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Sweetheart
Posts: **4**Member

in Gas Heating

I want to replace my 34 year old gas boiler and take advantage of good rebates currently available. My house has about 1000 sq feet of living space to heat. It is a split entry, living area on 2nd story, lower story is unheated slab for laundry, storage, garage.

I did my own two heat loss calculations. One followed steps specified by Chester on Feb 9 on this forum, using two months of utility bills, heating degree days for my town found online, and outdoor design temp for my locale (also found online). I assumed that my current boiler is 80% efficient. The other used the calculator available at the US Boiler web site. I estimated that my 34-year-old house is neither extremely drafty nor tight. My results were: 26,460 MBH and 29,869 MBH respectively. These are in line with the "rule of thumb" I saw somewhere on this forum of 30 MBH per 1000 sq feet.

I got proposals from three contractors for a total of 5 different boilers, each also with an indirect DHW. Here are the boilers and their MBH ratings:

1. modcon Burnham Alpine 080W : input 16-80 MBH

2. modcon Viessmann 100 : 37-91 MBH

3. modcon Buderus GB 142/24: output 22.7 - 75.2 MBH

4. conventional (not modcon) Weil-MCLain GV90-3: Input 70 MBH, Heating Cap 65 MBH, Net AHRI 56 MBH

5. Bosch Greenstar Zwb28 (I don't know if this is similar to the first 3 or not. Online specs say it has a modulating fan...): Input 101 MBH, Heat capacity 91 MBH, Net Rating Water 79 MBH

I don't know how to compare the specs for the Weil-McLain and Greenstar. This is what the web sites showed.

I am most interested in one of the first 3. BUT - I am concerned that even though each is the smallest of its model series, they may be oversized. That is, should I look for a boiler where the low number is lower, e.g. around 10 MBH? How do I compare the numbers when one manufacturer gives the input, another gives an output? I don't know which it is for the Viessmann numbers. I am considering putting an addition of approx 400 sq feet to the living space (and an unheated garage space underneath) but that is down the road.

Should I remove any of these choices due to size alone? Can anyone recommend a more appropriate boiler, esp. regarding size?

Thanks for any ideas! I know next to nothing about plumbing except for my recent research online.

I did my own two heat loss calculations. One followed steps specified by Chester on Feb 9 on this forum, using two months of utility bills, heating degree days for my town found online, and outdoor design temp for my locale (also found online). I assumed that my current boiler is 80% efficient. The other used the calculator available at the US Boiler web site. I estimated that my 34-year-old house is neither extremely drafty nor tight. My results were: 26,460 MBH and 29,869 MBH respectively. These are in line with the "rule of thumb" I saw somewhere on this forum of 30 MBH per 1000 sq feet.

I got proposals from three contractors for a total of 5 different boilers, each also with an indirect DHW. Here are the boilers and their MBH ratings:

1. modcon Burnham Alpine 080W : input 16-80 MBH

2. modcon Viessmann 100 : 37-91 MBH

3. modcon Buderus GB 142/24: output 22.7 - 75.2 MBH

4. conventional (not modcon) Weil-MCLain GV90-3: Input 70 MBH, Heating Cap 65 MBH, Net AHRI 56 MBH

5. Bosch Greenstar Zwb28 (I don't know if this is similar to the first 3 or not. Online specs say it has a modulating fan...): Input 101 MBH, Heat capacity 91 MBH, Net Rating Water 79 MBH

I don't know how to compare the specs for the Weil-McLain and Greenstar. This is what the web sites showed.

I am most interested in one of the first 3. BUT - I am concerned that even though each is the smallest of its model series, they may be oversized. That is, should I look for a boiler where the low number is lower, e.g. around 10 MBH? How do I compare the numbers when one manufacturer gives the input, another gives an output? I don't know which it is for the Viessmann numbers. I am considering putting an addition of approx 400 sq feet to the living space (and an unheated garage space underneath) but that is down the road.

Should I remove any of these choices due to size alone? Can anyone recommend a more appropriate boiler, esp. regarding size?

Thanks for any ideas! I know next to nothing about plumbing except for my recent research online.

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## Comments

1,370MemberType of heat (baseboard?)

Number of zones?

Length of finned baseboard for each zone?

Total loop length of each zone?

"30 MBH per 1000 sq feet" is quite high, a "modern" home should be about half of that....

Was your gas bill/usage for the period actual or estimated?

1,257MemberLocated in Staten Island NY

Servicing all 5 boroughs of NYC.

347-692-4777

[email protected]

ASMHVACNYC.COM

https://heatinghelp.com/find-a-contractor/detail/asm-mechanical-company

7,356Member315Member[email protected] yahoo.com

(207)229-7717

334MemberYour boiler at this point is, yes, oversized for the heating load. However, the biggest factor here in sizing the correct boiler is the indirect DHW tank.

Yes your heating load is small, but the DHW needs may be larger which can then influence the size of heating appliance.

Dave H.

1,139Member334MemberI'm ecstatic that a heat load was done in the first place! Even though we are looking at a 1000 sqft home, maybe there is a large DHW demand, large soaker tub.......car wash shower.......

Not derailing the thread, this conversation needs to happen more often than not. Heat loads are much smaller than most realize and lots don't believe the numbers.

Dave H.

113MemberI would rather see the storage size made larger and have a longer recovery than an oversized boiler.

7,356MemberA maximum rate of 80-85k will provide double the recovery rate of a standard gas tank water heater.

3,130Memberclick hereto visit their ad in Find A Contractor.2,629MemberI have an 1150 square foot CapeCod type house. I have a mod-con that runs 16K to 80K (input) Btu/hr, and also heats my domestic indirect hot water heater. I estimate it is about twice as large as it needs to be, but W-M who made it makes no smaller mod-con at the time I bought it.

