Boiler Replacement Dilemma

[Brendon]

Hi Professionals.

First off, a big THANKS for your advice! I am in HIngham, MA where it is a bitter 6 degrees F today. My original 1965 American Standard Gas boiler and direct hotwater tank circa 2004 are due for replacement.

Our house is approximately 2k sf of heating space, well-insulated. We have hard water, so the ultra efficient options and combi boilers worry me.

What would you suggest? I want something that is reliable first and efficient second. My options seem to be:

1. Combi boiler - brand unsure
2. HE wall mount boiler with indirect water heater. POssibly Buderus and SuperStor combo.
3. 2019 version of what I have now, cast iron boiler that is rock-solid with indirect tank or direct tank.

I could really use help on brands and combo. I want something that is easily servicable and reliable. I don't want something that only 3 people know how to repair if needed.

Thanks all!

 

[Dana]

Before doing any of the other analysis, start with a fuel-use heat load calculation. Almost all cast iron boilers that vintage in MA are 3-4x oversized for their space heating loads, and oversizing has costs upfront, and in efficiency & comfort thereafter.

When considering a higher efficiency boiler the MINIMUM firing rate is more important than the maximum, especially if the system is broken up into multiple zones. The boilers themselves have very little thermal mass to work with, and with low-mass heat emitters such as fin-tube convectors/baseboard it's easy to short cycle them into low efficiency and an early grave. The napkin math on that analysis is laid out here.

Once you have a reasonable handle on the load numbers and zone radiation it will be possible to make some recommendations (and caveats.) A right-sized modulating condensing boiler + indirect is the most likely outcome of the numbers crunch, but let's have the numbers. With sufficient radiation you may be able to pull this off with a stainless condensing water heater and an exterior heat exchanger for the heating end.

A "...bitter 6 degrees F..."??? And here I was thinking -5F was pretty comfy in Worcester this AM, fully 11F cooler than in Hingham. But perhaps you had more wind at the coast than I did inland. Even the short-haired puppy wasn't too miserable to do her business on her AM walk. Compared to my friends in Wisconsin a +6F (or anything in positive territory) would feel like a heat wave about now!

 

[Sponsor]

 

[Brendon]

Thanks Dana. I'm having a hard time finding a recent bill as wife has paid/shredded. Any other alternatives? Additionally, is there any local hvac expert that you trust can make the approrpriate calculations? Frankly, I'm not sure I have all the inputs and would prefer a professional make the proper calculations (without OVER-calculating of course).

Yes, it's 6degrees, Worcester always gets it worse it seems then along the coast.

 

[Jadnashua]

Most utility companies will have your bills online if you log in.

 

[Dana]

Thanks Dana. I'm having a hard time finding a recent bill as wife has paid/shredded. Any other alternatives? Additionally, is there any local hvac expert that you trust can make the approrpriate calculations? Frankly, I'm not sure I have all the inputs and would prefer a professional make the proper calculations (without OVER-calculating of course).

Yes, it's 6degrees, Worcester always gets it worse it seems then along the coast.

Don't let an HVAC contractor run the Manual-J. A RESNET rater or a professional engineer who make their reputation on accuracy rather than installing & maintaining equipment will give more accurate numbers. It'll cost several hundred USD though, and it's not really needed unless making extensive changes to the house or heating system. HVAC contractors tend to have thumbs on the scale, using outlandish design temperatures and insanely high air leakage assumptions, anything to avoid getting the 5AM call from an shivering irate customer. They thus calculate a higher-than reality number, and upsize the boiler from there, ending up with something than 2x bigger than what's appropriate.

The other approach is often to measure up the radiation and spec a boiler big enough to run it all at 180F. This too leads to 2x or higher oversizing on most houses.

Since you have a heating history you can use the existing boiler as the instrument to MEASURE the heat load. A few wintertime gas bills with exact meter reading dates and amounts is easier, cheaper and usually more accurate, as long as you didn't turn down the thermostat to 50F and head off surfing in Belize during the billing period, or make extensive used of auxiliary heating such as wood stoves, etc.

