working, HE-6 has heated the house very well over all these 27 years. I have no complaints with reliability of the Weil-McLain boiler.

Given the circumstances (venting, location, heat demand), what size and type boiler do you recommend? NH has run out of funding for 2015 rebates for boilers but may reinstate $1000 for 2016 installed boilers above 90% AFUE ($1500 for >95%). That said, 90% seems to imply condensing boilers, I believe, requiring a drain for condensation and the closet drain is near washing machine 30+ feet away. What are the pros/cons of condensing vice non-condensing boilers? Is the increased boiler complexity, failure/maintenance, etc, worth it? We only intend to live in this house another 5-10 years max so we have short ROI window.

I'm not sure I can wait for 2016 (think of last winter) and hate to have the system fail in the midst of another El Nino driven winter in New England. I'm also going to replace the 10 year old traditional water heater (sits next to boiler) with a indirect storage tank fed off the boiler. Any recommendations on brands and capacity with respect direct hot water storage systems?

Two boiler models have caught my attention. Burnham ESC and Weil-McLain GV90+. Given the above, are these too much boiler for my needs? Do you have other boiler recommendations? I (and my wife) are skittish about going to low and I want some heating reserve should I add a further addition to my house in the future.

Thanks,
 

Dana

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Yes, a 6 plate boiler is ridiculously oversized- the HE6 probably more than 5x your total space heating load.

The crude WM 2-pager reliably oversized by 30% or more. The Slant-Fin tool varies, but typically overshoots by ~25%. Fuel-use calculations tend to be more accurate, but if you're keeping the upstairs at 55F, use base 58F or base 60F for the whole house analysis to come up with the BTU/degree-hour constant, and project forward to what the load would be to keep the house at a code-min 68F.

If the boiler and all the distribution plumbing is inside of conditioned space (which they are, even though the foundation isn't insulated), use the nameplate efficiency, not a presumed lower value, unless a recent combustion analyzer test came in with a lower steady-state efficiency, not some presumed degraded number. (There are many 50 year old boilers hitting north of 75%.) That's BTU-in/D.O.E.-out, which is probably 80% for the boiler, could be as low as 70-75% for gas-fired stoves, but could be as high as 82%. If you were using one more than the other and the efficiencies are different, go with the efficiency of the heater that got the most use.

Unless you're using electricity for space heating, electricity use BTUs aren't normally used in the a fuel use derived heat load calculation, even though the heat output of the 24/7 plug loads like refrigerators are subtracted in a Manual-J type load calculation.

So, re-run the numbers using fuel-use only, at the boiler's nameplate efficiency and see where it ends up.

Even before re-running the numbers, it's pretty clear that your heat load at -3F is going to be under 30,000 BTU/hr. A load of 30,000 BTU/hr @ -3F for 1460' of conditioned space a ratio a bit more than 20 BTU /hr-ft^2 @ 0F. Most 2 x 4/R13 1960s type construction with clear-glass single panes + storms and some retrofit attic insulation come in at about 15 BTU per square foot of conditioned space @ 0F, give or take, if it's reasonably air tight. When it's 20 BTU/hr-ft^2 or higher it's usually due to quite a bit of uninsulated exposed foundation combined with high air leakage, both of which are fixable, or an unusually high glazing fraction.

If yours is an uninsulated CMU foundation, it's well worth insulating the foundation, even though you not actively heating it basement- the losses are still very real, and costing you real money. When you right-size the boiler the boiler room will be substantially cooler, which lowers the heat loss a bit, but nothing like putting 1.5" of foil-faced polyiso trapped to the foundation with an R13 studwall would. It's a double-digit percentage of your heating bill.

Both the smallest GV90+ and the ESC3 have a min-fire output that's about 2x your total load. The ESC3 has some smart-controls to not lose too much efficiency, but not exactly ideal. The amount & type of radiation you have on each zone matters, since that determines the minimum burn cycle, and the number of cycles per hour. For a cast iron boiler any burn shorter than 5 minutes cuts into as-used efficiency, and under 3 minutes or more than 10 burns and hour is something of an efficiency disaster, throwing away way too much fuel on ignition cycles. (Time the burns on the existing boiler on a continuous call for heat from just one zone starting with the zone that has the least amount of radiation.)

