Need New Boiler Sizing Help

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njb52

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Hello,
Our existing Utica PGB200A, gas fired boiler lists the following on its nameplate:

AGA Ratings:
Input Water: 200,000 BTUh
Output Water: 160,000 BTUh

Net I-B-R Ratings
Input Water: 139,100
Output Water: blank

I made a rough sizing calculation based upon :
Linear Fin Tube Feet x 650 BTUh = BTUh required
I came up with 180 linear feet x 650 BTUH = 117,000 BTUh
Is there a more accurate way, that a layman can use to size as well? I was thinking of measuring the lineal feet of my exterior walls and total window square footage, if that helps fine tune the sizing.

Thanks in advance :)
 

Dana

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You need both the heat load and the radiation to come up with the right sizing. You only gave half the information.

And 650 BTU/hr would be at a very high water temperature, say 200F out, 180F back for a 190F average water temp (AWT). The 650 BTU/hr per foot is just a specified output at a particular temp, not the output actually needed to heat the place. If you don't need an AWT of 190F to heat the place, you'll get more system efficiency and boiler longevity out of right-sizing the boiler to the load, running it at a lower temp to cut down on distribution losses. If you sized the boiler to the radiation presuming it needs 650BTU/hr THEN run it at a lower temp you'll short-cycle the boiler into lower efficiency & longevity.

For cast iron boilers running at a much lower temp only become problematic if the entering water temp (EWT) at the boiler is under 130F, but it's also possible to run the system at temps lower than 130F with some near-boiler plumbing to keep the EWT at the boiler in a safe range. (We'll burn that bridge if/when we come to it. :) )

If you have a heating history on the place, what's your ZIP code (for weather data and outside design temp) , and the fuel used over a couple of recent billing periods with the EXACT meter reading dates, it's easy to calculate an upper bound on the actual heat load by measuring fuel use against heating degree-days to come up with a BTU per degree-hour constant. Heat load increases fairly linearly with the temperature difference between the heating/cooling balance point (presumed 65F unless you keep the house cooler than 68F almost all the time), so with that BTU/degree-hour constant at the difference between your 99% outside design temp and the balance point it becomes simple napkin-math, a predictive model good enough for sizing the boiler.

A 117,000 BTU/hr IBR output boiler would be 2-3x oversized for most homes in the US. (You'd only use the IBR rating if the boiler is outside the thermal envelope of your house anyway.) A typical 2x4 framed 2500' house with some insulation and window upgrades will come in with a heat load between 35-45,000 BTU/hr @ 0F (or even less if it's brought up to IRC 2009 or newer code minimums). If the heat load at whatever your real outdoor design temp is 40,000 BTU/hr, your existing boiler is 4x oversized for the load, and won't hit anywhere near it's nameplate AFUE. AFUE testing presumes 1.7x oversizing, and with the boiler inside conditioned space (say an unfinished basement, but not in a garage on the other side of an insulated wall.) ASHRAE recommends no more than 1.4x oversizing for comfort & efficiency. That's enough to have you covered during extreme cold snaps, and would deliver long efficient burns.

With 180' of baseboard and a load of 40,000 BTU/hr the baseboard only needs to emit (40K/180'=) 222 BTU/hr per foot, which can be delivered at an AWT of about 125F, which means it would be a good candidate for a small condensing boiler, and it also means your distribution losses would be less than half that of running the system at an AWT of 190F. There are several small cast iron boilers that could fill the bill if that's what your design load is and at least dozen small condensing boilers.

Is it broken up into zones, or run as a single zone?

If multi-zones, what is the baseboard length per zone?
 

BadgerBoilerMN

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Naturally, Dana is right.

If your contractor is bright enough to know that the measured radiation is the potential maximum for sizing the boiler, he will come in with the smallest (lowest output) boiler of all the three bids you solicit. If he produces an ACCA Manual 'J' he will be your man.

It is however, accepted practice to over-size a low-efficiency cast iron boiler to 150% of the actual load. This number, DOE output, is more a reflection of the inherent inefficiency of a high-mass heat exchanger that allows for vast energy loss and generally beneficial lower burner cycles.