Contact your gas provider, or go online- most gas utilities in MA have options for creating a secure account from which you can view recent bills, make payments, etc.

If you can count, add, and use a tape measure you can do as-good or better job than the average HVAC contractor measuring your radiation. There is no rocket science involved.

Most cast iron boilers are oversized for the heat loads of reasonably tight 2x4 framed 2000' houses with some fluffy insulation in the wall cavities & attic and at least storm windows over double hungs, but some are more ridiculously oversized than others.

A typical 2x4 framed 2000' house above 1000' of basement in Hingham will have a design heat load of less than 40,000 BTU/hr at the 99% outside design temperature of +11F (yes it gets colder than that, but only for 87 hours out of a given year. The +11F temp is the 99th percentile temperature bin in Hingham.) Many will come in under 30,000 BTU/hr. If replacing it with cast-iron the biggest boiler that makes any sense would have an output of 1.4x that much. If the boiler is in the basement, and not a garage, size it off the DOE capacity, not the IBR or AHRI net BTU numbers.

For example, say your load calculations come in at 34,000 BTU/hr. The optimal cast iron boiler would have an output capacity of 1.4 x 34,000= 47,600 BTU/hr. That would be enough boiler to cover your Polar Vortex disturbance cold snaps with margin, but run long enough duty cycles during normal weather to run efficiently, and have enough extra to be able to run overnight setbacks during normal weather and still have reasonable recovery times from setback. There are a few cast iron boilers in that range, but most are north of 60K.

If installing a modulating condensing boiler the maximum capacity number doesn't matter as long as it's over 34,000 BTU/hr with a bit of margin. The more important consideration there is that it can modulate low enough in firing rate to run at much lower temperatures to reap the condensing efficiency without short-cycling the burner on/off. With a mod-con there is no need for a 1.4x oversizing, since they can and should be run under "outdoor reset" control to minimize water temperatures, maximizing combustion efficiency, and don't need rapid recovery from setback. With outdoor reset the boiler is programmed to adjust the water temperature up/down in response to outdoor temperature, which results in maximum efficiency and very steady indoor temperatures. That said, most mod-cons will still have ample capacity for setback recovery, if desired, even though the higher water temperature/lower combustion efficiency of operating it that way burns more fuel than simply dialing in the outdoor reset curve.

Wall hung combi boilers big enough to run a couple of bathrooms often can't modulate low enough to avoid short cycling on a houser your size.

 

[Brendon]

Ok Dana and professionals.

I am back. I've gathered some data and shared it below, now that I've received my most recent gas bill.

Therms: January, '19 (32 days) = 177 Therms
Current boiler: 120k BTU Input, 96k BTU Output

Looking at 65 degree days for same time period = 1083.5
Looking at 60 degree days for same time period = 895.5

Heating 99% dry bulb (South Weymouth) = 11

This is where I get lost. I did the calc of 177therms*.80 (boiler efficiency) and came to 141.6therms*100k = 14,160,000

For 65 degree days = 13,068 BTU
For 60 degree days = 15,812 BTU

Hopefully this is all that you need to calcualte from here, please advise.

Brendon

 

[Dana]

Using base 60F doesn't usually make sense for a 2x4 framed house unless you normally keep the house at 65F or cooler, so I'll run the base 65F numbers.

So you used 14,160,000 BTU/1083.5 HDD= 13,069 BTU per degree-day. There are 24 hours in a day, so that is the same ratio as 13,069/24= 545 BTU per degree-hour as the linear heating constant. That number means for every degree F below the heating/cooling balance point the heat load grows by about 545 BTU/hr. While the real load change with temperature isn't exactly linear, a linear approximation is a good enough model to use for sizing heating equipment.

With a presumptive heating/cooling balance point of 65F (the base temp), and 99% outside design temp of +11F you have 65F-11F= 54F heating degrees, so the implied heat load is then:

54F x 545 BTU per degree-hour= 29,430 BTU/hr

For a 2000 square foot house that's a very credible ratio of ~15 BTU/hr per square foot of conditioned space, in the right range for a reasonably tight, decently insulated 2x4 framed house.