I now heat my indirect to 140F (minimum, though it can go up to 160F sometimes), with a temperature control valve so it delivers at most 120F to the nearest tap (my shower). The indirect rarely runs more than 10 minutes at a time.

Design temperature is 14F around here, though I have seen the outside temperature go down to about 3F once this year and once last year. Perhaps for only an hour or so each time.

4,975MemberYou do not need to add the heating and domestic loads because the domestic can run as a priority.

Keep in mind that a typical electric water heater has an output of around 16k/btu hr. Most gas models are around 30k/btu.

All the boiler choices you are looking at will have more capacity than that. Yes, they are all too big!

If you need more hot water, get a bigger tank.

If you multiply the Input rating of the boiler by it's efficiency, you will get the output rating.

Albert Einstein

4MemberYesterday I asked all 3 contractors for their heat loss calculations. One gave me a Manual-J result of 61,434 MBH for an outside temp of 2 degrees F, indoor temp 70 F. He had estimated a floor area of 1440, for a house that measures 40 x 25 on the outside. OK, I could say 1440 is the size I will have if I extend one end with an addition. But comparing his calculations with the calculations used on the US Boiler site, the heat loss factors he used seem much too high. They seem to assume very little insulation. For example, his heat loss from 218 sq ft of glass windows and glass doors came to 18,861. This means he used a factor of 86.5. I have storm windows and a double-paned sliding glass door. The US Boiler calculator suggested a factor of 34 for storm windows. Big difference. Who's right?

The ultimate reason I doubt the 61K MBH is that my calculation based on actual, not estimated, usage (bills for Jan 14 - Mar 15) yielded 26,460 BTU/hr. This used a method recommended on this forum by Chester on Feb 9. I assumed 80% efficiency for my current boiler. That said, there are some reasons why my calculated result might be too low and that the best number is between the two results, but closer to mine.

First, I'm an energy miser. I use a programmable thermostat set to 63F at night but during the day have it at 69-70 about half the time. I'm retired but not always home. So my figure of 26.4 MBH might be a low estimate for the load on a modcon system that "wants" to keep the house at a steady temperature of 70.

Second, I should probably bump up my number to account for future addition of 400 sq ft. Yet it's hard to believe that adding 400 sq feet to my living area (assumed in the contractor's calculation, based on his square foot numbers) and keeping the indoor temp at 70 would double the heat loss. Oh, my actual usage includes gas-fired DHW. By how much should I bump up my heat loss estimate to account for a steady indoor temp and 40% living area increase?

Is it possible to choose a boiler such that it is not oversized but can still handle a future 40% increase (going from 1000 to 1400 total) in living area? No change in DHW needs.

To answer a previous question on this thread, I live in eastern MA about 20 miles from the coast and have baseboard heat.

Another factor is that I want my new boiler to handle an indirect DHW. How much would that add to the required MBH? There is no chance that I will share my house with teenagers; it's just me and often my boyfriend.

Thanks to all for the feedback so far!

113MemberThe average house in my area is about 1500 square feet. If you have any sort of insulation in the house the smallest boiler will still be too large.

As has been mentioned I would stay away from boilers that don't provide high turndown.

4,975MemberYou should never size the boiler this way, but it will give you an irrefutably maximum output. If you can't get rid of the heat, there is absolutely no reason to produce it....

Albert Einstein

2,462MemberTell me I can't, and I'll show you I can.

1,139Member3,130Memberclick hereto visit their ad in Find A Contractor.83MemberDefinitely no need to upsize the boiler for DHW, especially in your situation. I have a 30 gal indirect and a 57,000 boiler and have never come close to running out of hot water.

As Aaron says, choose the contractor, not the boiler. Make sure they either understand how to calibrate the outdoor reset curve or are willing to make sure you know how to tweak it yourself.

Or, if you go with a Greenstar or Buderus I can definitely recommend taking a look at Bosch's new line of thermostats. They modulate the boiler based on indoor temp and seem to work as advertised.

863Memberhttp://www.usboiler.net/control-system/sage2-1-boiler-control-system.html

9,264MemberHowever Btu requirements for DHW needs becomes ever more critical when low heat loss requirements are encountered IF you choose a small mod/con with a 7:1, or 5:1 TDR.

Example in this thread would be say the whn, or KHN 50. With outputs at high fire not much more than a standard 40 gal tank style water heater. Will it do the job? yes, but not like what an indirect can offer with a higher btu output from a larger boiler.

This where Loch shines because their 10:1 KHN 85 has a low end not much higher than the 7:1 KHN 50. So you get the low end for low loads in mild weather, and the top end for better DHW production. smart.

1,139Member3,130Memberclick hereto visit their ad in Find A Contractor.4,239Member1,139Member1,102Member912Member443Member297Member7,356Member5,725MemberLiterature say's "aux tap for air handlers"...why would a HEX be different? Plenty of DHW available.

3,325MemberI like the IBC HC 13-50...

Simple and modulates down to 13,500.

Its most important that the installer knows whatever you chose very well.

297Member297MemberSo plenty of showers, two loads of wash & drying a day and cooking meals equals plenty of gas being used.

3,056MemberWhich ever contractor quote the Viessmann, quoted the wrong boiler. Should have quoted a B2HA19 not a WB1B. I would always increase storage before I'd increase boiler size if that increased size is going to effect the low end of the modulation rate. If it's not then it's not a big deal, boiler is only going to go out to the high end of the modulation rate if the system demand needs it to.

1,139Member741MemberGreat boiler, ideal for small houses.

Absolute Mechanical Co. Inc.

www.AbsoluteMechanicalCoInc.com

5,725Member7,356Member741MemberAbsolute Mechanical Co. Inc.

www.AbsoluteMechanicalCoInc.com