Best options for replacing 23 year old gas/furnance & AC

[Z0neme]

So I'm in the middle of a whole house renovation which includes adding a 238 sq ft office to the 2nd story. My central AC unit died late last summer so our intention is to replace the whole HVAC system instead of just the AC unit itself. We never had any issues with heating the home but the upstairs typically is about 6-7 degrees warmer than the downstairs during the cooling season which can make it difficult to sleep at night. Our house faces the sun all day with almost no shade so it gets baked to say the least. All rooms have two supplies and one return except for the kitchen and bathrooms which have no returns and only one supply. The living room where the thermostat is located actually has two returns and one supply if that matters. In addition, there is no zoning of the house so all supplies are open when the HVAC is on. Our old AC was a 2.5 TON unit and the furnance is a Plus 90 Bryant 66,000 BTUs. The existing sq ft is 2,000 but will jump to 2238 sq ft after the office is completed.

That all being said, we are looking to have the house zoned. The recommendation is to pull the main trunk down and split it into two seperate trunks. One will be the first floor and the other will be the second floor. The office and basement will also be seperate zones. Per the HVAC tech, we would not need to go bigger with the furnance or the AC unit, but they will bump the AC unit to 3 TONs. From what I understand, the secret is in the duct work but I would still think a larger furnance/AC unit would be needed given the additional sq ft? I can see the system being fine when only a particular zone is calling for air but would it have enough flow to supply all zones at once? Let me know your thoughts when you have time. On a side note, what whole house humidifiers attached to the furnance are others having installed and can they be controlled from the thermostat? What are others takes on those electricstatic filters compared to the normal paper filters. Any thoughts on Ducane? This is the maker of the system the HVAC tech recommended saying it would be far cheaper to repair with off the shelf parts should it be needed in the future.

[PowerPlay]

You said the supply ducts for the house will be split into four zones -- 1st floor, 2nd floor, office, and basement, but they're all going to be fed by a single 3-ton A/C unit? If so, then I don't understand your statement about a "particular zone calling for air." If you have only one A/C unit, then all the zones will get air when the A/C unit runs. You can have a manual balancing damper installed in the supply duct to each zone, which can then be used to set the supply airflow for each zone.

Your finished square footage is going up by about 12%. You said the old A/C was 2.5 ton and the new A/C would be 3 ton. That's a 20% increase in cooling capacity, and you think it should be even bigger? The furnace fan will need to be sized consistent with a 3-ton A/C coil.

Whole house humidifiers are not controlled from a thermostat. They are controlled from a humidistat, with the sensor often installed in the return air duct upstream of the furnace/humidifier. AprilAire and Honeywell are two common mfg. of whole-house humidifiers.

[Z0neme]

You said the supply ducts for the house will be split into four zones -- 1st floor, 2nd floor, office, and basement, but they're all going to be fed by a single 3-ton A/C unit? If so, then I don't understand your statement about a "particular zone calling for air." If you have only one A/C unit, then all the zones will get air when the A/C unit runs. You can have a manual balancing damper installed in the supply duct to each zone, which can then be used to set the supply airflow for each zone.

Your finished square footage is going up by about 12%. You said the old A/C was 2.5 ton and the new A/C would be 3 ton. That's a 20% increase in cooling capacity, and you think it should be even bigger? The furnace fan will need to be sized consistent with a 3-ton A/C coil.

Whole house humidifiers are not controlled from a thermostat. They are controlled from a humidistat, with the sensor often installed in the return air duct upstream of the furnace/humidifier. AprilAire and Honeywell are two common mfg. of whole-house humidifiers.

