FURNACE BTU ADVICE PLEASE...

swihart_art

Hello,
After doing several load calculations from various sources here is what I've determined...



This calculator at coolcalc.com says I need 87,000 btu to heat my addition... it is VERY precise in that I could enter every detail of the construction even right down to the window energy values... I tend to trust this one after going through all of the details... this is also approved by the ACCA so again... I tend to trust it...

This one:



It has calculated 68,000 btu but in my opinion is not nearly as detailed as the first since I couldn't spec nearly as much so I'm thinking this is a rougher estimate...

I can buy a 125,000 btu furnace for $1,700 that will effectively produce 100,000 btu at 80%...
I can buy a 95,000 btu furnace for $1,500 that will effectively produce 76,000 btu at 80%...

My requirements are pretty much right in the middle of the ranges between these 2 furnaces...

Here's my question...
Do I go with too much or too little?... or is one sized better than the other?...

I'm going with the lowboy furnace due to the install location...

Thanks in advance for your thoughts...

Jamie Hall

Try one more for giggles. I really trust the Slant/Fin calculator...

hot_rod

Will it be for AC also, or just heat?

swihart_art

Jamie...It only comes in mobile platforms and I know this may sound funny but I can't find my phone.... lol... I rarely use cell phones as I work at home as a designer and being older like to be unplugged as much as I can... I spend so much time on computers that when I walk out of the house I try to be old school... I do have one though in case of emergencies but it's dead and who knows where I left it... lol... trying to get this all figured out so I can order the furnace and get it installed... cold in pa...

swihart_art

AC also... hard ducting, no flex... sealed as good as possible.. all ducts are contained between the 2 floors and not open to unconditioned air...

Zman

Can you give a general idea of construction type?
Both numbers seem high to be for a typical 2,500 sq ft building.

The Steam Whisperer

An poorly insulated, leaky home that size might need that, but well sealed and insulated it would need about half that in Northern Ill. I only needed about 52,000 btu/hr to heat a 3200 sq ft 1905 home in northern Illinois with about 800sq ft of windows, most original with aluminum storms. I used the airtight drywall method and r-19 or r-21 in the walls.

swihart_art

2x6 walls with r-19... "attic" (it's living area with cathedral ceiling) is r-19 so far but I may build down the rafters to get a higher r value up there... on stilts 10 ft above grade...
it will be drywalled and pretty airtight but I wouldn't say as good as foam throughout would be... dogs like to out 4 million times a day as do the cats so factor that in....lol...

can anyone explain why coolcalc is off so much... I punched every r value in, every window with it's specs and which direction every wall faces... of course sq ft of every room was given as well...

here is a photo for reference....

pecmsg

You have the construction value listed as Average, change it and see how much it drops.

I agree the #'s seem high for new construction.

GW

I love this topic 😀 as long as the report says it, it must be true😀😀😀
Your numbers seem very excessive. Your ducts are new too right? Why click “average” for ducts? And why did you click attic if the ducts are between the floors?
A bigger furnace is fine if you don’t mind the furnace cycling like a dog and then replacing it in 10 years
My house is a bit bigger than your addition, heat loss is around 32000 btu. I can heat it with a 3 ton heat pump

Zman

How is the main floor insulated? That detail will have a huge effect on heat loss and general comfort.
You can always do the heat loss in a simple spreadsheet if you want to eliminate the mystery. The building tightness (or lack thereof ) is usually the SWAG in the formula.

swihart_art

GW... Not saying it's true.. just posting the results... that's why I'm trying to dig up more opinions and advice before making my move... the "main floor" which is open underneath... I have a few ideas on that...

1. Insulate the living crap out of it... as much as I can stuff into 11" of i joist... probably r 38...

2. Run hydronic pex with a 27kw on demand I have for the hot water in the addition on the bottom of the subfloor with aluminum fins... and then isulate... if I do this my furnace could be WAAAAAAY smaller in btu's... just wondering how much energy I would use with that setup... it would definitely be more comfortable on the feet and overall room in the winter...

Yes on new ductwork but I did choose average cause... well... nothing special... just metal ducts...

I really didn't "click" attic I just listed that space as liveable cause it is... I didn't add in the ceiling / floor as exposure cause it says not to which makes sense... it's not open to outside air...

Many on here seem to think my numbers are too high so I'm thinking the answer is gonna be to buy the 95,000 btu lowboy...

This is by far the hardest part of my project as I'm proficient in all other areas of design and building but HVAC is my weak point and not easy by any stretch of my imagination...

Thanks all for your help, it is greatly appreciated...

pecmsg

There are sites that for a fee will run a calculation you just supply the information. Get the #'s verified, with all that insulation 40K may be closer!

swihart_art

Thanks pecmsg for your reply... this topic is maddening.... the info out there is so wildly different depending on source... there are folks out there who say if I don't get r40 in my rafters i will freeze to death...lol... some have 3" foam completely wrapping the entire structure which is great and energy efficient but it would takes years to recoup that initial investment... I'm sure I'll get flamed but IMO it needs to be good... not "the most energy efficient structure ever built" like some try to do...

