How do I compare boiler quotes?

[newhomeowner7]

We think we'll soon need to replace the 34 year old oil boiler in our home. We were told by the inspector when we bought the place that it's on its way out, and it's a tankless coil model that is not giving us nearly enough hot water. I can only fill the bathtub with about an inch of hot water to bathe the kids in the evening. So in addition to getting a new boiler, we're also going to install an indirect hot water heater.

We live in Massachusetts. Our home is a 4000 sq ft dutch colonial built in 1983. I'm told it has decent insulation but original windows. It has six heating zones (!). We currently have two adults and four children living the home and would like to have more kids.

I have gotten three quotes from angie's list-approved heating companies. But the language is so foreign to me and the recommendations and quotes we've gotten are so different that I'm having trouble comparing them.

Quote 1: Trio P4 (Net BTU of 106,000) cast iron boiler with SSU-60 water heater. All parts, permits, labor etc for $7,900, but he didn't give me a break down of exactly what components he'd be using or replacing.
This is the small, family-owned company that currently delivers our oil and serviced the boiler for the previous owners. When I asked how he calculated what size boiler I'd need he said he knew because he was very familiar with the house and its heating needs.

Quote 2: Buderus G215WS/4 (Net BTU 171,000) cast iron boiler with Tekmar 260 outdoor reset control and Riello burner and SuperStor indirect water heater SSU-60. Various sundries. He calculated the manual j load using Wrightsoft Universal. Parts, permits, labor, etc for total of $13,200

Quote 3: Burnham oil-fired boiler model Burnham MPO-IQ 159 (87% efficient) (Net BTU of 145,000) with 80 gallon hot water tank SSU-80. Lots of sundries in the quote that I don't entirely understand (like watts combo feeder valve and backflow preventer, burnham outdoor air option reset card, etc). He had me measure windows and doors and submit them to him in an excel sheet to do the manual J calculation. Parts, permits, labor, etc total $14,900

Where do I even start? How do I know how many BTUs I really need when the professionals give me a range of net 106,00 - 171,000? Is there a source for impartial boiler reviews? Is the high cost differential between quote 1 and quotes 2 &3 because the first boiler is undersized? Any advice would be so appreciated! Bonus points if you're able to explain your thoughts to me using lay language.

 

[Dana]

If you have any heating history on the place you can use the existing boiler to MEASURE the heat load, using the methods outlined in this bit o' bloggery. Even a couple of old oil fill up slips with a "K-factor" stamped on it would be enough to go on, if the fill ups happen in January-March time frame, reflecting primarily space heating use.

The oil delivery guy probably has that information from the prior customer, but may not be legally entitled to share it in detail, but ask him what the K-factors were for mid-winter fill-ups. He probably knows a lot more than the others about the heat load of this house, but upsized it a bit "just to be sure", but that's not what you want to do. The other proposals are in the "ridiculously oversized" range, but even his is suspiciously high.

Use only mid to late winter oil fill up quantities and dates, since the standby losses during the shoulder seasons for that high temperature standby needed for the tankless coil skews it by quite a lot.

A heat pump water heater will have a lower operating cost than an oil-fired indirect, and will double as a basement dehumidifier. I don't know if MassSave will subsidize that though.

A typical 4000' house 2x4/R13 house with 1980s vintage clear glass double panes that doesn't leak air like a tennis racquet would typically have a heat load of around or under 80,000 BTU/hr @ 0F outside, and most locations in MA have 99% outside design temps in positive digits. If it has an insulated foundation it'll usually be under, 70,000 BTU/hr @ 0F, under 60,000 BTU at +12F (Boston's outside design temp.)

If the boiler is in the basement, not the garage, size the boiler by it's DOE output BTU, not it's net-water IBR ratings. With a fuel-use based load calculation all of the distribution & standby losses are already included (since they're impossible to separate.) The net numbers are only appropriate if the boiler is located outside the building envelope, and on the other side of the insulation, where none of the standby losses are accruing to the heating of the house.

