Equipment Sizing Help

Crissie

I'm trying to size a wall heater for my lower level. I had two different engineers do a Manual J. One was at 25,000 BTU heat load, the other was at 19,700 BTU heat load. I have a choice between: a) wall furnace that outputs a max of 30,000 BTU's, but modulates down to a lower level of 8,000 BTUs, or b) wall furnace that outputs a max of 17,000 BTU's and modulates down to 4,000 BTU's.
The manufacturer told me to go with the smaller of the two because 2,700 BTU's (19,700 - 17.000) will not make that much of a difference. What do you all think? You guys have been so helpful, figured I get some opinions.

Zman

Well, which engineer do you believe?
Tell us a little about the climate you are in and the space. How big? How is it insulated? Lots of windows?
Are you sizing a hydronic unit or a stand-alone gas?

Crissie

Zman said:

Well, which engineer do you believe?
Tell us a little about the climate you are in and the space. How big? How is it insulated? Lots of windows?
Are you sizing a hydronic unit or a stand-alone gas?

I paid one engineer, he works remotely. This is what he does for a living. The other engineer came to my house, he work for a distributor here locally. Very smart guy, he was sizing for a boiler system and did a room by room load. I am not sure why there is any difference.
In northern IL, all had a concrete floor, 30% pf space is partially under grade by 2.5'. It is a split level house with a slopped lot, so one side end up at grade. The insulation on the part under grade is an R10 on walls, the rest is an R15. Most of it is 8ft ceilings with heat floor above it. Part of this space is a sunroom (14' by 21') with vaulted ceiling, it is an R13 on walls and I am not sure what is in the ceiling. The sunroom has 3 exterior walls.
I am putting a single gas wall furnace in. Most all of the area is open and connected. Been debating this for days. LOL

Crissie

I guess I think both engineers might be wrong on their sizing of this area because they are so far apart. And they were not far apart for other areas / rooms of my house.

EBEBRATT-Ed

Basements can be tough.

The part below grade doesn't loose much heat but concrete feels cold. What are you putting on the floor or is it staying bare concrete? Are you going to heat this all the time or only when you use the room?

The are about 22% different in their calculations which is a bit.

My gut tells me if you have carpeting or something on the floor and you heat it all the time and it's closed off tight from any cold utility area the smaller unit should be fine.

If heated sporadically cold concrete floor go with the larger unit.

As you can see from my last two sentences usage and a bunch of other factors go into this.

Crissie

I have ceramic tile and vinyl plank on the floor. Only 30% is partially under grade, the balance of the area is at / above grade. I would heat it most of the time to at least 65-67 degrees. The sunroom has 2 walls with huge windows. Thanks

Zman

What is the total square footage of the space being heated?

Crissie

Total Square Footage is around 1250 sq ft.

EBEBRATT-Ed

@Crissie

With big windows and that much wall exposure I vote for the larger unit especially because it has the ability to modulate down.

JMHO

ChrisJ

I'd go with the 30K.

2K won't make much of a difference but if the unit is already undersized, more undersized stinks.

Crissie

I decided to go with the larger unit. It might short cycle at 40-60 degrees, but I don't know of anything that will run well at both 50 degrees and 1 degree or less.