If you tell us where you are, the age and size of your house Dana can do a better heat load in his head than most professionals on a program, but don't stop there, since the Manual 'J' is the only way to tell for sure that your contractor even thought about it.
 

njb52

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I can’t believe how lucky I was to find this forum! Thanks so much, Dan and BadgerBoilerMN, for responding so quickly and comprehensively :).

Here are the answers to your questions for more information:

The is a small circular temperature gauge on the outside of the boiler reading 20 psi on the top half and 150 degrees on the bottom half.

My zip code is 01880, and the following is the fuel consumption/dates/meter readings for 2014/2015, since it was a bear of a winter compared to this year’s mild one:
  • 11/2014 reading 2304,
  • 12/2014 reading 2386,
  • 1/2015 reading 2507,
  • 2/2015 reading 2715,
  • 3/2015 reading 3113
Total used for this time period is 809. We keep the house thermostats at about 66 to68 degrees

We own a raised ranch (fancy name for a split level house) with a detached garage, so all of main house is living area. Each floor is 44 ft x 26 ft, times 2 floors equals a total of 2,280 sq. feet. An unheated loft/attic, approximately 26 ft x 20 ft, is above our bedrooms. Our home was built in 1975. We work out of two ground floor offices (slab on grade) in our home. Upstairs LR, DR & Kitchen have a cathedral ceiling - approximately 26 ft x 24 ft, and about 16 ft to ridge.

We have 6 zones assigned as follows:
  • Zone 1 - Downstairs office one, bath & hall (640 sq. ft), baseboard 24 lineal feet
  • Zone 2. - Downstairs office two (504 sq. ft), baseboard 38 lineal feet
  • Zone 3 – Upstairs Master BR & Bath, baseboard 25 lineal feet
  • Zone 4 - Upstairs LR, DR, Main Bath, & Kitchen baseboard 42 lineal feet
  • Zone 5 - Upstairs bedrooms 2 & 3, baseboard 28 lineal feet
  • Zone 6 – Future expansion to unheated loft/attic, projected baseboard 28 lineal feet

We are looking to get a plain vanilla boiler (like our present one) rather than a condensing boiler. My purpose for using a baseline calculation, is so that all contractors are bidding o the same criteria so I can compare apples to apples. It is good to hear how ACCA Manual J has a standardized calculation method and is a way to smoke out a knowledgeable contractor.

If you need any further info, please let me know, and I cannot thank you enough :D
 

Dana

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It only works with any accuracy if you have the EXACT DATES of the meter readings, not the month. One week of Polar Vortex or unusually warm weather that fell outside the exact period of that fuel use can skew the results by quite a bit.

That said, unless you spent a month in Belize with the thermostat cranked back to 50F your heat load isn't very big. Assuming the meter gets read mid-month, downloading base-65F HDD from weather station KMAREADI3 in nearby Reading MA and totaling the HDD from November 15 through March 14 (inclusive) it comes to 4479HDD, over which you used 809 therms in an 80% efficiency boiler, which means at most 0.8 x 809= 647.2 therms of heat was injected into the heating system.

That's 647.2/4479= 0.1445 therms per HDD

x 100,000 BTU/therm= 14,450 BTU/HDD

Divided by 24 hours in a day is 602 BTU per degree-hour as your constant.

The outside design temp in Wakefield is about +7F, which is 58F cooler than the presumptive 65F heating cooling balance point. The implied maximum 99% design heat load is then about

58F x 602 BTU per degree-hour= 34,916 BTU/hr.

As a sanity check, ~35,000 BTU/hr into ~2300' of living space is about 15 BTU/hr per foot of conditioned space, which is on the high side of normal for a 2x4 framed raised-ranch, but not outlandishly high. (That would be more typical at an outside design temp of 0F, not +7F, maybe 10% higher than I would expect, but still "in the range".) There are probably some cost-effective air sealing and spot insulation measures to be taken. If the upper floor is cantilevered it probably leaks air there, with a high likelihood of discontinuity in the insulation to boot.

