Calculating BTU Heat Loss for Oil to Gas Conversion Help!

PilotPat24

Hi Guys,

I am looking to convert over to Natural Gas from Oil in a 2000 square foot home with 2 bathrooms in New York.

The original system was a 89,000 BTU Hydronic Boiler with a Indirect DHW Heater. The Indirect tank was replaced with an independent on demand oil fired hot water heater.

I am looking to keep it simple and want to go with a cast iron natural draft boiler with an indirect hot water heater.

I have gotten about 5 estimates from plumbers and they spec'd out traditional systems all the way to a wall hung combi's ranging from 65,000 BTU's to 195,000 BTU's.

I really would like to have a properly sized system.

What do you think the best way is to spec out an existing house with hydronic baseboard heat? Any input would be greatly appreciated.

Thanks,

Pat

Hot_water_fan

How much oil did you use last year? And what part of NY?

PilotPat24

Hi,

The house is on Long Island. I just checked and I use about 600 gallons a year of fuel.

STEVEusaPA

<40K, although, are you sure about that 600 gallons?

PilotPat24

STEVEusaPA said:

<40K, although, are you sure about that 600 gallons?</p>

I'm positive. I log all of my tank fuel ups. That's 600 gallons for the entire year. I really don't use much from May through October. I shut the Peerless boiler off in the Summer and just have the On Demand HWH running.

I'm going to measure how much baseboard I have throughout the house next.

PilotPat24

I think I am going to go with the Burhnam ES2 Boiler. It's either going to be the 70,000 BTU or 105,000 BTU model.

Does the Indirect HWH needed to calculated for BTU useage since it will take priority when calling to make hot water?

Jamie Hall

And you stayed warm -- you were able to keep the house at the temperature you wanted? If so, there is no reason on earth to go bigger than what you had, is there?

Ideally one of the contractors would figure the heat load of your house -- it's not hard (in fact, using the handy calculator here: https://slantfin.com/slantfin-heat-loss-calculator/ you can do it yourself) and size the boiler to match that.

Hot_water_fan

Pretty low heat loss - I'd put you under 40kbtu as well. Which is conservative since you said the DHW was also oil fired. You want a boiler with an output as close to 40kbtu as possible. An indirect will not factor into sizing.

PilotPat24

Hot_water_fan said:

Pretty low heat loss - I'd put you under 40kbtu as well. Which is conservative since you said the DHW was also oil fired. You want a boiler with an output as close to 40kbtu as possible. An indirect will not factor into sizing.

I have that house pretty tight with having the sill plate insulated along with R49 of insulation in the attics.

Hot_water_fan

@PilotPat24 you have an on-demand oil water heater? Like a tankless oil heater?

Awesome job on the insulation. Do you have AC?

PilotPat24

Just wall units for AC. Here is what I currently have for the water heater.

https://toyotomiusa.com/product/om-122dw-toyotomi-semi-on-demand-water-heater/

EBEBRATT-Ed

@PilotPat24

The safe way and right way is to download the Slant Finn and do your own heat loss as most contractors are too lazy, Then measure your baseboards to compare the heat loss # to.

PilotPat24

Jamie Hall said:

And you stayed warm -- you were able to keep the house at the temperature you wanted? If so, there is no reason on earth to go bigger than what you had, is there?

Ideally one of the contractors would figure the heat load of your house -- it's not hard (in fact, using the handy calculator here: https://slantfin.com/slantfin-heat-loss-calculator/ you can do it yourself) and size the boiler to match that.

The current boiler is 25 years old. Never have had a problem with keeping the house at 70 degrees.

