Someone is wrong, either you or the dealers.

What size and efficiency is existing furnace?

What is your location?

Ask dealers to justify their sizing with a load calculation of their own. Then make comparison to your results.

Just an opinion based on the info you ave provided, but I think your calculation is too low but you certainly don't require an 80 KBTU furnace either.

BTW, your fuel source is nat gas?

Post back.

IMO

Currently have a Thermopride oil OL5-85 with 85k btu output nozzle, installed in 1998. This furnace rarely runs more than four or five minutes and then shuts off, and I recently learned about steady-state efficiency and how that plays into furnace sizing and how it is more desirable for a furnace to run longer at a lower output than shorter at a higher output.

Location is near Seattle. My delta T is 44. I've averaged 240 gallons of oil over the last 5 years, which if my calcs are correct is 21,152,000 btu's per heating season. This is in a house that still has zero insulation in the walls. That will change completely in the next two or three years.

No dealer has run a load calculation. They come in, shine their flashlight around, and hand me a brochure. I'm not being a know-it-all either. I keep my mouth shut and answer their questions and let them assess what I need. Right now, I'm just feeling like this is a huge disconnect from what I have learned over the last year about high-R houses, heating, cooling, and indoor air; however I still consider myself pretty ignorant about a lot of this.

The energy audit guy that came a few weeks ago said I probably could heat the entire house with a one or two-head mini-split system, but I'm still skeptical. I still favor the idea of using my existing ducting because of how the rooms and doors are configured and used.

p.s. Yes, I would be running natural gas for fuel, although it is not currently in the house. The cost of hookup would be negligible.

You will find it difficult to locate a small furnace.

Do you want 80% or high efficiency model?

No AC?

IMO

I assumed I would go with a 2-stage 95%+ furnace. I'm now trying to learn as much as I can about modulating units. I see some of them go as low as 20k'ish btu's on the lowest setting, but I don't know if one would be appropriate for me or not. I've read of some concerns about running them on low all the time. Don't know how valid these concerns are, although the idea of a furnace that adjusts to your heating load while maintaining its highest efficiency seems like a good one.

No A/C. It's just not hot long enough here, plus I'm by water and there is usually a breeze. I just turned the heat off two weeks ago. We heat 8 mo's a year and have 4 months of pure bliss.

Got another dealer coming at noon. I have four or five more I could call from my list.

By all means switch from Oil heat to lower cost Natural Gas.

With your winter design temps, I think your Whole House Load-Calc is quite accurate.

You can go 50% above the load-calc with heating.

I would stay in the 40,000-Btuh range or lower.

Longer runtimes provides a lot better efficiency.

Those contractors should be using A load-Calculator; here is as "Free Online Whole House Calculator" they can use; but they should use a room by room calc.

Here is a link that might be useful: Free Online Whole House Calculator

Thank you both tigerdunes and udarrell. I used that same calculator for one of my figures.

Well, the dealer rep from yesterday really knew her stuff. When I did bring up the subject of my load calc, she understood immediately and also agreed that my numbers were pretty close to what she would expect, but she still came back with a 60k 2-stage Coleman unit. This was after we discussed a mini-split system and telling me she would go with a two-head 18k Daikin system.

When I asked her why everyone previously was suggesting an 80k furnace and why the oil furnace was 85k, she said it's simply that my ducting was built for that, so I at least learned about that. She said running a smaller furnace would be absolutely no problem in a house my size with all the ducting in conditioned space.