Trying to determine the best boiler replacement

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sakle2k

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Hello all,
I've been reading this forum quite a bit in the past week and have realized how important it is to have a knowledgeable person installing your boiler. I'm hoping to get a better understanding of what my needs are so I don't make a mistake.

I have an old Weil-Mclain boiler that I'd like to replace in a very small house (about 875 sq ft). I have 46' of slant fin baseboard downstairs and 53' upstairs. I have no idea of the model of the current boiler or if it's sized correctly. A friend installed zone valves a few years ago and I've noticed at night when the downstairs thermostat is turned down, the boiler fires on and off every few (maybe 5) minutes. And, the room upstairs farthest from the boiler (which is on the 1st floor) is always cold.

I started to look at combi boilers as we only have 1 bathroom and 3 people living in the house. I'm getting ready to renovate the kitchen (very small) where there is a closet that hides our water heater. If I could use a combi boiler, I'd be able to get rid of the water heater & closet and build an additional storage cabinet in its place which would help with our small space constraints. I've also read about using a boiler with an indirect tank for DHW. Although I'd just be replacing the water heater with a different tank (and not benefitting from removing the closet), at least everything is new and efficient. The problem I see with that is the water heater is located almost 15 feet from the boiler and I don't have the room to install the new tank next to the boiler. Can the tank be this far from the boiler in this type of setup?

I was also looking at a replacement cast iron boiler like the Burnham ES2. Seems like simpler technology and less that could go wrong. I'd appreciate any information/suggestions that you may have.

Thanks,
Les
 

Dana

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Combi boilers are only a good fit for homes with high heat loads and low to moderate hot water needs.

To run at condensing temperatures without cycling the minimum modulated output needs to be about 200BTU/hr x baseboard length in feet, or lower. With a ~50' zone that would be 200 x 50' =10,000 BTU/hr. While the smallest Navien NCB 150E goes almost that low and can be set up to not short-cycle itself into an early grave the way your Weil McLain currently doing it's domestic hot water delivery in is pretty pathetic at NJ wintertime incoming water temps. There are several modulating condensing boilers that could handle it though, using an indirect water heater as a third zone. For a longer explanation of these issues, read this bit o' bloggery.

The smallest ES2 puts out 59,000 BTU/hr. Even at 180F output/170F average water temp the SlantFin is only emitting about 500BTU/hr per foot, so a 46 foot zone is only emitting 23,000 BTU/hr, which less than half it's output. Even if you cranked it to 200F out on the boiler the 46' zone would still be emitting less than 30,000 BTU/hr. Bottom line- it's going to short-cycle itself silly just like your current boiler, and it would need an indirect too.

Installing an indirect 15' away from the boiler isn't a problem- that's what pumps are for.

I suspect that if your walls and attic have any insulation at all, and you also have glass in our windows, doors that close, etc your design heat load is under 20,000 BTU/hr. Most likely your 99% design load is lower than the minimum fire output output of any combi boiler big enough for reasonable hot water service. Homes with loads that low are often better served by a condensing tank type water heater for both heat and hot water (isolating the heating loop with a heat exchanger) , or couple ductless or ducted mini-split heat pumps plus something else for hot water. The ~ 100' of baseboard it was probably installed to be able to not short cycle a small cast iron boiler to death, a strategy that was defeated when you broke it into two zones. But with that much baseboard you can probably heat the place adequately at domestic hot water temperatures.

Since you have a heating history on the place, run a fuel-use based heat load calculation/measurement and report back. (Or if you like, with a ZIP code and exact meter reading dates & amounts I could run those numbers for you here.)

Full basement, crawlspace, or slab on grade foundation?

The upstairs room that is colder than the rest needs more analysis. How many square feet of window, how many square feet of exterior wall (not counting the window area), and how many square feet of ceiling?

Is there an insulated attic above the second floor? If yes, are there air conditioning ducts up there?
 

sakle2k

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Combi boilers are only a good fit for homes with high heat loads and low to moderate hot water needs.

To run at condensing temperatures without cycling the minimum modulated output needs to be about 200BTU/hr x baseboard length in feet, or lower. With a ~50' zone that would be 200 x 50' =10,000 BTU/hr. While the smallest Navien NCB 150E goes almost that low and can be set up to not short-cycle itself into an early grave the way your Weil McLain currently doing it's domestic hot water delivery in is pretty pathetic at NJ wintertime incoming water temps. There are several modulating condensing boilers that could handle it though, using an indirect water heater as a third zone. For a longer explanation of these issues, read this bit o' bloggery.

