Adding a floor - can I get input on design choices?
Hi - because of my experiences with my own system (search my username if you want to hear the tales 8-}), I got asked to help my brother with his remodel.
They're in NY, I'm in New England, and they are paying contractors for the whole job. I'm posting because I want to make sure they are getting exposed to all their options. Anyway, the house is in the suburbs of NYC. It's a ranch built around 1950, with about 1800 sq ft on the main floor and a full basement. It has Forced Hot Air (gas furnace). It currently has an older (oversized) 100K+ BTU furnace with integrated (also somewhat oversized, though I don't have the exact #) A/C. My dad lives at the far end of the house (opposite end of the other bedrooms) in a separate 12'x 12' bedroom with its own full bath.
They're adding a 2nd floor of about 1300 sq ft to the house, and doubling the sq footage of my dad's bedroom, and doing some re-dividing of the bedroom living space on the main floor. One annoyance for construction is a cement slab between the main floor and the basement. (there used to be one between first floor and attic as well, go figure).
ANYWAY - The HVAC guy is recommending adding a 2nd system in the attic, properly sized for the 2nd floor (Carrier Infinity, 92+%, VSP) to cover the 2nd floor, and other than some opportunistic ductwork adjustments, leaving the existing system about the same. He's amenable to replacing the basement system. We're thinking of letting him replace the basement system (again, with a Carrier Infinity) so that we can zone my dad's room separately. Still, I have some questions for you experts...
1) I had suggested that perhaps the "right" solution was a new single system in the basement to cover the entire house. He says that is not the best choice, and that it's not even a comfort win because to cover the 2nd floor he'd have to run a plenum up to the attic and feed the 2nd floor down from there anyway. I was thinking the existing duct work could be expanded and added to (with much routed in the floor of the 2nd floor) to equip the house as if it were being built from scratch. Is he correct that if this was new construction, 2 systems would be the right approach?
2) Should I be worried at all about heat comfort on the 2nd floor since the heat is being pushed down? Contractor says that 2nd floor is somewhat warmed by heat rising from first floor, and that although most registers are positioned on the ceiling over windows, a few will be put lower down. He also said that whatever he does to position the registers for heat is fine for A/C because "for A/C it doesn't matter where the registers are".
3a) With all that ductwork (and possibly a system) in the attic, should we put an exhaust fan at one end of the attic?
3b) As you may note, the 2nd floor is smaller than the first, and so at either end of the house, there is a first floor room with a big unconditioned "crawl" attic space over it. In both situations these are bedrooms. Will a buildup of heat (or cold in the winter) in these crawl spaces cause issues for the room below it, or adjacent to it?
4) How important do you think it is to replace the basement system now? Should there be a significant cost advantage to having him replace the basement system now while he's doing other work?
5) If we don't replace the basement system, it was suggested that we could put a slightly larger system in the attic, and tap the new system to create a separate "zone" for my dad's bedroom. However, the bedroom will be 2 stories below the system (system in attic, bedroom on -first- floor). The space under the bedroom is unheated storage space, and so I 'm concerned about relying on pushing heat -down- 15-20' as the -primary- heat for the room. am I off-base here?
6) Anything else I should be asking or concerned about?
thanks guys, - you're the best
Where would he be putting the registers if he installs a new unit in the attic?
The end rooms with no room over top, will always need heat and cooling sooner then the other rooms. So they would be good reasons to zone.
If a new system is put for the second floor. The current furnace will be even more over sized then it is now. So it would probably be a good idea to replace now, if finance permits.
If the ducts are sized right. No harm using new second floor unit to handle the end room. As you know. Getting them sized right can be a problem.
>>Where would he be putting the registers if he installs a new unit in the
He said most would be in the ceiling blowing over the windows, but a few would be lower down.
>>The end rooms with no room over top, will always need heat and cooling
>>sooner then the other rooms. So they would be good reasons to zone.
Good point - had thought about the room at one end, but not the other (since it's part of the multi-room master bedroom suite)
>>If a new system is put for the second floor. The current furnace will be
>> even more over sized then it is now. So it would probably be a good idea
>> to replace now, if finance permits.
Interesting thought - not sure how it's "more" oversized - furnace is same as before, space it is conditioning (first floor) is same as before. I would think that the 2nd floor unit needs to be undersized because it benefits from the first floor. And "oversized" just means that main furnace short-cycles, not that it over-heats, I think?
>>If the ducts are sized right. No harm using new second floor unit to handle
>> the end room. As you know. Getting them sized right can be problem.
Yup - there's plenty of room to run big ducts from the new attic through the new crawl space over the end room.
In fact - HMMM - maybe the attic unit should service (as separate zones) the end rooms on the first floor on BOTH ends. Both have unconditioned space above and below. I was worried about how well the system could push heat -DOWN- 2 levels, but I guess that's ok. [of course, if we do this, then the original unit is way oversized for what's it has left to condition - ok, maybe it is best to replace basement system and use smaller system in attic]
Once a second floor is built. The first floor ceiling is below a conditioned space. No heat loss.
If he intends to put the registers in the ceiling. then weather he installs a unit in the attic, or zone a system from teh basement, its the same difference as for how the heat blows out.
Thanks for everything BT. We're putting a 2nd (96% infinity) system in the attic for the 2nd floor, and it will also condition my dad's room. we stopped there because of the difficulty of getting ducts to the master bedroom, and because of a concern that if we took too much away from the basement system, it might be too oversized and need to shed excess heat. I think he's struck good compromises between cost and comfort.
HOWEVER - i saw the ductwork plan for the attic, and noticed he was going with one central return, so I've asked to add returns to the bedrooms. He said that there was enough airflow under the doors because new construction (which the 2nd floor is) puts 1" of airflow at the door bottom. Does that make sense?
Also,, he splits his plenum in 2 directions from the main duct, and feeds about 500cfm in each leg. However, the plenums are only 10x12 and 10x10 - is that sufficient for 500cfm?
There may be enough under cut. Depending on how many CFM the bedrooms need.
As far as duct size. That needs to be determined by how long(total equivalent length)they are.
Almost sounds like he's guessing on size.
150 CFM per bedroom
about 12' of 12x10 length before they dump their first 150 - 200 CFM (out of 500cfm total)
Cutting it a little close on the return.
The whole duct system needs to be known, to determine TEL.
It may be close to sized right. Or it could be a bit small. It will probably work. But can't say how well.
Will all supplies have dampers in them.
I think all the supplies have dampers - there's a cross mark where they hit the main. What might be easier BT, would be to show you the plan. I'm not comfortable posting it, but can I PM you somehow?
You can email me.
PM function is restricted to pro members only.