BTU Load Sizing for Boiler versus Forced Air

Mike_Breault

while you have a complete heat loss, you will need a room by room done. (there is software to do this if you want), to determine how much baseboard you will need.

you also need to know how you are piping it, and controlling it.

are you doing zoning? room by room control, or all one zone, these are all factors in the design.

as many have said there is no difference in how the btu was produced
how big is your house? I havea 2 story, about 2700sq ft. my old 95% faf was 100k, I converted to water system, but I replaced the furnace with a hydro coil as step 1, the coil was 80k @180 water, (heat loss done, knew furnace was to big), I havent broken 135°F water in 2 years, meaning even 80k too big.

decide if you are doing DHW, my boiler is sized for that eventuality as it is actually the biggest load, I am doing a timed priority (most good control (tekmar - self promotion, full disclosure I work there) so I do not need the boiler to heat while the system is doing DHW,

good luck

Mike

Crissie

Mike_Breault said:

while you have a complete heat loss, you will need a room by room done. (there is software to do this if you want), to determine how much baseboard you will need.

you also need to know how you are piping it, and controlling it.

are you doing zoning? room by room control, or all one zone, these are all factors in the design.

as many have said there is no difference in how the btu was produced
how big is your house? I havea 2 story, about 2700sq ft. my old 95% faf was 100k, I converted to water system, but I replaced the furnace with a hydro coil as step 1, the coil was 80k @180 water, (heat loss done, knew furnace was to big), I havent broken 135°F water in 2 years, meaning even 80k too big.

decide if you are doing DHW, my boiler is sized for that eventuality as it is actually the biggest load, I am doing a timed priority (most good control (tekmar - self promotion, full disclosure I work there) so I do not need the boiler to heat while the system is doing DHW,

good luck

Mike

I did a room by room heat loss using the Slant Fin app. I have been told by several guys on this forum that I don't need extra BTU's for hot water, if I ever choose to go that route. Correct or not?

Tinman

You are correct and so is Mike. His DHW load is bigger than his heating load, so he's sizing for the DHW load. That is usually, but not always, the exception.

hot_rod

Those PTAC are notorious for leaking cold air in!

One thought would be to have a blower door test done, see where any or all air leakage is. Infiltration is a huge energy robber, and a hard number to pin down without testing.

www.dsireusa.org shows where they may be some energy upgrade incentives available, maybe even a blower door test..

Crissie

I did a blower door test in July. House is very tight, 25% tighter than code for new construction. Take 5.13 hrs for one full air exchange. We walked around with an thermal camera and the PTAC did not jump out. If... the blower door test was done correctly. You never know these days.

pecmsg

Crissie said:

I did a blower door test in July. House is very tight, 25% tighter than code for new construction. Take 5.13 hrs for one full air exchange. We walked around with an thermal camera and the PTAC did not jump out. If... the blower door test was done correctly. You never know these days.

With that please get a whole house dehumidifier hooked to outside air.

Crissie

@EBEBRATT-Ed @KC_Jones @Hot_water_fan @hot_rod @Jamie Hall @neilc @ratio and everyone who commented on this post.
Just to update you all, if you are interested. I had an independent professional Manual J done for each room in my house. The heating load is 73K BTU's of which 6K is for a constant ERV ventilation system bringing in 250 CFM. So without the ventilation, it is 67K BTU's. That is a far cry from the 175K BTU boiler my contractor sized for me.
I can't thank all of you enough for saving me from the nightmare that would have unfolded if they installed this 175K boiler in my home. You gave me an education and an incredible amount of support. Saved my life!!
I told this contractor I was revisiting the size load on my house. Without my asking, they came back with an email sizing my house again and recommended the 130K BTU version of the boiler, one level down from the 175K. Here are the credentials of this point who did this 130K BTU sizing. This gives you a feel for the #@*# I am dealing with. It is dangerous being a consumer these days.

Service Operations Manager
LEED Certified HVAC
D.E.T Certified Multi family verifier ID #
B.S Mechanical Engineering

Rich_49

Service Operations Manager
LEED Certified HVAC
D.E.T Certified Multi family verifier ID #
B.S Mechanical Engineering

I just love guys with letters after their names . Fact is , most of em are not very good even with the education . Ask me , and others here how we know .

Crissie

Rich_49 said:

I just love guys with letters after their names . Fact is , most of em are not very good even with the education . Ask me , and others here how we know .

Okay, how do you know?

sunlight33

The reason is most average consumers will not be bothered by short cycling, but they will be by inadequate heat, so contractors err on the high side.

Jamie Hall

sunlight33 said:

The reason is most average consumers will not be bothered by short cycling, but they will be by inadequate heat, so contractors err on the high side.

Oh so true... !

And Rich, how about Jamie, OSB, PE, PhD?

ChrisJ

Jamie Hall said:

sunlight33 said:

The reason is most average consumers will not be bothered by short cycling, but they will be by inadequate heat, so contractors err on the high side.

Oh so true... !

And Rich, how about Jamie, OSB, PE, PhD?

If you earned the PhD I say go for it. Otherwise, no.

Jamie Hall

I earned it, @ChrisJ -- just not in engineering. In climate physics/global thermodynamics, to be exact. Before climate change was a popular thing...

ChrisJ

Jamie Hall said:

I earned it, @ChrisJ -- just not in engineering. In climate physics/global thermodynamics, to be exact. Before climate change was a popular thing...

What's OSB besides a product used for sheathing?

Jamie Hall

ChrisJ said:

Jamie Hall said:

I earned it, @ChrisJ -- just not in engineering. In climate physics/global thermodynamics, to be exact. Before climate change was a popular thing...

What's OSB besides a product used for sheathing?

Order of Saint Benedict -- or Benedictine in common terms.

EBEBRATT-Ed

@sunlight33
The reason is most average consumers will not be bothered by short cycling, but they will be by inadequate heat, so contractors err on the high side.

How true

Crissie

EBEBRATT-Ed said:

@Crissie

I wouldn't be surprised if you ended up in the 60-70K range

What you really need is a contractor that knows what he is doing. Yours don't sound like top shelf to me.

Check "find a contractor" on this site and post your location. Someone may have a recommendation .

A contractor is the most important decision

As an FYI, I did reach out to a couple of contractors from the 'Find a Contractor' on this site. They are no better.

BobZmuda

The third contractor may have the right idea. Not enough info though. After not reading 3 pages of posts I'll interject my opinion

The important thing: How low can the boiler fire? How small is your smallest zone?

Also, why not use some runtal style radiators? Radiant heat is best heat IMO.

mzstein

I know this is an old post , but doing some research on boilers vs forced air. When looking at boilers they list several different BTU ratings - input, output and net output . So when someone says they have an 80000 btu boiler is that input ? Just making sure I am comparing apples to apples and not getting mixed up in the description.

ChrisJ

mzstein said:

I know this is an old post , but doing some research on boilers vs forced air. When looking at boilers they list several different BTU ratings - input, output and net output . So when someone says they have an 80000 btu boiler is that input ? Just making sure I am comparing apples to apples and not getting mixed up in the description.

Usually input