The smallest ES2 puts out 59,000 BTU/hr. Even at 180F output/170F average water temp the SlantFin is only emitting about 500BTU/hr per foot, so a 46 foot zone is only emitting 23,000 BTU/hr, which less than half it's output. Even if you cranked it to 200F out on the boiler the 46' zone would still be emitting less than 30,000 BTU/hr. Bottom line- it's going to short-cycle itself silly just like your current boiler, and it would need an indirect too.

Installing an indirect 15' away from the boiler isn't a problem- that's what pumps are for.

I suspect that if your walls and attic have any insulation at all, and you also have glass in our windows, doors that close, etc your design heat load is under 20,000 BTU/hr. Most likely your 99% design load is lower than the minimum fire output output of any combi boiler big enough for reasonable hot water service. Homes with loads that low are often better served by a condensing tank type water heater for both heat and hot water (isolating the heating loop with a heat exchanger) , or couple ductless or ducted mini-split heat pumps plus something else for hot water. The ~ 100' of baseboard it was probably installed to be able to not short cycle a small cast iron boiler to death, a strategy that was defeated when you broke it into two zones. But with that much baseboard you can probably heat the place adequately at domestic hot water temperatures.

Since you have a heating history on the place, run a fuel-use based heat load calculation/measurement and report back. (Or if you like, with a ZIP code and exact meter reading dates & amounts I could run those numbers for you here.)

Full basement, crawlspace, or slab on grade foundation?

The upstairs room that is colder than the rest needs more analysis. How many square feet of window, how many square feet of exterior wall (not counting the window area), and how many square feet of ceiling?

Is there an insulated attic above the second floor? If yes, are there air conditioning ducts up there?

Dana,

Wow, I can't thank you enough for the information and insight you've provided.

The house has a crawlspace. As for the cold room, the windows total 21.65 sq/ft, exterior wall (not counting the window) is 45.93 sq/ft and the ceiling is 79 sq/ft. Insulated attic above with no air conditioning vents.

The heat load calculation is 27,731.6 BTU/Hr, based on base 65°F heating degree-days for dates 1/22 - 2/20 with 146.69 therms used. The 99% outside design temperature used was 13 F. Zip code is 07734. The boiler has an input rating of 133,000 BTU and output rating of 109,000 BTU.

Based on these figures are you still leaning towards a condensing tank type water heater for both heat and hot water? And is there a way that I can still use 2 zones? Do you need any additional information from me?

Thanks again Dana for taking the time to help.

Les
 
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Dana

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Without checking the math (could be right, but seems really high, by about a factor of two for a house that size), 27,732 BTU/hr divided by 99' of baseboard is 280BTU/hr per running foot. Looking at the Slantfin output specs it means your baseboard would deliver that much heat at an average ater temp of about 135F, which could be 140F out, 130F back, or if pumped more slowly 145F out, 125F back. With an entering water temp at the boiler of 125F you're already in the 91% efficiency range. The other 99% of the hours in a year you won't need water that warm, and could be operating well in the condensing zone at 95% efficiency.

An inexpensive condensing boiler (cheaper than an ES2-3) that is easy to retrofit and would fill the bill is HTP's UFT-080W, which modulates down to about 7600 BTU/hr in condensing mode, but can crank it up to over 70,000 BTU/hr at non-condensing temps. It's a bit overkill at high fire, and will probably work out better if programmed to limit itself to 75% if it's max fire. That boiler comes pre-plumbed with and extra port and some control features for supporting an indirect water heater. Being a fire tube heat exchanger design has low "pumping head", designed to take higher flows than water-tube mod-cons, and can be plumbed to pump direct, taking the full flow of the radiation rather than set up primary/secondary. That saves the cost of a secondary pump, and simplifies the plumbing by quite a bit. The exact same boiler is sold under the Westinghouse nameplate as the WBRUNG080W. There are floor mounted versions of this boiler too, for $100-200 more, if that makes installation easier, but the wall hung version is usually under $2K.

This could also be done with an HTP Phoenix Light Duty PH76-50 (50 gallon tank, 76KBTU burner) and an exterior heat exchanger plate, if it fits in your water heater closet or boiler room. Not all hydronic designers are up to the task but it's not rocket science. Richard McGrath at Langans Plumbing and Heating down in Toms River NJ could certainly handle either approach, if you can't find anybody more local to you.

