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Hypothetical Situation:
(not really but let's assume so in order to establish creative constraints to the following)
1,600 sq. ft. ranch home is given a second floor addition of 1,600 sq. ft.
Existing 96,000 BTU output Burnham Series 2 gas boiler is 14 yrs. old; well maintained (no damage & clean), baseboard heaters and plumbing to them in great shape.
Existing central AC (in attic w/ flex vents) requires replacement.
Adding second floor increases height of single-flue chimney by about 9'0".
Owners prefer to install gas furnace for 2nd floor heat.
Questions:
1) Would basement or attic furnace be able to share existing chimney with existing boiler since the boiler flue would have to be decreased and, thereby, allow possible 'double-flue' liner?
2) What safety/efficiency considerations are there to a gas furnace being in basement vs. attic?
3) How would one address sizing if furnace and A/C is to share venting...considering the A/C would also vent to 1st floor but heat wouldn't need to?
1,600 sq. ft. ranch home
Existing 96,000 BTU gas boiler/ baseboard heaters
Existing central AC (in attic w/ flex vents) requires replacement.
(that's convenient as it'll have to come out anyway)
a second floor addition of 1,600 sq. ft.
Owners prefer to install gas (downdraft FHA) furnace for 2nd floor heat.
1) Would basement or attic furnace be able to share existing chimney ...?
Probably not.
2) What safety/efficiency considerations are there to a gas furnace being in basement vs. attic?
so long as their designed for the purpose... no safety/efficiency issues apply
3) How would one address sizing if furnace and A/C is to share venting...
considering the A/C would also vent to 1st floor but heat wouldn't need to?
This is where the HVAC **professional** earns their salt.
Find a real good one.
I can just about guarantee that every HVAC person looking at this would suggest:
a) Do not use forced air furnance if you already have hydronic heat -- it would be super easy to add on sufficient underfloor zones to warm second floor addition
b) put in new A/C to handle cooling he whole new space and existing
If home owners absolutely do not want hydronic heating for second floor the obvious "alternate b" is to used combination forced air furnace and A/C for addition and separate A/C only for first floor.
Given relatively modest size I suppose a "split system" might also be an option but if there are more than 4 bedrooms and 2 baths being added in the new space the cost of split systems is going to get nuts...
1) Would basement or attic furnace be able to share existing chimney with existing boiler since the boiler flue would have to be decreased and, thereby, allow possible 'double-flue' liner?
Not sure about the boilers but I've seen a lot high efficiency gas furnaces vented using PVC, you don't need a chimney.
I've consulted with four HVAC contractors and only one thinks having a gas boiler and a gas furnace is problematic.
The other three thinks it's a great idea since the existing boiler was somewhat oversized for the existing house, but would be undersized for the expansion, and since it probably has another 7-12 yrs. remaining of it's expected lifespan (with diligent annual maintenance).
They recommend the new gas furnace be located in the basement with a centrally located 24x24 supply to the second floor and branch supply from there (with one furnace vent serving the main room of the first floor to lessen the cycling of the boiler).
The floorplan allows this with no branch supply needing to be over 10' long (there are 5 second floor rooms and only one is over 5' from the supply shaft).
But they are split with the chimney venting. It's an unusual set-up but being that we use only gas for fuel and both units compliment rather than compete with each other (room heat from one mixes with room heat with the other).
As for the A/C. Since we rarely use it, usually just at peak times (ie. 12PM-5PM) we decided on having it in the attic and, by use of manual plenums and louvred vents, have it share the furnace ducts but using A/C vents at the ceiling (the heat vents at the floor would be manually closed). This way the A/C (unlike the heat pump option) can be sized for just the 2nd floor but by-pass the rooms least used.
This may be problematic for the next owner if they're A/C reliant but by then the unit will be about 10 yrs. old and their problem.
They recommend the new gas furnace be located in the basement with a centrally located 24x24 supply...
The floorplan allows this with no branch supply needing...
This is a highly unusual approach (to me at least)...
No comment beyond that.
Quote:
But they are split with the chimney venting.
This is the odd part.
If the new furnace is a modern high efficiency model...
then venting should be a non issue as you can vent it through a side wall...
and leave the existing flue set up for the boiler as it is.
Call me cheap, but this is what I would do. I see you are in NY, so A/C demands aren;t too incredibly high.
Assuming your expansion is going to be better insulated than the rest of your home and since it is on teh second floor, heat rises. Your heating requirements on the second floor will be less than your first floor and it will always be warmer on the second floor since it is new, more insulated and where the heat will rise.
A/C Get a Split mini System for the second floor. Look into it if you are not familiar with them. You can get them with heat as well. You can get a 4 zone split mini-system (allows 4 rooms to have different temps) for around $4k and could so the install yourself (easy). These systems are electric, but seeing how you can turn down the temp in rooms you are not using... it may end up being a wash.
Doubt you'll want this system... .but it is an alternative!
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