And with a boiler output of 96,000 BTU/hr, the original boiler is a bit more than 3x oversized for the ~30,000 BTU/hr load, (which is unfortunately the norm rather than the exception.) At that oversizing factor the boiler room is probably the warmest place in the house all winter, and by being warmer increases the actual load. Replacing the boiler with a right sized boiler will reduce the load, and reduce the standby/idling time losses as well.

If going with another cast-iron boiler the biggest one that makes any sense would have an output of 1.4 x 29,430 BTU/hr= 41,202 BTU/hr, which is a tough bill to fill- most cast iron boilers are much bigger than that. With smarter controls a 60K output boiler can still pretty much hit it's AFUE numbers, but the oversizing isn't buying you anything.

For a modulating condensing boiler you really care more about the minimum-fire output relative to your radiation size. All combi boilers have much higher heating capacity than you need, as do almost all modulating condensing boilers. So the next step is to measure up all the heat emitters such as baseboards, convectors, radiators etc in the house, zone by zone if multi-zoned. That will affect the system design and control even if going with another cast iron boiler.

It's usually the case when the previous boiler was 3x oversized the radiation would be at least 2x oversized if operated at 180F boiler output, but right sized for operating at condensing temperatures. Let's find out if that's true in your case!

 

[Brendon]

OK, thanks Dana, I'll take some additional measurements!

I should add that in the years we've lived there, we've replaced the windows with double-insulated glass, replaced the doors, added blown-insulation in the walls and in the attic, all the way up and slightly over the rafters. The result has been our heating bills have reduced by 1/2.

We've also replaced the original thermostat with a programmable one. During awake/home times, we keep the upstairs at 65degrees. During away/night time, the temperature drops to 62 degrees. The second zone is the lower level, which is a couple of rooms and a bathroom. Again, it's 65 degrees in use and 61 degrees when not in use.

I tell you this as I'm not sure if this is an adjustment to your calculations or not.

 

[Dana]

The lower-than-68F temperature settings affects the numbers slightly. Using base 60F we have a heating constant of 15,812/24= 659 BTU/degree hour, and 60F-11F= 49F heating degrees for an implied heat load of 32,291 BTU/hr. Add three more degrees to guarantee it can make 68F (the code-legal minimum) instead of 65F (your as-used daytime temp) adds another 659 x 3= 1977 BTU/hr, for a total of 34,268 BTU/hr.

That's about 17 BTU/hr per square foot of conditioned space, which is also a very credible, reasonable number. If it were over 20 it would usually mean there are more upgrades to make. It sounds as if you've already been pretty diligent on that front. The original boiler might have only been a bit more than 1.5x oversized for the pre-upgrades condition of the house, if 3x oversized for the current load. If the foundation hasn't yet been air sealed and insulated it might be the next big heat loss thing to fix, even if you're not actively heating the basement. (There are details to get right to avoid creating mold problems when going that direction.)

 

[Brendon]

Great thoughts.

In calculating the radiators, it breaks-down as follows:

Zone 1 - Main living area, 1350 SF: 79 Linear feet of baseboards. With programmable of 65 degrees day/62 night
Zone 2 - Lower level, 650 SF: Currently 20.5 Linear feet of baseboards. I'd like to add an additional room with 12' more baseboard. Manual thermostat we turn up to 65 when in use, turn down to 61 when not in use.

With that, let me know what you suggest!

Thanks.

Brendon

 

[Dana]

Typical ~2" fin-tube baseboard emits about 200 BTU/hr per running foot when the average water temperature is 120F (125F out of the boiler, 115F return), which is what is necessary to hit the low mid-90s efficiency. The 79 foot zone would then be able to deliver ~15-16,000 BTU/hr at condensing temperatures. If the minimum modulated output of the boiler is less than 16,000 BTU/hr that zone would be fine. There are a several low mass combi-boiler options out there that can work with that amount of radiation, but also many that can't.

