First time doing heat load calc: Whoa, is my boiler 2x oversized?

On the coldest day/night of the year, does it run constant? Or 20 min on, 20 min off?

My old furnace only ran in short spurts. Plenty of heat, a good chunk radiating to the basement at every shut-down.

It was a 100kbtu box which we used to run with a 60k nozzle if there was one on the truck, so it was more often run 80k.

I replaced with a 39k furnace. The very coldest day/night it runs 21 hours without stopping and that is not a problem. There is far less waste heat in the cellar. When I replace it I'll get the 60k rating; it is clear this 39k is an oddball, too small for the frame or fan, a bit more pick-up would be nice.

There is a whole house heat loss form that I used to use Called the form 1504 WH from the Hydronics institute (now part of GAMA and AHRI). Here is a link to this form

https://centralboiler.com/pdf/FORM-1504WH.pdf

It is simple to use because it just uses the exterior Walls, Roof and floors. You select your building type, insulation amount and use the factors that apply to the actual square foot of eaxh component. It is for heating only so there are no latent heat and sensible heat that Manual J needs for cooling load calculation.

I think you will find this easy to use.

I agree with @Steamhead that you calculation sounds too low. It may be accurate but I would do further calculations by some other methods to cross check

Don't know what your local energy costs are or how adventurous you are for the install. If your existing emitters are oversized (or can easaly add some more) and can run at low enough temperature at your outdoor design temp, you can look at a air to water heat pump. Your heat load and design temp put you well within the range of a 5 ton unit.

The equipment is definitely more expensive but since you are not dealing with combustion or refrigerant, you can DIY the install. Around me a heat pump operating cost is about 1/3 of oil, so ROI is only a couple of years.

modular homes are built tight. Infiltration will be lower. How thick are your walls? You will be able to tell if you have 2x6 walls or 2x4 walls. So if you have 2 x 6 walls, you’ve got R-19 insulation. Can you see the attic insulation? How thick is it?

I assume by conservative you mean erring on the side of leaky drafty high btus?

Yes. I'm in climate zone 4 and the 2017 energy code is enforced for new construction. Cold ceilings have to be R60, walls require continuous insulation. Air infiltration has to be tested with a blower and has to be 3 ACH50 or less, which works out to about 0.15 air changes per hour. There's nothing even close to that on the form.

@Hot_water_fan is spot on. The only unknows with a fuel based calculation is your actual boiler efficiency and solar gain effects. Unless designed for something like passive solar, older houses with reasonable sized windows don't have enough solar gain to change numbers much.

When you are doing a man J or any other heat loss, you have a whole pile of thing to guess or estimate. One estimate on top of another guess and before you know it, you can make that sheet show any heat loss number you want.

For giggles, lets work backwards. Assume 75000BTU is correct. You might have 6000HDD based on your design temp.

So same 56F delta, so 75000BTU/56F=1340 BTU/degreeF

so 6000HDD to hours 6000*24 to therms 6000*24*1340=1929therms.

Assuming 85% efficiency, you need 1929/0.85~2300 therms or 2300/1.35=1700 gallons of oil. At say $4/gallon, that is $6800/year. I think at that cost, you would be rethinking your housing and heating choice.

@seized123 , if you're going with Weil-McLain, I'd use their 3-pass Ultra Oil model UO-3R. The efficiency of this unit is slightly better than the Gold series, but its 3-pass design makes it much, much easier to service. Basically, you open a large door on the front where the burner is mounted, and this gives you access to pretty much everything that might need to be brushed and vacuumed. No more removing the flue collector and brushing from the top to clean the little pins. OK, it probably costs a bit more, but the extra bucks are well worth it.

Some other good 3-pass boiler series are the Burnham EMP and MPO-IQ, Biasi, Buderus G115WS, ForceOil (Ferguson's house-brand of the Burnham EMP, price might be better), Solaia (similar to Biasi) or Trio (similar to Biasi also).

The most important factor is a good installing contractor. Go here to find one near you:

https://heatinghelp.com/find-a-contractor/

@seized123 … this is called overthinking…. You have all the information you need…. a WM or Williamson will use the same footprint as the old one. The pipe connections will be near identical. You want to save on installation costs and you can DIY this one. You don't expect to be there after 4 years so any extra you spend to get a lower operating cost will be for someone else's benefit. So what is holding you up? After you get sticker shock from the EK estimate. Order the boiler and put it in. By the time anybody needs to make a warranty claim, you will be long gone and the plumber that the new homeowner calls will do all the necessary paperwork to get the warranty claim handled. No one will ask if a homeowner installed it in 5 or 10 years.

If you have a DOA boiler or accessory part, when you open it up, then you go to the supplier and work with them. It is against the law to sell you something that is clearly non functional without some kind of guarantee by the manufacturer that it will operate when installed according the the manufacturer's instructions. Just be sure to follow the instructions. They are online, so you can get familiar with them before you even purchase one.

If you show us a photo of your existing WGO, we can tweak any changes you might want to make. You can decide to replace all the accessories or use all the accessories that exist and replace only those that you may find defective as time goes on. I believe the new WM will come with the HydroStat with the built in LWCO so you done even need to get a new one of those. (LWCO is the only thing I would replace when doing a "push pull" even if it was only a few years old).

Hope this helps to push you off the fence.

Mr. Ed

As a point of reference, I maintain a 4-unit condo building in the Boston suburbs. We have two WGO-5 boilers that burn a total of about 1200 gals/yr, which happens to be almost exactly double your consumption rate. I've calculated our building heat loss 4 different ways, 3 of which used actual oil consumption or boiler run times at known outdoor temps, and all the heat loss numbers agree within 10%, so I have high confidence in them. Our heat loss is 90,000 BTU/hr, and since the math is basically linear, by comparison your heat loss (in a similar HDD zone) would be half, since you consume half the amount of oil. Which puts your heat loss at 45,000 BTU/hr, quite close to the 50,000 BTU/hr that @EBEBRATT-Ed predicted earlier.

We also have massively oversized boilers, and I plan on downsizing to WGO-2's when the time comes. But since you say you're doing the install and weight matters, you might want to look at the Biasi B10/3 or B10/4. Those are around 300 pounds, much smaller and easier to wrangle than the 800-pound monster UO-3. I do not have any experience with the Biasi, but I have done some reading on it in the course of looking for alternatives to our WGO-5's. There are a number of people here who have experience with Biasi's, and there are a number of threads discussing their reliability, etc.

https://qhtinc.com/wp-content/uploads/2022/03/B10-Lit-0321.pdf

The downside of the Biasi is that the plumbing is all different from your existing WGO setup, so if you can wrassle a new WGO in there like @EdTheHeaterMan said, that would be simpler plumbing-wise.

Invest in a hand truck with pneumatic tires.

Yes, a WGO-2 will be plenty for your application. We have double your heat loss, and I plan on eventually downsizing to two WGO-2's. Those will still be oversized, but not as badly as the WGO-5's we now have.