There are several modulating condensing boilers out there that can modulate down to less than 10,000 BTU/hr. They're not as cheap as the GV90+, but they're a better fit to your loads, and will deliver not only higher efficiency, but when the outdoor reset curve is dialed in, higher comfort too. One of those that's becoming a favorite among some installer it HTP's UFT-80, which it set up to make installation a lot simpler than some others. It's designed to be pumped direct- it won't need primary/secondary plumbing for most systems, and is internally plumbed with a separate port for hooking up an indirect. With a 10:1 turn down ratio makes it a good fit for 19 out of 20 homes out there (even homes with too many zones prone to short-cycling even a right-sized boiler.) But there are others that would fill the bill too, once we know your real heat load.

There is some dumb cast-iron that would probably fill the bill too, but I'll wait for the revised fuel-use derived numbers.
 

Dana

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I guess that's what they were thinkin'. What are the odds that there is even enough radiation to emit the full 137K into the house?

That's more than 100 BTU/hr-ft^2 for the original 1300' house, approximately the load ratio of a greenhouse in Fairbanks AK.
 

ColoradoKid

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Dana, Thanks for great feedback. We had another HVAC contractor in today who also said our 27 year old WM HE-6?boiler was near its end of life. He saw evidence of rust under and leakage between the six plates of the boiler so we are definitely looking to replace it. It is running now but we are on the cusp of failure. It's still using its original circulating pump, zone valves, and power exhaust fan. Again, I cannot complain about Weil- McLain reliability here for sure.

This HVAC tech was more thorough and measured the linear feet of baseboard radiators in our house. We have about 100 feet of baseboard radiators; 56 feet on zone 1, 32 feet on zone 2, 11 feet on zone 3 (the 4 season room). Using an assumption of 600 BTU/hr per linear foot I found somewhere on the Internet, that puts the max heat delivery capacity of the radiators (180 deg) at ~ 60,000 BTU/Hr, still only half of what the HE-6 boiler can produce. We are going to add a 40 gallon HTP Suprstor DHW tank to replace a ten year old traditional HW heater for our new system running off the boiler and I'm not sure what BTU demand is that will drive.

Per your direction, I modified my calculation of consumption using Heating Degree Day data for Nashua to use my natural gas consumption figures only. This is the equation I used:

BTU/Hr=(((((Therms Gas)*100000)/HDD)/24)*Efficiency)*(HouseTemp-(-3))

Where Therms = 692 for Oct14-Apr15

HDD for Nashua = 6057 for 7 Oct 14 – 11 Apr 15, -3 degrees for 99% number, 65 Degree home temp

Weil-McLaine HE-6 boiler Manual states following specs: Input BTU/Hr = 167K, DOE Heating Capacity BTU/Hr=137K, Net I-B-R BTU/Hr=119K, DOE AFUE 82.2%

Per your suggestion, I Used 60 for average house temp the 82.2% for efficiency. The result is 24,652 BTU/Hr for last winter, way under the max heat output the radiators and boiler can theoretically provide.

Assuming the worst case conditions and driving the HTP Supestor DHW, the latest contractor is recommending a Burnham ESC-4 85% AFUE, with outdoor reset capability, 4 circulators in lieu of zone valves, and external air supply kit. He (and other contractors) have recommended more efficient modulating condensing solutions (Burnham K2 and Lochinvar Knight) but we are skittish about the complexity, maintenance, and procurement costs. The reviews for the Knight seem pretty bad in some cases. These high efficiency units seem like the Lamborghini's of the boiler world, high performance with high Ops & Maintenance and tuning demands. We are looking more for your basic Chevy or Ford. All three contractors have agreed they perform more service calls on the high efficiency units.

Our house is near 50 years old and probably could use some tightening up. Only about half of the windows have been replaced with modern units (Harvey double hung) but that is another project. The foundation insulation work is an idea I had not heard about before so I'll have to think more on that.
 

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You do not normally include the indirect WH as part of your overall heating load. If you were running a spa or something that used hot water constantly, yes, but not in a typical home use. Almost any boiler you can buy has excess capability, and the indirect is usually installed as a priority zone, meaning, it gets all of the boilers heat when needed. In most all circumstances, that gets satisfied long before you'd ever notice the house cooling off any. In reality, don't worry about it...size the thing for the static heat load of the house.
 

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You do not normally include the indirect WH as part of your overall heating load. If you were running a spa or something that used hot water constantly, yes, but not in a typical home use. Almost any boiler you can buy has excess capability, and the indirect is usually installed as a priority zone, meaning, it gets all of the boilers heat when needed. In most all circumstances, that gets satisfied long before you'd ever notice the house cooling off any. In reality, don't worry about it...size the thing for the static heat load of the house.
 

