Do you need a bigger btu output on a boiler than a furnace?

grantiman

I have a new built in-law apartment, I was given a quote for a Lennox 95% efficient 45,000 btu gas furnace, long story short I'm unable to use forced air and decided to put in a h/w boiler. They gave me another quote on a 90 % efficient 50,000 btu boiler, why are the boilers btu's more than the furnace btu's?

Jamie Hall

Mostly, probably, because that is what's available. Neither boilers nor furnaces are made to measure -- any more than pants or shoes are any more. You choose what fits best.

pecmsg

Lower the water temperature and you lower the BTU output!

DZoro

Good install of a Modulating 95% boiler will provide you with the exact amount of BTU's your house needs on a daily basis. Not just the maximum amount on the coldest day of the year.
D

HVACNUT

> @DZoro said:
> Good install of a Modulating 95% boiler will provide you with the exact amount of BTU's your house needs on a daily basis. Not just the maximum amount on the coldest day of the year.
> D

As long as outdoor reset is used.

The Lennox was probably the smallest furnace available.

grantiman

So if I was to check out boilers and their output online is the DOE Heating Capacity the output rating I should be looking for? If the apartment called for a 45,000 btu furnace then I should look for boiler rating at 45,000 or close to it as long as I don't go under 45,000?

Jamie Hall

grantiman said:

So if I was to check out boilers and their output online is the DOE Heating Capacity the output rating I should be looking for? If the apartment called for a 45,000 btu furnace then I should look for boiler rating at 45,000 or close to it as long as I don't go under 45,000?

More or less. Some boilers also quote net BTUh output, which is an even better guide. Do not go by input or gross!

EBEBRATT-Ed

@grantiman

You should buy a boiler or furnace base on the heat loss of the apartment. Just because they quoted you on a 45k furnace your heat loss could be 30k or 35k.

grantiman

I have a floor plan I drew up, I'll have to try finding a Plumbing and Heating supply store that will help me figure out the actual heat loss of the apartment, a lot of places won't figure your heat loss out unless you buy the boiler from them. I'll keep you posted and let you know if I find one or not...

EdTheHeaterMan

Sounds like your professionals are offering you the smallest equipment available in the brand that they offer. Your actual load may be 20,000 BTU. That said, you want to deal with a pro that knows his stuff. look at reviews on the pro you are thinking about using. I did not ask for customer reviews when I was in business, but I still got a few positive ones. If I found a negative one, I went to the customer to see if I could resolve their complaint. Most of my growth was by word of mouth.

You might try this: https://www.slantfin.com/slantfin-heat-loss-calculator/
It has changed since I used it years ago.

here is another way to go.... "really old school"
https://www.weil-mclain.com/sites/default/files/wm-easy-loop.pdf

This is not rocket science. don't overthink it. the difference between the boiler and the furnace rating is because that is what is available. If the Furnace company made a 45,000 BTU and the furnace company made a 50,000 BTU unit. you would still ask the same question on this forum and you would get the same answers. Some over-thinking and some practical.

The smallest boiler available from Bosch Greenstar line is 57,000.
Carrier, Bradford white and Dunkirk all have 50,000 as the smallest, I always liked Buduers but the smallest they offer 80,000.

If you are going with a modulating boiler and the load calculation that says you need anything between 16,000 BTU to 40,000 BTU, then the 50,000 is the right one.

grantiman

Thanks for all the help, I'll check out the links you sent me and let you know what I come up with..

HVACNUT

> @grantiman said:
> I have a floor plan I drew up, I'll have to try finding a Plumbing and Heating supply store that will help me figure out the actual heat loss of the apartment, a lot of places won't figure your heat loss out unless you buy the boiler from them. I'll keep you posted and let you know if I find one or not...

Yes, download the Slant Fin heat loss calculator. DIY. Its fairly simple and you'll learn a few things. Keep in mind when researching boilers, a mod con with outdoor reset will adjust BTU input depending on outdoor temperature. With a smaller space, the boiler might never run at 100% firing rate. A mod con might not be the best option, cost wise for a smaller space. Short cycling could be an issue if not installed correctly. i.e. a buffer tank. You might not recoupe the extra costs from increased efficiency. And if the boiler is not condensing, it's no more efficient than a standard atmospheric boiler. The same could be said for an atmospheric boiler but it wont bankrupt you. And outdoor reset can also be used on an atmospheric boiler.
Also, maintenance and repair can be a lot more than a standard boiler.

grantiman

Got started with my heat loss with the Slant Fin calculator, fairly simple but I have a couple of questions. The calculator asks for door sizes and then door factors, is this just for exterior doors? What about the interior doors such as the bedroom, bathroom, laundry room, and also doorways, should I just put in the size but "0" as the factor if the doorway leads into another heated room?

KC_Jones

grantiman said:

Got started with my heat loss with the Slant Fin calculator, fairly simple but I have a couple of questions. The calculator asks for door sizes and then door factors, is this just for exterior doors? What about the interior doors such as the bedroom, bathroom, laundry room, and also doorways, should I just put in the size but "0" as the factor if the doorway leads into another heated room?

Exterior doors only, the program is trying to calculate losses to the outdoors.

Jolly Bodger

What climate do you live in? what is the configuration of the apartment? Construction type? What distribution method is the boiler going to use? Fuel available and cost? There are many options available to you. If you are questioning a 5000BTU discrepancy, it does not seem like you don't have much faith in your contractor. Might want to get several options.

Load Calc is defiantly the first step. Might find a gas fire place would heat the space.

John Mills_5

If you only need a little bit of heat, I think Carrier family has a 26K 95% model. Most start at 45-50,000 BTU that would do a 1000 sq ft house just fine. I heated a MIL apartment with 5kw or 17K for a friend.

grantiman

Long story short I can't use a furnace so I decided to go with a boiler. Did my heat loss and came up needing a 52K DOE btu heating capacity boiler, I found 2 so far a Slant Fin 52K and a Weil McLain 51K. I like the Weil McLain but it falls a little short of what I needed, should I go up one size bigger or do you think 51K will do the same job as the 52K I needed?

Jamie Hall

The difference between 51K and 52K is trivial. It's much more important that the installer do the job right and set it up properly.

pecmsg

You’ll only need full capacity maybe 1% of the time. If theses a problem tighten the envelope.
As @Jamie Hall said the install makes or breaks the job.

The Steam Whisperer

That's a lot of btu's. We heated a circa 1906 3200 sq ft frame home with 800 sq ft of windows ( most original but weatherstripped) in Northern Ill with that amount of heat. The home was completely insulated and quite airtight, but still that a lot of heat.

John Mills_5

Using baseboard radiation? Look at output of baseboard vs output of boiler and heat loss. Without excess baseboard compared to heat loss, you pay be running water above 135° much of the time meaning your 90+ boiler will be 85% or less. Mod cons are great for low temp radiation, may not save much on baseboard and the good ones are pricey. You might be better off with a cast iron. Burnham 202 is 84% AFUE and probably near 1/2 the price of an Alpine.