Assistance in sizing a new hot water boiler?

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John Gayewski

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Homeowners can abate asbestos in lowa then it ends up land fills. Asbestos in the 50s and 60s was in everything. Floor and ceiling tile, pipe covering, fire brick, plaster, vermiculite insulation and boiler cement just to mention a few things.
https://www.google.com/url?q=https://humanrights.iowa.gov/sites/default/files/media/Asbestos%20in%20Homes.pdf&sa=U&ved=2ahUKEwicyfOJptTzAhXJZc0KHWluDwYQFnoECAAQAg&usg=AOvVaw0o7lHWS6bmirgc57oiBNnc
Most homes built in the fifties here did not insulate their piping. By the fifties people realized asbestos was dangerous, by the late 70's it was illegal to make many products out of asbestos. Plus the people thought "why would I pay to insulate piping if I'm trying to heat the house?"
 

Dana

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Since you have a heating history on the place, use (wintertime only) gas bills and the nameplate efficiency of the boiler to MEASURE the heat load on the place. It's quicker and more accurate than doing a full Manual-J based on guesswork about construction, insulation, air leakage, etc. A (relatively) quick method for running those numbers lives here. Oversizing by no more than 1.4x will be enough to cover Polar Vortex disturbance cold snaps, yet still run at an efficient and comfortable high duty cycle at normal winter temps.

With cast iron boilers you really need to get a better handle on the radiation, and with oversized bulky cast iron rads there is a high risk of destroying a right-sized cast iron with return water too cool, causing corrosive condensation on the heat exchanger plates. But right-sizing a modulating condensing boiler turns that risk created by excessive radiation into a reward, in both lower fuel use and higher comfort if you take the time to dial in the outdoor reset curve. Your system has enough thermal mass in the radiation on each zone that it won't short-cycle even if oversized, but right sizing it will still produce marginally higher comfort, if not higher efficiency.

In many cases it will be CHEAPER to install a right sized modulating condensing boiler than a right-sized cast iron boiler, due to the cheaper venting materials, and not having to design & build protections against low return water temps.
 

John Gayewski

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Since you have a heating history on the place, use (wintertime only) gas bills and the nameplate efficiency of the boiler to MEASURE the heat load on the place. It's quicker and more accurate than doing a full Manual-J based on guesswork about construction, insulation, air leakage, etc. A (relatively) quick method for running those numbers lives here. Oversizing by no more than 1.4x will be enough to cover Polar Vortex disturbance cold snaps, yet still run at an efficient and comfortable high duty cycle at normal winter temps.

With cast iron boilers you really need to get a better handle on the radiation, and with oversized bulky cast iron rads there is a high risk of destroying a right-sized cast iron with return water too cool, causing corrosive condensation on the heat exchanger plates. But right-sizing a modulating condensing boiler turns that risk created by excessive radiation into a reward, in both lower fuel use and higher comfort if you take the time to dial in the outdoor reset curve. Your system has enough thermal mass in the radiation on each zone that it won't short-cycle even if oversized, but right sizing it will still produce marginally higher comfort, if not higher efficiency.

In many cases it will be CHEAPER to install a right sized modulating condensing boiler than a right-sized cast iron boiler, due to the cheaper venting materials, and not having to design & build protections against low return water temps.
Too much of a re-pipe job. Changing out boiler for boiler is easier than changing out boiler for totally new design. Not to mention the short lifespan and complexity that leads to issues with mod con boilers. The venting is done.
 
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Dana

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Too much of a re-pipe job. Changing out boiler for boiler is easier than changing out boiler for totally new design. Not to mention the short lifespan and complexity that leads to issues with mod con boilers. The venting is done.

The venting may or may not be done, even if moving from a 75-80% boiler to a mid-efficiency boiler of the same burner size.

The venting would in many or most cases NOT be done if downsizing the cast iron boiler for a better fit to the actual load, and would need a right-sized flue liner to improve stack velocity in order to reduce risk of backdrafting &/or flue condensation.
 

John Gayewski

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The venting may or may not be done, even if moving from a 75-80% boiler to a mid-efficiency boiler of the same burner size.

The venting would in many or most cases NOT be done if downsizing the cast iron boiler for a better fit to the actual load, and would need a right-sized flue liner to improve stack velocity in order to reduce risk of backdrafting &/or flue condensation.
Real life. If there's a vant there (which there is) the homeowner won't want to pay for a new one unless totally necessary. Generally it's not done.
 
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