The other zone is tougher. With 20' of baseboard you're looking at 4000 BTU/hr of heat being emitted at condensing temperatures, which is well below the minimum fire output of even the smallest combi boilers or modulating condensing boilers. There are several suitable boilers that can throttle back to 7000-8000 BTU/hr out though. If the radiation on that zone gets bumped up to 32' with the additional room on that zone it would still cycle with one of those mod-con boilers, but it can be set up to not short-cycle itself into low efficiency & low reliability.

An inexpensive but quite decent mod con that fills the bill would be HTP's UFT-080W, which coupled to a SuperStor indirect would work just fine. The UFT series boilers have a pre-plumbed secondary port and control hooks to make installing and supporting an indirect dead-easy. The -080W can back off to 7600 BTU/hr 95% efficiency at minimum fire (which would balance perfectly with 35-40' of baseboard), but can ramp up to 70,000 BTU/hr even at non-condensing temperatures if needed. There are several others at a range of price points, with different control features & options, eg Lochinvar WHB-055 or -085, Navien NHB-55 or -80.

Assuming most of the ~34,000K load is zone 1, that a ratio of 34,000/ 79'=430 BTU/hr, which would take average water temp of ~155F to deliver. While that is above the condensing zone, you only need water that hot 1% of the time or less. Using the outdoor reset controls that come with every modulating condensing boiler you should be able to dial it in to where the water temperatures are always nearly exactly what's needed to cover the heat loss at any time of day or night, and let the thing run constantly at it's lowest temperature/highest efficiency. That results in extremely stable and comfortable room temperatures, and uses less fuel than using overnight or away-at-work setback strategies, since the recovery ramps from setback require much higher water temperatures (=lower efficiency) to be able to recover from setback in a reasonable amount of time.

With 79' of primary zone the ideal cast iron boiler would have no more that about 40,000 BTU/hr of output, or it will cycle excessively during extended calls for heat. Cast iron boilers with more output than that can still be made to work if it has heat purged controls to maximize burn times. The dumb-as-a-box-o'-rocks Weil-McLain CGi-25 or CGa-25 would be reasonably sized for both the radiation and heat load, but would be on the small side for serving an indirect water heater. That's fine if your hot water needs are modest- the recovery times would be about the same as a 50 gallon standalone water heater. The next step up CGi/CGa-3s would be fine for hot water, but sub-optimally oversized for the space heating load. Burnham's ES2-3 or ESC-3 (direct vented version) boilers are oversized for the space heating load, but come with the more sophisticated controls that would allow it to hit near their AFUE numbers despite being 2x oversized for the space heating loads.

Since any mid-efficiency cast iron boiler would require a right-sized flue liner to avoid backdrafting & flue condensation, the cost of these right-sized or nearly-right-sized boilers will usually end up being higher than a robust simpler mod-con boiler. In MA there is a $2750 rebate subsidy (more than the retail cost of an HTP UFT-080W ) for installing a 95% AFUE or higher condensing boiler, which sweetens the pot considerably.

 

[Dana]

It occurs to me that at 180F boiler output your existing baseboard can't emit more than about half the old boiler's 96K output even with all zones calling for heat, so the "before upgrades" picture of the space heating load was probably more like 48-50,000 BTU/hr, not 60K. That means that even on the the coldest of days the boiler never delivered more than a 50% duty cycle.

With such a low duty cycle the as-used AFUE is substantially lower than the steady state thermal efficiency, so the fuel use based load calculation is going to be an upper bound, not that it changes the boiler recommendations at all.

 

[Brendon]

Terrific perspective, thank you Dana.

Based on your well-stated and eloquent thoughts, the answer seems obvious, which is to go with an option like the Mod/Con HTP. Given that I have 3 young kids and warm baths are every night, we've had many issues running out of hot water, which makes for a less then enjoyable bath time for the kiddos! This is an area I'd be willing to spend more money to prevent this from re-occurring, so given your point on the Weil Maclean dilemma (being good for heating, but not for indirect water heating) I think I'll rule that one out. Combine that point with the MassSave rebate seems to make the Mod Con option a no-brainer! I'm sold.

Currently, we have a 40-gallon direct fired water heater, but I'd be in favor of expanding that (if needed, Dana) to say 80-gallons indirect SuperStor in order to accommodate all of our hot water needs.