My concern is that the fan size may not be adequate (even if matched/size for a 3 ton AC) if all zones are calling for heat or AC at the same time. I'm sure it can push the air just fine for each zone individually (for example, just the first floor is calling for air). It's my understanding, that not all zones will get AC or heat if the thermostat in that zone is not calling for it. Unless I'm missing something here? However, let's say we come back from vacation where we held the temperature to say 65 and now bump it back to 70. Will the fan have enough 'push' to supply all zones at once adequately without issues? I should also mention that the basement will be finished at some point and that will add another 750 sq feet. I realize it won't necessarily need AC, but will certainly need heat which again brings me back to the fan having enough push to supply all zones at once. I should mention that the zoning will be controlled automatically by dampers in the duct work in the basement. Base on what I know, I should not have to manually close them.

Yes, I still question the tonage of the AC unit. Like I said, the 2.5 ton unit struggled to keep the home cool and would never turn off when set to 75 degrees and an outside temp in the upper 80s or lower 90s. Again, the upstairs was consistently 6-7 degrees warmer (81+) than the first floor. It was significant enough that I installed window AC units in the upstairs bedrooms as a temporary solution until we could address the HVAC system as part of the current project. I even tried closing all the supply vents on the first floor to see if that would improve the cooling of upstairs but found it to only help slightly. I should also mention that the additional 238 sq ft will also have cathedral ceilings unlike the most of the other house which has 8 ft ceilings. The only exception is the master bedroom also has cathedral ceilings and is about 350 sq feet.

Ok, so can I presume the humidistat and thermostat are one of the same unit? I would imagine the humidity sensor sends a signal to the humidistat so you can adjust the level of humidity if so desired? In short, similar to the way the temperature works.

Hopefully I didn't muddy the waters too much here. On a side note, I really have no clue if a 20% bump in AC cooling is enough to bring the upstairs in line with the downstairs. This is why I'm posting here so I can get educated either way.

[PowerPlay]

I should mention that the zoning will be controlled automatically by dampers in the duct work in the basement.

Isolating air to multiple zones could cause insufficient airflow through the A/C coil, and you could get into the freezing coil issue. It would also not be good for a furnace to have airflow shut off to multiple zones.

Ok, so can I presume the humidistat and thermostat are one of the same unit?

I don't understand this statement. The thermostat and humidistat are two separate devices. The humidistat will turn water on & off to the humidifier, but water only flows when the furnace is running.

[Z0neme]

Isolating air to multiple zones could cause insufficient airflow through the A/C coil, and you could get into the freezing coil issue. It would also not be good for a furnace to have airflow shut off to multiple zones.



I don't understand this statement. The thermostat and humidistat are two separate devices. The humidistat will turn water on & off to the humidifier, but water only flows when the furnace is running.

Ok, maybe I'm missing understanding how zoning works here? Here's how I understand it and please correct me if I'm wrong. The main trunk is to be split into two seperate trunks. One trunk will serve the first floor and the other will server the second floor. Let's just leave the Office & basement zones out of the equation to keep this simple. So both trunks come back to the box above the furnance. If the first floor is calling for cooling and the 2nd floor is not then wouldn't a damper automatically close the trunk to the 2nd floor and so air is only pushed to the 1st floor. The fan on the furnance is a 2-stage or variable and therefore should have enough smarts to increase/decrease the speed to prevent damage or icing of the coils. I would imagine the AC "talks" to the furnance as well and can vary the cooling capacity depending on what it's calling for in terms of zones. Make sense or am I'm assuming way too much here on the intelligence of today's HVAC systems.

As for the thermostat and humidistat, I expect the sensors to be seperate but the control panel to be one unit. For example, I have an outside temperature/humidistat monitoring device that reports back to a control unit inside the house the outside temperature & humidity. I would expect the same exact thing with a HVAC system. Therefore, I would go to the same control panel to increase temperature and/or humidity.