Using a programmable "smart" ECM drive pump is highly recommended to be able to optimize the system flows and minimize pumping power. When the "outdoor reset" function is dialed-in to perfection the thing will run almost constantly, which can add up to a lot power use if using a dumb low-efficiency pump. There are several out there that would work, but impress upon the contractor that you want a high efficiency pump, and will be adjusting the outdoor reset to where it will run extremely long burns. An ECM drive pump will pay for itself several times over in lower electricity use, running mayb 12-25 watts instead of 75-100 watts on a system like yours.

Using weather data from station KLDJ at the Linden airport it logged 908.7HDD from 1/22 through 2/19, and 901.1 HDD from 1/23 through 2/20, so averaging them it's about 905 HDD.

146.69 therms/905HDD is 16,209 BTU/HDD. Divided by 24 hours that's 675 BTU/degree-hour source fuel energy. At 82% efficiency that's 553.5 BTU/degree hour (indeed a high constant for a house that size.) At 13F you have 65F- 13F= 52 F heating degrees, for an implied load of 28,782 BTU/hr. That's the same ball park you came up with (probably a different weather station), but it's really pretty high, implying high outdoor air infiltration or major gaps in the insulation or something.

There is probably some very cost effective low-hanging fruit to pluck on the air-sealing and insulation front on this house. Is it a vented crawlspace?

How much baseboard is there in the cold room?

Edited to add:

Ignore the attachment- not sure how it snuck in there or how to delete it, but it's meaningless
 

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sakle2k

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I used the Belmar - Farmingdale (KBLM) weather station - 939.8HDD, and you're right on the money when it comes to the air-sealing of the house. It's old with poor construction. The kitchen is especially drafty through the cabinets but that will be solved once the kitchen is redone. There are some other areas that can use some work as well and they will be addressed. The crawl is not vented - the house sits on a stone foundation with an access panel on 2 sides. And the cold room has about 17 feet of baseboard. The windows in that room definitely need to be replaced, but the heat output from the baseboard just isn't as good as the other rooms (it's also the farthest from the boiler).

I appreciate you pointing me to specific units so that I can get a sense of what I need, as well as suggesting a competent plumber. Between the 2 you suggested, do you have a preference? As they're different types of units, would you predict one outlasting the other? Are the pro's of one better than the other? And with either of these would having 2 zones work or should I just keep the whole house on a single zone?

Thanks,
Les
 

Dana

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I'd need to see the house to get a better sense of which approach is more suitable. I suspect the boiler + indirect would last longer, but I don't know that for sure- if you pick a lousy indirect the indirect may fail long before the boiler does. The all-stainless Phoenix Light Duty should go for at least 20 years in a residential application, even when used as a combi heater. The burner modulates between about 25K and 75KBTU/hr, which means it has more "extra" burner capacity above your heat load than most 50 gallon water heaters. It's only "Light Duty" relative to the burner sizes of other commercial water heaters, and would outlast most residential gas water heaters by more than 2x. You can get the same water heater under a Westinghouse label too, if they're better supported locally than HTP.
 

sakle2k

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Thanks Dana, sounds like I’m ready to start talking with plumbers. Any opinion on continuing to use separate zones? Before we added the zone valves, in order for the upstairs to be comfortable, downstairs would be too warm.
 

Dana

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Thanks Dana, sounds like I’m ready to start talking with plumbers. Any opinion on continuing to use separate zones? Before we added the zone valves, in order for the upstairs to be comfortable, downstairs would be too warm.

Zoning floor by floor is fine. With 46 feet of baseboard on your shortest-radiation zone it'll do OK with a modulating condensing boiler as long as it can throttle back to under 10,000 BTU/hr. If you go nuts on it and break it into a gazillion tiny zones so small that it can't be managed with modulation the tank type water heater version will do better, since it has enough thermal mass to work with to keep it from short cycling.

In my oddly shaped home there are two rooms on the first floor with heat gain/loss characteristics different enough from the others that it didn't work well as a single zone so I ended up installing buffer tank to have enough mass to manage the cycling issues. Had I been starting from scratch it would have made more sense (and cheaper overall) to do it with a water heater. In my smallest zone there is only nine feet of cast iron baseboard, which has at least a modest amount of thermal mass, but the lowest my burner modulation output is about 18,000 BTU/hr.
 

sakle2k

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That’s great, my wife thanks you! Seriously, thanks for taking the time to answer my questions, I understand so much more than I did yesterday.
 
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