If you assume that's the actual heat load (it's actually an upper bound, and without exact billing dates it could be off by 5% but not 10%) applying the ASHRAE 1.4x maximum oversizing, the biggest boiler you would be looking for would a DOE output in the range of (1.4 x 34,916 BTU/hr=) 48,882 BTU/hr

Among dumb-controls no frills cast iron beasts out there the Burnham P202X fills the bill, as does the Weil McLain CGi-3 (though the CGi 2.5 is probably a better bet if you can nail the load down with better accuracy) or the SlantFin S-60. There are others.

With any of these boilers, if all zones are calling for heat at once, 50,000 BTU/hr into 180' of baseboard is about 280 BTU/hr per foot, which balances at a water temp of about 135F, so it's likely that the installer would have to install a bypass branch at the boiler to keep the EWT high enough to mitigate condensation risk. This is very standard stuff, but it's important to ask the contractor what approach they are going to take, if only to tip them off in case they don't usually do the napkin-math analysis. If you didn't mind the electronic controls, the Burham ES23 is internally plumbed with cool return water protection down to 110F EWT, but it's 59K DOE output is more like 1.7X the output, and a bit more problematic for your zoning configuration:

Looking at your zones, the shortest are 24-25' long. 50,000 BTU/hr into 25' of baseboard is 2000 BTU/hr per running foot, which is 3x as much as the baseboard can actually deliver even with the boiler temp set to it's maximum. That's a recipe for short-cycling the boiler into lower efficiency & higher maintenance but it'll probably work, since most of the time you'd likely be having 2-3 zones calling at any one time if you right-size it. If you put anything BIGGER than a 50K boiler it becomes something of an efficiency disaster, unless it's an unusually high mass boiler. The Utica's 160,000 BTU/hr output into 25' of baseboard is exactly this sort of disaster, which means your real heat load is likely to be LOWER than the fuel use calc indicates.

Ideally the smallest/shortest zone would be at least 50' of baseboard, and 75' would be even better, since a 50K output boiler could even balance with the radiation output delivering a single burn, not cycling at all during a continuous call for heat from just one zone. Zoning it by floor rather than room-by-room micro-zones is usually better, but the baseboard in each room needs to be proportional to the calculated room loads for that to work without big temperature differences between rooms. (Again, a room by room Manual-J would be able to suss that out.)
 
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Dana

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P.S. Just about any 50-70K input modulating condensing boiler with a stainless steel heat exchanger would be a slam-dunk on this place if you consolidated some of the zones to keep it from short-cycling at low water temps.
 

njb52

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Sorry I did not get back to you earlier – I had a lot to digest from your detailed calculation. I am very appreciative of you providing me with the calculation and your expertise. Your commentary was very helpful for me to understand the numbers, their implications on the entire system and the perspective. That 280BTU/hr per foot is such a huge difference from the 650 BTU/hr per foot that I was using.

Thanks also for the boiler model recommendations and also on adding the bypass loop for the cooled water return- I will make sure to include it in the scope of work.

As far as the ACCA Manual J calculation, I want to be very considerate and not have bidding contractors waste their valuable time using such a labor intensive calculation. Instead, I am thinking of first using your fuel consumption load calculation as a baseline for pricing. After awarding the work, I can then have the contractor that I hire refine the load calculation by using Manual J. What is your opinion on that approach?

When using the Manual J calculation, what data points can I supply the contractor with to facilitate his calculation in a less labor intensive way (total square footage of walls, windows, or whatever else)?
 

Dana

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A Manual-J is useful when designing the whole system, radiation included. But for sizing a retrofit boiler when you have a heating history it's overkill.

