How much does size matter?

scatgo

I plan on replacing my 97 year old furnace with a Weil-McLain AquaBalance Heat-Only Wall Mount Gas Boiler. My option is 65,000 97000 or a125000 BTU unit. After a quick calculation I should need a 93000 BTU unit. Just how close do I have to be to the 93000 for thing to work right?

383-100-045 - Weil-McLain 383-100-045 - AB-120H AquaBalance Heat-Only Wall Mount Gas Boiler, 97,000 BTU (NG)

mattmia2

Is 93,000 btu/hr the heat loss of the house that you calculated? Since it is a modulating boiler it can fire at a lower rate much of the time. I would go with the 97,000 btu/hr output model though the actual output may be a little less depending on what type of emitters you have and how well they deal with low temp water. Are you heating DHW with it or just space heating?

scatgo

Just space heating with cast iron radiators.

scatgo

Boiler BTU Calculator - SupplyHouse.com

hot_rod

A 97,000 input will be around an 84,000 output, derate for efficiency.

If you trust the load calc, go with the 125, it will turndown about the same.

mattmia2

it is 97,000 output, but that is in a lab test. how close it comes to that will depend on how cool you can run the ci radiators and get the design output. if the house is 2 story that calculator is way off because it will be guessing about 2x the roof area you actually have.

hot_rod

That is a pretty simplistic load calculator. I would run another load calc to tighten up that number.

What size is the existing boiler?

GGross

that calculator gives me a load of 103,554 BTU/hr. My boiler is an 85k mod/con and when I checked up on it this past winter when it was -15f outside it satisfied a call for heat in less than 10 minutes. That calculator is pretty far off, that being said a 125k with a 10:1 turndown is going to be fine, I just don't think your actual load is anywhere near what SH suggests.

EdTheHeaterMan

How much does size matter?

This question is better asked in private with your significant other.

As far as the boiler size is concerned…

If you do the load calculation based on I=B=R form 1504WH or ACCA Manual J, you will find a much lower requirement. I have a feeling that you will be able to use the WM AB-80H with a 65,000 NET output, and have the most efficient setup. If you don't want to go to the trouble of an actual Load Calculation for your home where you measure the actual amount of doors, walls, windows, ceilings and floors, then you will be guessing that you have something less that 97,000 NET and I would choose the WM AB-120 with the 97,000
NET output. Either way, the boiler will only use the amount of BTUs required to heat the home and rarely fire more than actual load requirements. Where you will waste fuel is at the low end when you only need 6000, BTU on a spring evening and the turndown ratio only goes to 9700 BTU so the boiler will cycle on and off to provide the lower BTU requirement.

EBEBRATT-Ed

@EdTheHeaterMan

When I read the post title, I had visions of what your comment would be😊

DCContrarian

I recommend the process detailed in this article for sizing based on actual fuel usage:

https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

EdTheHeaterMan

https://forum.heatinghelp.com/discussion/comment/1863964#Comment_1863964

I need to tread lightly here. Some of my comments have been removed in the past and I don't want to get banned. I think I did this one properly.

GGross

https://forum.heatinghelp.com/discussion/comment/1863971#Comment_1863971

Need to start a mailing list for your spicy jokes. 'Ed Young: Too Hot For The Wall"

ZackR

https://forum.heatinghelp.com/discussion/comment/1863968#Comment_1863968

That is what I did and was able to size down my boiler and save approx. 8% on my gas usage. 2 heating seasons so far and no problems keeping things warm in the winter.

The Steam Whisperer

https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

I've used a method like this for decades and it works quite well. However, some of thier assumptions are quite off.

Heat loss of a structure is not close to linear. Heat movement through most materials is nearly linear at typical design temperatures. However, air leakage rates increase nonlinearly, so heat loss accelerates as it gets colder. The old ASHRAE charts show the design air leakage rate of a medium "tight" construction as 0.6 air changes per hour when its 50F outdoors. At 0F , the number of air changes per hour increases to 1.1, nearly double the air leakage and at big delta T for that same structure If you have a leaky building, this can really increase heat load dramatically.

Also, the other big error is the efficiency of equipment. A typical 80% gas efficiency atmospheric boiler without a damper that is 3 times oversized is running about 62% efficiency on a typical winter day ( which is about 1/2 of design load). A 3 times power burner boiler boiler tends to fare much better at about 68% efficiency.

This is why I really like running staged boiler heating plants in larger buildings Sized and controlled properly, you can take the typical day 62% or 68% efficiency and bump that up to 80% to 85% with a properly sized heating plant with two smaller boilers. On the buildings I've done this, fuel usage reduction typically runs from about 20% to 60% and you still have very simple long lived equipment that is inexpensive to repair.

ratio

@The Steam Whisperer, I'm curious about the staging. Do you often (or ever) do more than two stages? Asymmetrical stages (like 1/3 & 2/3)? Pressure control vs temperature vs time?

The Steam Whisperer

I typically do 2 stages, unless I'm trying to stick with residential pre assembled boilers on larger loads. I have used asymmetrical stages, especially in unusual load situations like church structures that are in setback must of the time and only are heating a small space. For control on Hot water systems, I've used Tekmar for staging controls and reset, but its been quite awhile since I've done these. For steam, I use two stage thermostats quite often with orificed two pipe systems. On some steam systems you need both boilers to prime the system and then you can drop to one boiler. An aquastat at the end of the slow to heat main usually can take care of that. R&D controls makes a 2 stage steam control that will drop out the 2nd stage after the temp rises to about 60% of the differential. I probably will be trying one of these soon.