Multiple appliances on flue stack

[jimbo]

I am trying to evaluate a proposal our condo management has from a heating installer for new furnaces, and it is more complicated than I have had to deal with. Any comments?

Currently there is a 3" round B-vent which extends from the water heater closet on the first floor, straight up. On the second floor, the other water heater connects into a 3x4x3 tee. The B-vent continues as 4" straight up thru the roof.

The proposal is to connect a gas furnace, probably not larger than 45,000 btu input rating, into that 4 B. Is that too much? Because that upper section of B would then have roughly 120,000 btu in it.

We have previously disapproved any proposal to put in a furnace requiring thru-the-roof because our roof is SACRED ( read $$$$$) and we don't mess with it! It seems to me this new proposal would require that the roof section be enlarged to 5". Is that correct???
All of the present furnaces are direct vent thru the sidewall, but they are Suburban, which can't be repaired , and we can't find a new one of that type to replace it. ( They are "pancake" style furnaces due to soffit-mount in the first floor.)

[Dana]

You don't give the total B-vent vertical & lateral lengths and number of ells, which also make a difference. If this is a 2 story with a shallow roof angle, the 4" section could be as short as 8' from where Tee to vent-cap, or it could be as long as 15'. But realistically, best-case, to handle the whole 120K with a 4" stack you'd need something like 30' of vertical 4" stack from the last tie-in, no ells or lateral runs. If the contractor is proposing going with any less stack than that, make them show their math.

Would it simplify the design issues to run a hydronic coil in an air handler running off the HW heater rather than replacing the gas furnace? Even if it meant putting in a bigger-burner HW heater to handle the combined load, something like the condensing 75K Vertex is quite flexible in combi applications, and more than enough water heater for most homes. You have 2 HW heaters here. If an existing HW heaters is reasonably located it might cut the mustard for the heating end, depending on what the actual heat load is. If the new furnace needs to be a heating/cooling HVAC unit there may still be options, but it takes a bit more design. I'm thinking the heat load of a first-floor condo in San Diego can't be anywhere NEAR 45KBTU/hr, (or even 1/3 of that), unless it's a truly huge space with a heluva lot of single-pane glass or something.

[jimbo]

Thanks for the input. That is what I was wondering about. It is a flat roof. The attic space between the ceiling of the second floor, and the roof, is about 4' height max. The flue extends approx 36" above the flat roof. It is a straight vertical rise, but the TOTAL from the first floor to the top of the flue is maybe 25'. The 4" part starting near the ceiling on the second floor, is may 15' max.

We have been considering going to a HW airhandler off the WH. I think that is going to be our best bet. Thanks very much for the input. You're right about the heat load. I will have to look for my manual on the Suburban, but I think they were something more like 24,000. Our A/C is simply thru-the-wall and we are not going anywhere witht that. There is mininal ductwork installed for the furnace...and access, especially on the ground floor, to put in a DX air handler is ....well, just forget it@!

I am going to have management company insist on better plans from the HVAC contractor.