A lot of HVAC contractor Manual-Js are pro-forma slop jobs of it using rediculous air leakage assumption and incorrect window & wall U-factor information to steer them toward their preconceived ideas (sad but true), and excessive assumptions about system distribution losses. Not everybody is as careful as BadgerBoilerMN. A fuel use analysis on mid-winter fuel bills has fewer fudge-factors. It's not a WAG or an estimate, it's a measurement of the heat load, using the pre-existing boiler as the measuring instrument. The instrument isn't super accurate, and usually lower (never higher) efficiency that the nameplate efficiency, but it's WAY more accurate than a sloppy Manual-J or a crummy rule of thumb approach. And it includes all of the distribution & boiler jacket losses (since that can't be separated out.)

To do a Manual-J right you need to know the R-values and material stackup of the walls (including the type of sheathing, interior & exterior) the published or calculated U-factors of the windows & doors, the power of the 24/7 electrical loads, the number of human occupants, the R-value of the attic insulation, the foundation, the relative air-tightness of the house (which is usually a WAG, though some will use blower door test data to estimate infiltration losses.

The 650BTU/ft is the absolute maximum that the radiation can emit. Like most houses, you have a lot more baseboard than you actually need to heat the house in it's current condition. Even 280 BTU/ft is more than you need to heat the house- that's just how much it would be delivering with 50,000 BTU/hr of boiler input. Your actual heat load at +7F outdoor temps is less than 35,000 BTU/hr, which would only take 200 BTU/ft. The water temperatures required for 200 BTU/ft output is well into the condensing zone for condensing boilers, and you WOULD get the benefit if you went that route, unlike a house that actually needed the baseboard to emit 650 BTU/ft to cover the load.
 

njb52

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Thanks again for all of your help. I will most likely post again as we get closer to getting bids.

Thanks also for the specific conventional boiler manufacturers. You mentioned that there were others as well.
Can you give me a few more manufacturers of cast iron units that you have had success with?
 

Dana

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First, I'm not directly in the HVAC biz, so the number of models that I personally "...have had success with" is a vanishingly small subset of all available boilers. In MA when specifying cast iron boilers proposals from Burnham installers seem to win half or more of the contracts I've been even tangentially involved with, but again, not huge sample size. The bigger problem you're likely to run into is installers pushing you toward larger boilers that you clearly DON'T need or want.

Both Burnham & W-M are well supported by contractors and distributors in our area- I'm sure you could get competing bids on a P202X or CGa-3 if that's really the way you want to go. They're dumb boxes of rocks and somewhat oversized for the load, but they'd work. The gas version of the Biasi B-3 would be OK too, but I'm not sure how much local support there is on it. (The importer is located in Portsmouth, NH, so you could always drive up there and scream at them if it craps out, but the product reputation is pretty good.)

Any right-sized atmospheric drafted boiler is going to require a right-sized flue liner for it's BTU ratings. The existing flue is probably even oversized for the old 200,000 BTU/hr behemoth, but it didn't matter too much at 80% efficiency. With the higher efficiency requirements of new equipment the exhaust output is cooler, and in an oversized flue becomes a flue-condensation problem. It's sometimes better to go with a side-vented forced draft model and just seal up the old flue. The Burnham ESC-3 is a direct-vented side vented model that would be about right, but if you can't tolerate the electronics in the controls, it's a non-starter. And it's not always cheaper than a modulating condensing boiler since it requires expensive stainless steel venting as opposed to plastic.

Seriously, with your zoning setup, design heat load, and the water temp requirements you'll be better off using a condensing hot water heater as the boiler than cast iron. The HTP Phoenix Light Duty would work- the manufacturer is located in MA, and there is good local support.
 

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Thanks very much, Dana, for making me aware of another critical component. I did not know that a properly sized unit would produce cooler exhaust temperatures that would lead to condensation in our existing over sized flue.
The current balance worked perfectly for years because of the over design of our existing boiler. The duct from the boiler is 8 inches in diameter and enters the flue along with a 4 inch exhaust duct from our conventional domestic hot water heater, that we replaced last year.

