Reply45,000 BTU furnace has a 3 ton blower which is good for 36,000BTU of A/C.
Looks like a 45K furnace with a 2 ton A/C would be enough.
I've NEVER seen an undersized gas furnace in the real world...
Regular Guest
I have an existing home that I have completely remodel, demo and replaced. It is approx 1600sqft on the 1st floor with 750sqft basement. I have had the ductwork reworked when I tore off and replaced and added on approx 1000sqft of the home.
I had a load calc done to see for myself what size of furnace/ac I should be expecting from quotes I am waiting on.
My load calc came out with the following
Heat Gain-Latent and Sensible-14000
Heat Loss-44000
I have recieved one quote which is for a 100000 BTU 2 Stage 96% Furnace with a 4 Ton Heat Pump. I was told that part of the reasoning for the size was due to the size of blower I needed.
I had another contractor stated he was thinking 70,000BTU with a 3 ton Heat Pump, I am still waiting on his quote to see if it sticks.
Any suggestions welcomed
Professional Member*
45,000 BTU furnace has a 3 ton blower which is good for 36,000BTU of A/C.
Looks like a 45K furnace with a 2 ton A/C would be enough.
I've NEVER seen an undersized gas furnace in the real world...
Regular Guest
So do I show the HVAC contractor the load calc I had done, or keep searching till I find one that sizes it accoerding to the information
Professional Member*
The contractor should do a load calc of his own, otherwise he is going by rule of thumb and you need to find a contractor that will perform the calculations necessary for a correct install of your system. If they cut corners on the load calc where else are they going to cut corners?
Regular Guest
My bad I forgot to mention that I would be using a heat pump, would that change how it is sized to be done so by heat loss
Professional Member*
Some will size the H/P to the heat loss of the home. The problem is that you'll have too large of a cooling system in the summer, even if you went with a 2-stage H/P.Originally Posted by remodel80
If ur heat loss and heat gain formula is correct, then all you need is a 2 ton H/P. On the heating size, it sounds like a 60,000 btu input, 2-stage would do.
Make sure to get a variable-speed blower and a 4" filter rack assembly.
Professional Member*
You can get a 2 ton HP with 6 kw electric resistance heat. Your indoor blower will need can be 800 CFM. If your ductwork is complex, maybe a 1200 CFM blower.
You can call me Sam
It should be a crime to be a mechanical engineer in San Diego
Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)
Professional Member*
I have run into many houses that have been remodeled and sealed up so the load goes down 25 - 50%. So that makes it easy to size the equipmetn to match the needs of the house BUT, you need to have a blower big enough to fill the existing duct if it is not going to be changed out. If the duct and registers are sized to move twice the amount needed by the new equipment you won't either get the air spread through out the house as needed or it won't have enough velocity to properly stir the air. Either way your comfort will go down. Long story short if you don't have properly sized equipment, ducts, and registers you have a problem. Make sure your contractor checks all 3.
Regular Guest
I had the ductwork replaced when I had completed my remodel/addition. At the time the existing furnace was more than big enough is what i was told. I had my ductwork designed and built by a commercial shop that I work with daily insulating there pipe and duct.
At the time I was told the ductwork was sized to the home not the system, since I wasnt replacing the system at the time.
I personally took the time to show both contractors the entire duct system (duct,registers) which neither one stated there was problem. I told them that if a component of the system needs to be replaced even though it was recently done that is what I was wanting done to make sure I have a hvac system that can do the job right and effieciently.
I guess I will just have to start asking on the front end how they size there equipment there quoting.
Professional Member*
So, as I read this, ur home is approx. 2,600 S.F. (ranch?) with a 750 S.F. basement? Is that right?
Regarding the contractors looking at ur ductwork.........did they test the return air side? I've been to homes where the homeowner told me the ductwork was fine, according to the contractors before me, and, after a quick test, I found the return side was 1/2 of what was needed.
Regular Guest
No my home is approx 1,600sqft with 750 sqft basment. My home is a two story with the second floor being heated and cooled by minisplits, there is a cellar basement as well but is moreless my mechanical room.
On my return side i have the following
2-32x8 return grills (2-16" stud cavities per grill)
1-16x12 ducted return with a 16"x20" grill
So i should roughly have 1250 cfm of return.
I personally showed them all the ductwork, I told them what i knew but did not comment on whether it was fine or not. Thats there job to make that call.
Professional Member*
We are getting closer...........are you planning on heating and A/C (or H/P) the whole home or are you going to let the mini-splits continue to take care of the 1st floor?
How large is each level of the home (not the basement)?.......800/800?
Regular Guest
Heating and H/P the entire first floor and basement
The First Floor is the 1600sqft, the second floor is not in this number since it is heated and cooled by the mini splits.
Basement-750sqft
1st Floor-1600sqft
2nd Floor-600sqft (Not to be included since it is served by the mini splits)
Professional Member*
Thank you for the clarification. The guy that suggested the 100,000 and 4 ton must be smoking something. Absolutely no way. You will hate the 4 ton blower noise, not to mention it won't work.
Your 1st floor basically has no heat loss or gain. According to your calculations, a 60,000 input btu (96%) furnace and a 2 ton H/P is in order.
Regular Guest
I personally have done all work on my home and have in reason made a goal of making an efficent affordable home. I paid to have the calculation done and supllied all information, the results are what I posted and was looking for a second opinion to confirm this wasnt out of reason. Even rule of thumb the system he is trying to sell me is to big.
Thanks for you advice on this, know to find someone to do the job right and install what is needed not what is thought to be needed. I believe that this could be a challenge, so I will be getting with my commercial HVAC guy and have him design the duct needed for the connnections and buy the unit myself and have a service tech do final setup and connections.
The other disappointing factor of this is this contractor that suggested the 100,000 BTU 4 ton system was a contractor i found off of this site in my area.
Professional Member*
The other disappointing factor of this is this contractor that suggested the 100,000 BTU 4 ton system was a contractor i found off of this site in my area.
That is too bad, but that goes to show that one needs to do their homework. I've been told the wrong info enough that (I've been in the business 35 years) I've always done my own homework.
The last company that I joined didn't know they could use 2" PVC, they were putting tax on the labor (installation jobs) and they were not setting the set-up switches on the furnace correctly.
This is along with not selling (until I joined) geo-thermal, mini-splits, zoning or ductwork to room additions or modifications. And it's a large company.........
Regular Guest
I agree always do your own homework.
Again Thanks to all who have commented to help me achieve finding the right equipment. I dont mind paying the price that comes with a job done right.
I installed the mini splits myself becasue I couldnt find a contractor that knew enough about them to recommend them. That was thanks to another commercial HVAC friend of mine that recommended them and also hooked me up with a distributor that would sell me the equipment.
It pays to ask questions.
Posting Permissions