Per ASHRAE the most that you would want to up-size the burner would be 1.4x (using the DOE number, not the net) so if the heat load at your 99% outside design temperature is 75,000 BTU/hr, the "ideal" boiler would have a DOE output of no more than 1.4 x 75, 000= 105,000 BTU/hr, which would make every one of those proposals sub-optimally ovesized, and a some of them RIDICULOUSLY oversized (do you really expect to see -70F weather? Is there even enough radiation to deliver that much heat?) Even the Tri0 P4 has a DOE output of 122,000 BTU/hr

NEVER oversize the boiler for heating hot water. Make the indirect the priority zone- it'll recover well before the house loses even a one degree F. Size the indirect for the largest tub you'll need to fill.

Having six zones can be a potential short-cycling problem. Measure the radiation (I assume it's fin-tube baseboard?), zone by zone and report back. A heat purging controller like the one on the MPO-IQ or the Hydrostat 3250 on the Trio can fix some of that, but it won't fix everything. At 180F out, 160F back most fin-tube can only emit 500 BTU/hr per running foot. Even the smallest boiler proposed, the Trio P4 delivers 122,000 BTU/hr into the heating system (the DOE rating), and would need on the order of 122,000/500= ~250' of baseboard for the boiler output to balance with the baseboard output. So any zone would need 250' of baseboard to avoid cycling at all, and any zone with under 100' (which is probably all of them) would cause the boiler's burner to cycle pretty rapidly, enough to cut into efficiency.

You may or may not even have 250' of baseboard in the whole house, and if that's the case every one of these proposals is pretty lousy, but that's probably what you have. A DOE output of 122,000 BTU/hr for a 4000' house is a ratio of over 30 BTU/hr per square foot. That would be about right for a basically uninsulated house with single pane windows, or a mid-century modern house that's basically mostly-glass walls with clear-glass double-panes. The glazing area of most Dutch Colonials is pretty reasonable, and I'll bet you have at least R13s in the walls (R19s if it's 2x6 framing.)

If you want to do it by the book, instead of relying on HVAC folks (who spend their time and make their money on installing equipment), pay a P.E. or RESNET rater to run an aggressive room by room Manual-J heating & cooling load calculation on the place. That typically runs ~$500-700 in MA, (sometime more in some "gold plated" communities such as Nantucket or M.V.), but it'll pay back right away in lower up-front equipment costs, and several times over in lower fuel use, and higher comfort.

Sticking a wet finger up into the wind, without even knowing your location I'm going to hazard that both the fuel use calc and Manual-J will come in somewhere between 60-70,000 BTU/hr, and that a boiler with a DOE output of 80-85,000 BTU/hr like the G115WS/3 would be the right size. Even the Trio P3's ~91K DOE output is probably going to be a bit oversized, but at least it wouldn't be ridiculous. The MPO IQ84 might be undersized for the Polar Vortex days (or not- depends), but the MPO IQ115's DOE output of 101 would probably be sub-optimally oversized (especially with your zoning.)

 

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[newhomeowner7]

Dana, This is so incredibly helpful! And also mind-boggling. It startles me to read that three highly-rated HVAC companies could oversize by so much!

The blog you linked to was helpful in explaining design temps, etc. We're in a southwest suburb of Boston with a 99% outdoor design temp of 9 degrees F. Our boiler is located in the basement.

I'm not sure if asking the oil company for historical data will help in calculating heat load. The previous owners were an elderly couple and I know they spent months at a time in Florida. I don't know when they were or were not in the house or how hot they kept it. I do know that we're on an automatic oil delivery schedule and this morning we ran out of oil and had to call them to deliver it ahead of when they assumed we'd need it. We've only been in the house for a few months so I have no old oil delivery slips to go by.

We do have hot water baseboard heat which I guess is fin tube? You hit the money on the head when you assumed we had less that 250 linear ft of baseboard heaters. This is what I measured:
Zone 1 - 79'
Zone 2 - 23'
Zone 3 - 19.5'
Zone 4 - 19.5'
Zone 5 - 29'
Zone 6 - 66'

Total ~236 linear ft of baseboards

Does it make a difference if we use multiple zones at once? For example, with a range of kids, it would not be unusual to have two playing in the basement, a baby sleeping upstairs, a parent working in the office and another in the kitchen (four zones). But even with all of them on, it's still less than 250'

One thing that confuses me is when I initially started reading about boiler sizing, I found a lot of resources that suggested estimating appropriate boiler size by multiplying the sq ft of the house by some factor, depending on climate. The x factor generally ranged from about 30 to 55 for cold climates. So going in to this process I assumed I'd need a boiler with a BTU of between 120,000 - 220,000. Is there a different rule of thumb for gas boilers vs oil boilers? Could I have been conflating the two? Or is this a classic case of "someone is wrong on the internet"?