Lastly, and frankly the biggest hurdle: finding the right technician, that thinks like you (and me)! I've had 3-4 quotes from various HVAC folks. Honestly, 99% of their customers defer fully to their recommendations and the majority of the recommendations I've received are along the lines of Buderus GB142 (higher output) and SuperStor. Their sales pitch: Buderus are "German" (translation: "quality") and I've seen several with the 100k btu attached as well. I'm obviously not one of their typical customers that will go along with their recommendation. However, with me, I'm perceived as the fickle customer, telling him/her how to do their jobs, which isn't ideal!

Being the data-driven guy that I am, I'd like to find someone that thinks similarly and is competent installing HTP. None of the 4 bids I've received mentioned it and all stated higher recommended btu performance. I could push back on them and say "i've done the analysis and here it is" and the problem is they may something like "you're taking a risk and don't call me when it fails!" That is not the person I want to hire! Instead, I want to hire the person that reviews the data, agrees with it, and agrees with the recommendation here.

Dana, professionals, any suggestions for someone South of Boston? I know this may go into a possibly gray area/touchy subject, so feel free to PM me if preferred. I just want to find the right hire. My neighbor (house same age as mine) was "sold" a combi boiler 8 years ago that failed 3 different times, the last time the company had gone out of business! They're now on their 4th boiler in 10 years....I don't want to be them.

 

[Dana]

Your best resource would be to call HTP (you could drive there I suppose- it's only about an hour's drive from Hingham to
their HQ in New Bedford ), tell them the equipment you think you need, and ask them to recommend a few contractors/installers in your area. They know better than anyone which contractors are ordering them in dozens per year with no bogus warranty claims submitted, and which ones are tying up their technical help line to answer stuff already clearly spelled out in the manual. They may refer you to a local distributor that, if they don't have any high volume direct sales contractors in your area.

A couple of years ago a contractor gave the "I won't install anything smaller than..." speech when a friend in CT was pushing back on an oversized proposal, to which the response was, "That's right, YOU won't be installing ANYTHING at my house with an attitude like that." Hopefully it doesn't come to that.

I'm guessing none of your contractors to date did even the most cursory heat load calculation on the place, maybe one or two of them measured your baseboard (?). Too many contractors are installing mod-cons and combi boilers who don't really have an inkling about the related sizing issues, relying on some crude rules of thumb that maybe kinda sorta worked in some other universe. The number of combi boilers in MA short cycling themselves into an early grave on insufficient radiation are many.

 

[Brendon]

Done.

I followed up with HTP who then put me in touch with a local distributor. I asked for some recommendations in my area and got a couple names of those HTP techs.....of course, they could just be giving me the names of those that pay their invoices on time!

We'll see, I'll need to vet them each and I emphasized proper sizing. I want to see what they recommend for sizing and performance.

Stay tuned!

 

[Dana]

Contractors who pay their invoices on time are usually the more-professional types- don't discount that. The distributor isn't likely to hand you the name of somebody with a track record of a gazillion warranty claims either. A lot of equipment failure is due improper installation, but it's often hard to sort that out after that fact.

It's usually better to just tell them what you want and let them try to disabuse you of your position if they disagree rather than play coy to see what they come up with. Almost no HVAC contractors run a proper load calculation- it takes time/costs money. They make their real money installing & maintaining equipment, not running load calculations. Many will just measure up the radiation and spec something that can still hit 180F with that radiation with room to spare. With 100' of baseboard that number would be about 50-60,000 BTU/hr of output, so it's still UFT-080W territory.

The "right" solution (nearly every time) is to size the boiler for the heat load, and size the indirect for the hot water load. An 80 gallon indirect is probably overkill even with back to back tub fills, even if a 40 gallon unit might fall short if the bath times are short. Sizing the pump for the indirect zone to be able to deliver 100% of the boiler output when the indirect is given priority is sometimes a weak point, but most manage to get it right. If you have a 40 gallon indirect on that 96K out boiler it had plenty of burner capacity for fast recovery, so if it was often running short it may have been either the pumping rate or an issue of not giving the indirect priority.