Hot_water_fan

The math behind this: 138,000 btu/gallon * 600 gallons/year * 85% efficient = 70MMBtu output into the house. Divided by 4475 heating degree days (base 65 degrees)/year gets you 15,731 btu/HDD_65. At a design HDD of 50 (average temp of 15), that gets you a heat loss of 32kbtu. Add in a fudge factor of 20%, and you get 38.5kbtu/hr. Obviously, this includes all oil, so it's pretty conservative as the DHW consumes a good deal of energy too, bringing down the heat loss.

random12345

Non-pro here. I've said this before, I will say it again, the Slant/Fin calculator no longer exists, either on their website or on the Apple app store. The Google Play store may still have it, but I don't have Android so can't confirm. If I were in your position, I would ask a third party to do an ACCA-approved Manual J calculation for you. They should be using one of the approved software packages:

https://acca.org/standards/approved-software

If you do a Google search for this, you will find many companies that charge a couple hundred or maybe even less to do this calculation for you, so you don't have to a) spend a lot on one of those software packages and b) learn how to use it correctly. They should be able to calculate the heat loss of the whole house as well as that of every room in the house. Both are valuable I would think.

Hot_water_fan

I don’t see the value of a manual J for this decision- CI boilers come in very few sizes, especially at the heat losses discussed here. Basically it’s either a 32kbt output boiler or a 58kbtu output boiler. 32kbtu would be extremely close to the heat loss, perhaps too close for comfort. 

pecmsg

Hot_water_fan said:

I don’t see the value of a manual J - CI boilers come in very few sizes, especially at the heat losses discussed here. Basically it’s either a 32kbt output boiler or a 58kbtu output boiler. 32kbtu would be extremely close to the heat loss, perhaps too close for comfort. 

It will help with results of tightening the envelope!

pecmsg

random12345 said:

Non-pro here. I've said this before, I will say it again, the Slant/Fin calculator no longer exists, either on their website or on the Apple app store. The Google Play store may still have it, but I don't have Android so can't confirm. If I were in your position, I would ask a third party to do an ACCA-approved Manual J calculation for you. They should be using one of the approved software packages:

https://acca.org/standards/approved-software

https://slantfin.com/slantfin-heat-loss-calculator/

Hot_water_fan

@pecmsg will it? A blower door test would I'd say. Still, the CI options are either 32kbtu or 58kbtu. Unless the heat loss comes in sub 32kbtu, it's not worth it - just get the 58kbtu output.

random12345

pecmsg said:

https://slantfin.com/slantfin-heat-loss-calculator/

I know the page exists. Once you get there, the link to the actual calculator in the browser gives this page. It's broken. The app on the Apple app store no longer exists either. Slant/Fin is gone.

https://forum.heatinghelp.com/discussion/189146/r-i-p-slant-fin#latest

PilotPat24

random12345 said:

pecmsg said:

https://slantfin.com/slantfin-heat-loss-calculator/

I know the page exists. Once you get there, the link to the actual calculator in the browser gives this page. It's broken. The app on the Apple app store no longer exists either. Slant/Fin is gone.

https://forum.heatinghelp.com/discussion/189146/r-i-p-slant-fin#latest

I already have the app on my phone so hopefully it still works!

PilotPat24

What size Indirect Tank would you guys recommend for a 2 bathroom house with up to six people living in it?

Jamie Hall

Well that's interesting. Almost looks like the link has been hijacked... wonder if the company is aware of that? It redirects to a very shaky website now.

R Mannino

600 gallons isn't out of line for Long Island. That's pretty much what I have used over the last 15 years. I too have had my home weatherized.

Hot_water_fan

What size Indirect Tank would you guys recommend for a 2 bathroom house with up to six people living in it?

Depends on if you're talking two showers or two large tubs. 40 is reasonable unless you have a huge, sudden draw - like a jacuzzi.

Dave Carpentier

I tried to get the pc version of the Slantfin app mid-summer, found same "highjacked" page.
The android version works good on the phone though, from googleplay.

PilotPat24

Hot_water_fan said:

What size Indirect Tank would you guys recommend for a 2 bathroom house with up to six people living in it?

Depends on if you're talking two showers or two large tubs. 40 is reasonable unless you have a huge, sudden draw - like a jacuzzi.

No huge sudden draws on this house. Just a two bathroom house with two dishwashers and a clothes washer.

HVACNUT

PilotPat24 said:

What size Indirect Tank would you guys recommend for a 2 bathroom house with up to six people living in it?

That's where you might consider DHW with boiler sizing. If your heat loss is low, but have high DHW needs, then that has to be accounted for. Any jacuzzis or rain heads now, or in the future?