I'm thinking hiring a RESNET to do an aggressive manual j load will be a tough sell for my husband. I wish we had some old heating bills! Maybe I should just wait until February to calculate myself.... Just thinking out loud here. Thank you for your advice!

 

[Dana]

With 236' of baseboard the absolute maximum DOE output you should be looking at is in the range of 236 x 500= 118,000 BTU/hr. You don't have enough radiation to deliver the higher boiler output into the house unless you run it at 200F+. Even at 210F then the Buderus and Burnham proposals are oversized for the radiation, but your oil-delivery guy's proposal isn't oversized for the radiation, at 122,000 BTU/hr out, even if it's oversized for the house. A 200K DOE output boiler won't heat the house any faster or better than a 120K boiler- it'll just cycle more often.

Heat load isn't proportional to the square footage of the house- its more proportional to the square footage of the exterior surface area. Shape matters- a sprawling ranch with bump-outs an ells has a LOT more exterior surface area per square foot of conditioned space than a rectangular footprint 2 story house with a gable roof (which is similar the description of a Dutch Colonial). The difference in interior to outdoor temperature also matters. Got a ZIP code? (For estimating your 99th percentile temperature bin.) Using a BTU per square foot ratio rule of thumb is sorta-OK if using a hot air furnace, since even 4x oversizing doesn't impact efficiency (although it is a down-grade in comfort) but with a hydronic boiler oversizing results in significantly lower efficiency, due to the standby losses and thermal mass involved.

Even your largest zone only emits not even half the DOE output of the smallest proposed boiler, and except during the very coldest hours of the year there isn't going to be enough overlapping calls for heat to keep it from doing a lot of cycling. The shorter zones aren't enough radiation for even the smallest oil boilers out there. In micro-zoned low-mass radiation situations like this it's often better to have the radiation pulling it's water from a buffer tank, and have the boiler's only "zone" serving the aquastat on the tank. The calls for heat from the zones pull water from the tank, but the boiler only fires when the tank's temperature drops to it's low-limit, and the boiler is compeletely agnostic of whether one or even any of the zones are calling for heat.

There are buffer tanks with internal coils to be used as heat exchangers for potable water, sometimes called a "reverse-indirect", making it completely a one-zone system as far as the boiler is concerned. One such manufacture is Ergomax, another is Turbomax, but there are others. The thermal mass of the water in the tank and the difference between it's setpoint temperature, and the low-temp limit of the boiler establish a minimum burn time, independent of how much heat is being emitted, or how much domestic hot water is being used.

These are more expensive than a stanadard indirect, and since the domestic hot water calls aren't separate it's OK to oversize slightly from the space heating load to accommodate the domestic hot water load, but not by more than 2x.

How many bathrooms? (How many simultaneous showers need to happen at once?)

 

[Dana]

BTW: "The x factor generally ranged from about 30 to 55 for cold climates. "

That would would be RIDICULOUSLY oversized for a house your shape and vintage in a MA climate. Reality for 2x4 framed houses with clear-glass double-panes rarely has a load/area ratio of more than 20 BTU/hr p wer square foot @0F outside unless it has more than a 15% window/floor ratio. If it's reasonably air tight and the foundation is insulated it'll be in the 15 BTU/hr per square foot range. Even without foundation insulation a reasonably air tight Dutch Colonial could come in at 15 BTU/hr per square foot at say, Boston's 99% outside design temp (+12F.)

Note, even a factor as low as 30 x 40,000=120,000 BTU/hr , which is about the maximum your baseboard can even emit without resorting to higher than usual water temperatures, and it probably sails through Polar Vortex double-digit sub zero outdoor temps without losing ground.