HVAC sizing for large remodel
As part of a large remodel of a 2200 sq.ft house, which involves re-insulating the interiors with green fiber, replacing all ducting and insulating with R6, new energy efficient windows, I am looking to arrive at the ideal configuration for my HVAC. I plan on dividing the single-story house into 3 zones based on occupancy and exposure to sunlight.
Based on the heat-loss calculations done by an energy consultant (see attached pic), it requires 46000 BTUH.
I have a HVAC quote for a 5-ton day&night furnace (92% AFUE, 100k BTU), 2000 cfm capacity to be installed horizontally in the attic, with 12 supply registers, 2 return air ducts, and 2 zone damper system with bypass, using 3 white rogers thermostats. I am not planning in cooling, but the system is going to be AC-ready.
Based on the calculations, for 12 vents @100 cfm each, I would only need a 3-ton unit at 60000 BTUH which would still have a 15-20% safety margin. Both my general contractor and HVAC contractor fee; 60k is too little especially when all vents are open. Also, the contractor does not recommend going with a 2-stage furnace due ECM motor reliability issues.
I think that the unit may be oversized and wanted your opinion on what should be the correct size. The ducting is already in place, and I have asked them to hold off on the furnace.
Last edited by tsranga; 04-23-2009 at 10:38 PM.
So what did the GC's hvac contractor's heat load look like, to justify his equipment selection. Also why did you wait till the duct work was installed to question his selection?
Originally Posted by tsranga
As you can guess, it based on "experience" i.e. rule of thumb based on sq ftage.
Originally Posted by ascj
I did not get the quote until this morning, so did not have the details. Initially, this was supposed to be just for the addition of 600 sq.ft, but since the existing ducting needed work (which we found out yesterday), they felt that upgrading to a larger single unit was more cost-effective.
Is it a typo that you sad 2 zone system with 3 thermostats.
A 60,000BTU 90%+ furnace will be plenty by that load calc.
Your GC and HVAC contractor should know that not all of your rooms will need 100CFM.
Some will need more, some will need less.
Few bathrooms need 100CFM for heat.
A bathroom may only need 37CFM(2,000 BTUs at 50° temp rise), and a 10X12 bedroom may only need 72CFM(3,888 BTUs at 50°temp rise).
Next, a 100,000BTU furnace will probably short cycle on a 3 zone system. And use more gas then needed.
That's correct .. it's a 3 zone system.
Originally Posted by beenthere
What is the consensus on 2-stage vs single-stage systems? I am mainly concerned about reliability, and the ECM sounds very much like my car's ECU (which failed a couple of times ).
Are there any additional areas that I need to check on, like the number of return air ducts? Do I need to have one per room?
Last edited by tsranga; 04-24-2009 at 03:18 AM.
You don't need one per room. But you do need at least one in every zone.
ECM blowers are reliable, when the system is installed right.
The attachment may help you decide how to proceed.
An ECM motor attached to a duct system that is undersized will cost significantly more to operate and could have reliability problems because it is designed to deliver the programmed CFM, regardless if the filter is plugged, all dampers are closed, the ducts are 1/2 size, etc. All of those issues can be detected by a simple static sytem check. Fixiing the high static problem could be a simple as replacing an air filter or as complex as replacing the duct system.
Ask your contractor why he put a foundation under the home? (I'm assuming your home/addition has a new foundation?) If he provides the appropriate answer, it's so the house won't fall down in a short period of time. The foundation ties all the other components together and keeps them together. So it is for a load calculation on the home. The load calc is the foundation for the HVAC system. Once established, you know the total load for the home as well as the load for each and every room under all conditions. Once you are armed with information, the appropriate duct system can be designed and the appropriate equipment selection can take place. Abesent the load calc, there is no foundation and aboslutely nothing to base the system on. That means it could be subject to failure from day one. If the GC insists on forging ahead, then demand at least a static check for total static when the system is started. If it's any greater than .5, reject the system as being too restrictive.
If YOU want change, YOU have to first change.
If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!
Thank you all for the valuable education.
Assuming that we go with a 3 ton unit at 60k BTU, 3 zones, I have 4 bedrooms (3 of them 10x13 - shaded by trees NE facing, and master at 15x25 SW facing), 3 1/2 baths adjacent to each other, and a great room of about 900 sq ft including the kitchen. See attached layout.
There will be 12 vents.
The ducting install was completed yesterday, and I have asked them to hold off on the work.
What should I check for in the ducting system as far as sizes go to ensure that adequate sizing based on CFMs/room and matching returns/duct sizing with a 60k/3-ton unit?
Ask them how they determined what size dust to use.
Manual D, or just rule of thumb.
Rule of thumb again . I went to the site today, and there was a duct layout plan that I have converted to a jpg file (see attached). It does appear that the vents and ducts are of different sizes and lengths based on room size.
Originally Posted by beenthere
He has a list of the following -
7 units of 14x8x8 B boxes
2 units of 10x4x4 b boxes
2 units of 10x6x6 b boxes
1 unit of 12x12x8 b box
1 12" bypass damper
I have also asked for a whole house fan and an optional Honeywell 2000 CFM media type air cleaner.
Based on the duct sizing shown in the pic, how would I calculate the cfm for each room? If the vent is 14"x8"x8', how does this translate to cfm output?
More importantly, can someone tell me if the ducting can work with either a 3-ton/60kBTU unit or a 5-ton/100k btu unit?
Can't really tell from just seeing the sizes.
Length of run(TEL) would need to be known.
But, from the looks of it, it is better suited for 3 tons of A/C then 5 tons.
I will try and post some pics tomorrow. Considering 3 zones, should I get the whole house fan and Honeywell 2000cfm media type filter (quoted as optional) as well?
Originally Posted by beenthere
How about choosing between VS and PSC, or 1-stage vs 2-stage? The unit will be mounted horizontally in the attic.
What should be the size of the coils for a 3-ton/60kBTU unit?
I am sure these questions have been answered in the past, so if you can point me to the relevant posts, I'll try to read up more.
The coil has to match the outdoor unit.
VS blower will help with humidity control in cooling mode.
And help set the proper furnace temp rise in heat mode.
You may want to do searches on duct size, and Manual D.