Zman

I would recommend taking a broader view of this.
If you are doing new construction and you cannot get your heat loss under 20 BTU/square foot, you are doing it wrong. Stop thinking about furnace size and figure out how to insulate better and tighten up the building envelope.

pecmsg

Well too add to the confusion once the structure is closed in have a blower door test performed so you'll know how much outside air needs to be introduced too maintain healthy air. A tight structure requires a complete air change every 6 - 8 hours to exhaust indoor pollutants and humidity and replace it! You will need some type of mechanical means. ERV, HRV.

swihart_art

Will do!... My goal is to get the furnace sized and installed with all of the ductwork so that it will be nice and warm while I wire and finish the insulation in the walls, plumbing etc this winter... I also prefer to wire after hvac so I don't run into issues with wiring in the way while trying to run ductwork... easier to re-route wires than it is ductwork...

pecmsg

All that construction is going to kill that new furnace!

swihart_art

@pecmsg ?????? ... I don't understand why...

Zman

I think he is referring to the construction dust. The unit should sealed combustion and should pull it's air directly from the outside. Be sure to change the filters frequently during construction and consider turning the unit off when there is a lot of dust being generated. Drywall sanding is probably the worse.

pecmsg

Zman said:

I think he is referring to the construction dust. The unit should sealed combustion and should pull it's air directly from the outside. Be sure to change the filters frequently during construction and consider turning the unit off when there is a lot of dust being generated. Drywall sanding is probably the worse.

Exactly.

Even if you were to put a media filter on there dust WILL get into the motor and squirrel cage assembly. It doesn't do any of the components any good. Can they be cleaned yes to a point but I don't know how your going to clean the internals of a ECM motor that costs several hundreds!

rent a couple of construction heaters during construction. After the painting and trim work is completed then you can run the primary heater!

KC_Jones

I don’t know how many rooms that addition has, but you really need to do a room by room heat loss. You need this to be able to design the duct system to get proper airflow to each room so the heat is even.

I’m far from an expert, but the math can’t be ignored.

I agree with others that have mentioned the slant fin app, it does things room by room.

JUGHNE

For construction heaters we have always used electric heat.
A tear out used electric furnace/air handler sitting on saw horses, connected to the electric range rough in, add restriction/filters to air inlet to get blower up to speed; this would heat most of the house once it was closed in. (North NE -10 design)

It was always surprising that the 10-15 KW (34 - 50,000 BTUH) would heat the house once the rock was on. It would cycle on tstat even.
And the house would end up with a 60-80,000 BTUH oversized furnace.

Bed rooms would get a 1500 watt cube heater set on low.
The SR mudders would unscrew the 300 watt temp bulb installed on a pigtrail in the bd rm because they got too hot working.

Oil or LPG heaters add a lot of humidity to the new construction, making mud and paint drying difficult.

Solid_Fuel_Man

During construction vented or electric heat is the best. Helps mud and paint dry! I've piped a wood stove through a sheetmetal window cover more than once. Great way to get rid construction scraps too. I never feel bad about opening a window or two to let dust and humidity out.

GW

If we are talking oil furnace, them there things only come so small anyway (roughly 70000 btu).

Ductwork- yes that’s an important step. It’s good to have some familiarity with the steps. Or maybe hire some person to design the system

What heats the rest of your home? This topic could go on and on 😀 Maybe do a small boiler and run an air handler unit.

pecmsg

> @JUGHNE said:
> For construction heaters we have always used electric heat.
> A tear out used electric furnace/air handler sitting on saw horses, connected to the electric range rough in, add restriction/filters to air inlet to get blower up to speed; this would heat most of the house once it was closed in. (North NE -10 design)
>
> It was always surprising that the 10-15 KW (34 - 50,000 BTUH) would heat the house once the rock was on. It would cycle on tstat even.
> And the house would end up with a 60-80,000 BTUH oversized furnace.
>
> Bed rooms would get a 1500 watt cube heater set on low.
> The SR mudders would unscrew the 300 watt temp bulb installed on a pigtrail in the bd rm because they got too hot working.
>
> Oil or LPG heaters add a lot of humidity to the new construction, making mud and paint drying difficult.

So how exactly do they ADD humidity?

Jamie Hall

They add humidity because they aren't vented to the outside -- and a big part of the combustion gasses is water vapour (for perfect combustion, roughly a 3 to 2 ratio water to carbon dioxide; for LP closer to 1 to 1 for diesel or #2 fuel oil).

pecmsg

I would never vent a heater in a construction site. Vent them out a window!

JUGHNE

Product of combustion to where the windows sweat like a pig, as Jamie said.
A lot of things happen in construction sites.

Also I have been told that unvented "Nipco" type oil burning heaters produce fumes that interfere with concrete curing correctly. Maybe SR mud, IDK.......also if LPG has same effects IDK either.

swihart_art

Ahhhh.. I get that point now... drywall will get done after the heating season... I want heat so I can wire and insulate but for sure will put filters on the returns to keep as much sawdust and fiberglass as I can out of the furnace... drywall will get done once weather warms up and we hope to move in in the fall... we've lived in that lower house forever so it's no big deal to wait... at least the flood won't threaten to make us homeless anymore... Merry Christmas all!!..

JUGHNE

Another point of concern other than dust/dirt which is very hard to keep out of a new furnace is the return air temp.
House furnace is designed to heat a "warm" house, return air temp to be seldom down to 50 degrees....more like 60 plus.
If you hit the furnace with low RA everyday then you may get condensation in the heat exchanger.

If you do end up with what seems to be an oversized furnace, it would be good to try for a 2 stage. Low fire only about 60% of high fire.