Help Sizing Replacement Boiler

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Scott Thomas

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Need advice on sizing a replacement boiler for my home. I believe I need a smaller boiler, however I am not a professional and obviously could be wrong. My contractor is insisting on installing a new boiler of the same size. He believes the replacement must be equal in output to the existing boiler due to the size of the piping and radiators in my system. He didn't go around and count/measure radiators, however he did see the size of the basement piping, much of which has an OD of 3.5 inches. I'm not aware he did any kind of heat load/loss calculation.

The details are as follows:

I have a 1920's era brick 2.5 story home in the Reading, PA area that currently has a HydroTherm HC-145 gas boiler. I estimate it is 40+ years old, so I think I should replace it. The HydroTherm is rated new at 145k BTU/hr input/116k output (80% efficiency) and an IBR Rating of 100,900 BTU/hr. The tech who cleaned it last fall measured the output at about 100k (about 70%). When I moved in some 25 years ago, I replaced all 30+ windows with double pane and insulated the attic floor. I heat the first and second floors only, about 1800 square feet.

The boiler has always seemed more than capable of heating the house. In fact, I keep three of four bedroom radiators set to about half open all the time. Even so, at night the boiler overheats the house if I don't turn the thermostat down 2-4 degrees depending on the outside temperature. The colder it is outside, the more I need to turn down the thermostat before going to bed. If I forget to do this, the radiators become extremely hot and the temperature overshoots the thermostat setting. At that point it takes several hours for the radiators and house to cool down. We end up cracking windows. (Possibly I have a thermostat issue also. I don't know. But the system keeps a nice constant temperature during the day.)

The contractor quoted the following options: 1) a Bosch floor combi (152 MBH max, 36 min), 2) a Lochinvar wall combi (150 max, 15 min) and 3) a Weil-McLain standard cast iron (CGa-5 at 117 MBH heating capacity).

I decided I would go with option 3, a standard cast iron replacement boiler. However, if the current boiler seems oversized outputting at 100 MBH, a CGa-5 would likely be too large at 117 MBH. I suggested the CGa-4 (88 MBH heating capacity) at most, maybe something smaller. His answer was:

"A CGa4 would be too small for your system. At one time when the original radiators and piping was installed, it would have been a gravity system with larger piping and larger radiation. If you would install a CGa4, the radiators would not heat up enough to heat the house due to the larger radiation."

He may be right. However, I did my own calculations using Dana Dorsett's "Out With the Old, In with the New" (blog post on GBA) based on my natural gas usage over the past two winters. My heat load estimates came to between 35-38k BTU/hr (at about 70% efficiency; 65 deg base temp) and after applying the 1.4 ASHRAE sizing factor came to about 49-53k BTU/hr. If these numbers are reasonably accurate, even a CGa-3 with a heating capacity of 59 MBH might be acceptable. However, if the piping size is an issue requiring some allowance on the upside, the CGa-4 at 88 MBH might be sufficient.

Weil-McLain has a "Boiler Replacement Guide" for sizing boilers on its website that attempts to estimate a building's total heat loss. Using the Guide, I came up with an Adjusted Heat Loss of 71,000. According to the Guide, the Net AHRI Water Rating of the replacement boiler must be equal to or greater than the total heat loss calculation. The AHRI Water Ratings are less than the DOE Heating Capacities: the CGa-5 has a heating capacity of 117 MBH and a Water Rating of 77 MBH; the CGa-4 has a heating capacity of 88 MBH and a Water Rating of 77 MBH. So apparently the CGa-4 would have a sufficiently high Water Rating to heat my home. However, it also appears that an additional allowance must be made for "unusual piping and pick-up loads." Unfortunately no guidance is provided for calculating this allowance.

Sorry for the length of this post. Thank you in advance for any input.
 

DIYorBust

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Your house your call. Most contractors would rather deal with all the callbacks they get for a slightly too big boiler than a slightly too small one
 

PeterDux

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I don't know anything about large diameter pipes and radiators, but I can vouch for Dana's heat load estimator. My thread is here: https://terrylove.com/forums/index.php?threads/replacement-boiler-sizing-in-se-ma.81532/ along with a post-mortem on the first season's heating cost with a new boiler. The short story, however, is that I replaced 151K BTU boiler with an 84K BTU boiler: House has been perfectly comfortable and our fuel costs were reduced dramatically last winter. I had to push pretty hard for that boiler with my heating contractor, who was concerned it would be far too small for the house, but that's not the case. If anything we could have used an even smaller boiler, or a condensing boiler if natural gas were available to us.

See what folks say here about delivery piping and radiator size, but I suspect you'll learn that you've got better data on heating demand than the contractors. Good luck!
 

Sylvan

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In my home I did use smaller boilers as I have 11 rooms over 10 house built in the 1920 brick 4ft thick rock foundation 2" supply and 2" return piping


Your "peanut" boiler should be replaced same size as your not going to gain much going even smaller

If you have a two pipe system and put a non electric zone valve on each radiator then you can easily down the size the boiler.

If something is not broken stay with the same BTU input
 

Fitter30

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Your having to turn down the thermostat when it gets colder has to be the set up of the heat anticipator of the the stat.Thermostat might not be correct for your application. Boiler water temperature should vary on outside the temperature called outdoor reset. If your flue from the boiler just dumps into a brick flue might consider a flue liner.
 
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