Our existing flue is encased in a chimney, exposed on all sides in heated living space. It runs inside of our home for about 21 feet (in the heated space) and then another 5 ft outside above the roof ridge. I was wondering if having the flue in heated space, for the majority of its height, would minimize the risk of condensation and allow us to reuse the existing flue. If not, the boiler/utility room has a suspended ceiling, so we could route the new exhaust duct up then over between the 2 x 10 floor joists. The manufacturer spec sheets, that you provided, show exhaust ducts of 4 inches in diameter, based on units that corresponding to the 50 k load that you calculated. This new exhaust duct should fit fine with about 3 to 4 inches of clearance under the sub flooring. I am assuming that the exhaust temperature will be to high to use PVC. And that we will have to use a sheet metal duct. Another item would be re routing our existing domestic how water exhaust the same way (up and over between the joists to the outside) if we cannot use our existing flue. Is there any maximum run for the exhaust duct work if we cannot reuse our flue? The reason I ask is because the horizontal run to the exterior wall is about 8 ft.
 

Dana

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Any newer boiler (right-sized or not) produces lower exhaust temps, an artifact of higher minimum efficiency standards.

A 4" liner for the existing flue with 4" B-vent between the boiler and chimney would work just fine.

The far better choice would be to buy this cheap mod-con boiler (it's even cheaper than the ESC-3 or ES2-3), and side-vent it with plastic vent pipe. This particular mod-con is easier to install than most, and the installed cost should be LESS than another cast iron boiler, since it has cheaper venting materials. (It's exactly the same boiler as the HTP 080W.) In MA it would even qualify for a substantial rebate subsidy, further reducing the out of pocket cost. Even if it only lasted 10 years it would save more than it's purchase price in fuel costs over the small cast-iron, even at current low natural gas pricing. With the outdoor reset controls tweaked in it would also be a LOT more comfortable.

At min-fire the thing puts out about 7600 BTU/hr. Into a 24' zone that works 317 BTU/hr. That balances just fine with 140F average water temps, which is just slightly above the condensing zone. While that would still cycle a bit at 120F AWT (where it would be running 95% combustion efficiency) it wouldn't be terrible. If you can combine some zones to where the shortest zone is at least 35' of baseboard it would run at condensing temps with no cycling. But even if you kept the zoning scheme as-is, at low boiler output temps the calls for heat would be long and overlapping- you probably don't need to change a thing (other than the boiler and a smarter system pump that uses less than 1/4 the total electricity.)

With a modulating condensing boiler you use less fuel if you keep constant room temperatures or only modest overnight setbacks, since that lowers the average operating temp requirements, for more condensing efficiency.
 

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Thanks very much for your recommendation of using a new 4 inch liner and B vent so I can reuse our existing flue. I am assuming that the liner would also need to be insulated to avoid condensation and that I would need a Tee section to connect both the new boiler and existing hot water heater to the flue liner – am I correct?. Can a plumber do the new liner and associated duct work install, or would I need a chimney sweep to do that work and get separate pricing?
In an earlier post you mentioned return water protection. Do you favor a boiler bypass or a system bypass?
 

Dana

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System bypass, with a tweakable ball valve. You don't need a lot of protection, since it's low-mass radiation and only nears the danger zone when literally ALL zones are calling for heat at once.

Adding flue liners for mid-efficiency cast iron is standard stuff for boiler installers. If it's not spelled out in the quote, ask about it. If they respond with a blank stare or use it as a pretext for raising the quote, use a different contractor. It's not an "extra", it's a requirement under MA code.

Get some quotes on those cheap 80K 10:1 turn-down mod-cons too. You may be surprised (the MA rebate subisidy almost cover the purchase price of the boiler itself.) Another inexpensive mod con with comparable modulation range is the Navien NHB-080.

If you don't combine some zones you'll be running a lot more burn cycles with the cast iron boiler (even a tiny one), which will take a toll on both efficiency and maintenance. The ignition systems of new boilers won't take 1,000,000 cycles, and with your zoning they won't last anywhere near as long as an old-school standing pilot ignition (that no longer meets regs for new equipment.) With a modulating boiler that can cut back to 8000 BTU?hr the burn times will be long, and calls for heat from zones will usually overlap.
 
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