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The base temp isn't the average indoor temp- it's the heating /cooling balance point, which is ~5-10F lower than the indoor temp. (The better insulated & tighter the house, the bigger the interior temp & base temp difference.) So, don't use 60F as the average house temp, use it as the heating degree-day base.You might even run another calc using a 55F base temp to set an upper bound. You can download HDD data on 1-F base increments from degree-days.net from a nearby weather station. From that, derive your BTU per degree-F, but note that to meet code-requirements that it be possible to heat the house to 68F at the 99% outside design temp. Use (65F- -3F=) 68F as the heating degrees, even starting with your base- 60F derived constantant, since to meet code you'd have to run the house warmer than your have been.

But this isn't going to change the boiler size, since you're guaranteed to already be at the smallest size of most boiler series out there.

The ESC-4 has enough output to run a 1.5gpm shower 24/365 AND cover your heat load, so what's the point of the SuperStor if you go that route? The ESC-4 is also totally & completely ridiculously oversized for you space heating load, and makes your undersized radiation even more problematic. Size the indirect for the biggest tub you need to fill, size the boiler for the space heating load. If going with that series, even the ESC3 is ~2x oversized for the load at 60K out, but could still be OK (provided the radiation on the smallest zone won't short-cycle it to death, which it probably will), due to it's smarter electronic controls.

Outdoor reset would improve comfort, but will decrease efficiency, since it'll doing a lot more on/off cycling at low temp. DON'T go there, and here's why:

Baseboard only puts out 600 BTU/ft-hr at average water temp of ~180F or a bit more. With 11' of baseboard as it's own zone you're kinda screwed, since even at 210F AWT it can't emit more than ~15% of the output of the ESC3, a recipe for extreme short-cycling. ( See this output spec sheet for a rough estimate of how output changes with AWT.) If you dropped the temp to 140F with outdoor reset the zone could only emit 6% of it's output. The 32' zone can't emit more than about half the ESC3's output. The 56' zone is the only one that won't have a cycling problem with the ESC, even though it doesn't quite balance even at high temp. To avoid excessive cycling on zone calls you'll either need some thermal mass , or add sufficient radiation to each zone so that any single zone can emit at least 2/3 of the boiler's output on it's own. Either that, or use a modulating boiler that can throttle back to under 10,000 BTU/hr. But there's no way you should be installing outdoor reset on any cast iron boiler with your zone radiation.

The completely dumb & old-school Weil McLain CGA-25 has 44K of ouput, which is about 1.5x oversized, and would hit it's AFUE numbers. Depending on the size of your radiation you'll probably have to plumb it with a bypass branch at the boiler to protect it from cold water return, and you'd have to install a narrowing flue liner to get it to draft properly. Even though it's a cheaper-dumber boiler the installed cost might be higher than an ESC3. The CGa-25 is small enough to not be in ridiculous-cycling mode on the 32' zone without adding smarter controls even though it'll be cycling more than is ideal, but it still is not small enough to not short-cycle on an 11' stick o' baseboard even WITH retrofitted smart controls.

The UFT-80 is still a reasonable boiler for your situation, and it could still run fine with outdoor reset with your 36' and 56' zones. It would probably be cheaper than installing a buffer tank to cut the number of cycles down to size with a cast-iron boiler. The smallest Lochinvars go that low too. HTP is a smaller operator, but has a pretty good reputation with good local support. (Their headquarters & factory are within easy day-trip driving distance of Nashua if you need to go down to scream & rant in their face, for instance! :) )

Using your Slant-Fin load tool, break down total loads by zone. If they're all about the same BTU/ft-hr of load to radiation they'll all behave similarly under outdoor reset control. To really max out the comfort you'll want to be able run it a the lowest temp possible, down to maybe 110F AWT (120F out, 100F back) below which the output of fin-tube baseboard is too non-linear to be useful. With a probable load of ~25-30K and 99' of baseboard you're already able to run in condensing mode even at -3F outdoor temps on the 2 bigger zones, but probably not on the 11 footer.
 

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Only if the load to radiation ratios are close would that be a good idea. ADDING radiation to make them close is a good idea, even if it's still 3 zones.

The min-fire output of some of these mod-cons are getting down there, but there are limits to modulation. An 11' baseboard zone is going to short cycle on just about anything but a high thermal mass source like a water heater.
 
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