sizes for exhaust and air return

[George2]

My home seems to be well insulated; I see it in attic and exterior walls. Also have double paned windows. Our climate here is mild I only use the A/C about 6 weeks per year in Aug and Sept when temps get to max of 100 F outside, even then the nights always cool down below 85 F. We use the furnace much more frequently maybe 5 months per year, but outside temps rarely get below 30 F.

So is it fairly certain that I would be better off with a 3 ton system for my home? If George2 gets 2.5 tons to work adequately in homes that are 10% bigger than mine in climates that are more extreme that almost seems proof enough that 3 tons would work for me. And as udarrell stated that should make the current vent and return opening issues less severe?

How many supply registers does your home have on the second floor and how many registers on the first floor? Where are the registers on each floor located? Thanks......that will help determine the airflow. Can you control the airflow other than at the register itself?

[cjccmc]

Is there a forced power vent on the water heater and the furnace? Power venting increases the capacity allowed through the vent. Also the vent material makes a difference. Since it was used in multiple houses in a tract setting, chances are it was an engineered design calculated to show the code official that it would work. The code allows that option. Apparently it does work for the system that was installed. 33 years is decent life expectancy for that type equipment. Same goes for the return air. As long as the total external static pressure at design CFM of the fan is not exceeded, the return opening can be small. And again, that is probably the case because of the tract situation. The design was probably engineered, calculated and shown to work so it could be used multiple times to save first cost money, with little thought to energy savings. Not to say that larger or more openings wouldn't work better especially in the case of the RA grille.

No forced air on the vent, just a mostly vertical run of 5" (4.25 ID) pipe.
The return register is on a wall in my family room about 2 ft away from the furnace that is behind that wall in the garage.

[cjccmc]

How many supply registers does your home have on the second floor and how many registers on the first floor? Where are the registers on each floor located? Thanks......that will help determine the airflow. Can you control the airflow other than at the register itself?

Only that one 30 x 10 register on the bottom floor. All the air pulled by the fan has to pass thru it (and tiny cracks that ants like to crawl thru )

[vangoghsear]

How's the noise level when the system is running? Do you hear air noise (hiss) from the register? Everyone saying that it should be noisy at correct CFM for 4 tons are right. It would run about 35 NC through that register, which is at the high level of acceptability of the design standards for residential. A larger one would likely run quieter, all else being the same.

[cjccmc]

Yes, it's kind of noisy, my command central station (Lazy-Boy recliner) is right next to it and my finely tuned reflexes quickly hit the Volume up button on the remote when the fan kicks on. The register actually makes a humming sound.

[vangoghsear]

Yes, it's kind of noisy, my command central station (Lazy-Boy recliner) is right next to it and my finely tuned reflexes quickly hit the Volume up button on the remote when the fan kicks on. The register actually makes a humming sound.

Since the unit is so close, that could be machine noise from the unit. Reducing the unit to better match the load could improve that. Making a larger opening might not, so holding off on that is not a bad thought.

[George2]

How many supply registers does your home have on the second floor and how many registers on the first floor? Where are the registers on each floor located? Thanks......that will help determine the airflow. Can you control the airflow other than at the register itself?

I'm sending this to you again...........I'm asking about the SUPPLY registers. Please re-read. Thank you.

[cjccmc]

I'm sending this to you again...........I'm asking about the SUPPLY registers. Please re-read. Thank you.

Ah, my bad, supply registers mean the vent / louver that supplies air to individual rooms then? If so there are:
6 upstairs, (1 per bdrm and 1 per bath)
4 downstairs: (kitchen, family rm, dining rm, living room)
10 total
all located just below the ceiling
only way to control the flow is the louver thing on each register
we typically close off the ones in rooms not being used, probably 1 or 2 closed anytime system is on for both heat and cool.

thanks for your help George

[George2]

Ah, my bad, supply registers mean the vent / louver that supplies air to individual rooms then? If so there are:
6 upstairs, (1 per bdrm and 1 per bath)
4 downstairs: (kitchen, family rm, dining rm, living room)
10 total
all located just below the ceiling
only way to control the flow is the louver thing on each register
we typically close off the ones in rooms not being used, probably 1 or 2 closed anytime system is on for both heat and cool.

thanks for your help George

Okay, thank you. Sounds like all high sidewall supplies.....just like my home built in 1946. It works great for A/C. Hopefully, you don't have ductwork in the attic. It probably all comes from a main duct run through the ceiling of the main floor. Anyway, you are only moving on the supply side, with all of them open, about 1,200 cfm at .5 static.

This is not good for a 4 ton system. Did the salesmen look over the ductwork? Meaning: count the number of supplies?

[cjccmc]

Los Angeles 2.5% summer design is 89F & 70F wet bulb; so let's not oversize the A/C.
Reducing the required size of the A/C will help make the duct system problems easier & less costly to resolve.

Before doing anything else, I'd have a Home Energy Efficiency Audit performed; the Power Company may have a program.
After the retro-work is done, do a heat gain/loss load-calc & size-down as much as possible.

The oversized A/C is creating a lot of costly problems that would be somewhat ameliorated if it can be sized-down enough....

A Free online Whole House Load-Calc might be helpful...

I went to the website udarrell posted above and gave it a shot (I probably know just enough to be dangerous )

When I put in closest area, Los Angeles, it entered some default temps for winter and summer. To be conservative I made the summer temp 100 F and winter 30 F. I used R-11 for my stucco walls and picked average quality for ducting, etc. Anyway results I got in rounded values were:

Cooling: 33,000 btu with 30,000 btu sensible
Heating: 37,300 btu

Does that seem about right for 2000 sq ft, 2 story house?
What decides the required tonnage cooling or heating? Both come out close to 3 tons if 12,000 btu/ton is the formula. Anything else to consider?

[cjccmc]

Okay, thank you. Sounds like all high sidewall supplies.....just like my home built in 1946. It works great for A/C. Hopefully, you don't have ductwork in the attic. It probably all comes from a main duct run through the ceiling of the main floor. Anyway, you are only moving on the supply side, with all of them open, about 1,200 cfm at .5 static.

This is not good for a 4 ton system. Did the salesmen look over the ductwork? Meaning: count the number of supplies?

Ducts for upstairs do run thru attic, gets plenty hot up there in summer.
One of two salesman did look at all the rooms, both said 4 tons without making any measurements, just I think going from size of existing unit and 2000 sq ft value I gave them.