[PowerPlay]

Ok, maybe I'm missing understanding how zoning works here? Here's how I understand it and please correct me if I'm wrong. The main trunk is to be split into two seperate trunks. One trunk will serve the first floor and the other will server the second floor. Let's just leave the Office & basement zones out of the equation to keep this simple. So both trunks come back to the box above the furnance. If the first floor is calling for cooling and the 2nd floor is not then wouldn't a damper automatically close the trunk to the 2nd floor and so air is only pushed to the 1st floor. The fan on the furnance is a 2-stage or variable and therefore should have enough smarts to increase/decrease the speed to prevent damage or icing of the coils. I would imagine the AC "talks" to the furnance as well and can vary the cooling capacity depending on what it's calling for in terms of zones. Make sense or am I'm assuming way too much here on the intelligence of today's HVAC systems.

As for the thermostat and humidistat, I expect the sensors to be seperate but the control panel to be one unit. For example, I have an outside temperature/humidistat monitoring device that reports back to a control unit inside the house the outside temperature & humidity. I would expect the same exact thing with a HVAC system. Therefore, I would go to the same control panel to increase temperature and/or humidity.

Prior to this, I don't think you mentioned the AC or furnace would be 2-stage. What you are describing is not typical for a 2200 sf residential installation. You need to bring in a HVAC contractor to see what iif what you are envisioning is workable and how much it would cost.

Very large houses (say 4000 sf and up) may have zoned heating & cooling, but that typically means a separate heating & cooling system for each zone, rather than redistributing supply air from a single unit by means of motorized dampers.

I'm not aware of a combo humidistat/thermostat controller, but that doesn't mean it doesn't exist.

[beenthere]

Where are they going to add/get the extra return a 3 on will need from?

The office will be its own zone? or it will be on with the basement zone.

Doubtfull the addition will add more then 2000BTUs if that to the cooling cooling load. plus it will reduce load to the room its attached to, and the room below it on the first floor.

[Z0neme]

Where are they going to add/get the extra return a 3 on will need from?

The office will be its own zone? or it will be on with the basement zone.

Doubtfull the addition will add more then 2000BTUs if that to the cooling cooling load. plus it will reduce load to the room its attached to, and the room below it on the first floor.

There will be 4 zones:

1st floor
2nd floor
Office
Basement

The floor between the office and basement will be insulated (mostly for sound) so I would think any load reduction between the floor for that part would be minimal. I don't
understand your other question regarding the return.

[beenthere]

There will be 4 zones:

1st floor
2nd floor
Office
Basement

The floor between the office and basement will be insulated (mostly for sound) so I would think any load reduction between the floor for that part would be minimal.

Since both areas are conditioned, there is no heat gain to either, so the load on the existing section is reduced because of the addition.

I don't understand your other question regarding the return.

3 tons needs more return then a 2 ton. So where are they going to get the extra air from. that little office space isn't going have 200 CFM of return. Which is how much more air flow a 3 ton needs over a 2.5 ton.

[Z0neme]

3 tons needs more return then a 2 ton. So where are they going to get the extra air from. that little office space isn't going have 200 CFM of return. Which is how much more air flow a 3 ton needs over a 2.5 ton.

I guess that would depend on the size of the return, right? The current returns are between wall joist which are 16" x 8". So are you saying a return of that size in the office won't be enough? This is purely a guess but there could already be too many returns for all I know. That being said the intention at this point is to just put one return in the office.

[beenthere]

I guess that would depend on the size of the return, right? The current returns are between wall joist which are 16" x 8". So are you saying a return of that size in the office won't be enough? This is purely a guess but there could already be too many returns for all I know. That being said the intention at this point is to just put one return in the office.

A 2X4 wall cavity is barely big enough for 140CFM, under ideal conditions.

Its also doubtfull you office will need 200 CFM of supply. A load calc should be done to know what then house needs.

[udarrell]

I guess that would depend on the size of the return, right? The current returns are between wall joist which are 16" x 8". So are you saying a return of that size in the office won't be enough? This is purely a guess but there could already be too many returns for all I know. That being said the intention at this point is to just put one return in the office.

You're not going to get too much return-air...
Also, I would set it up so there are at least two large Return-Air Filter racks in the conditioned space.

What size is the wall studs; 2X4 or 2X6; that provides the depth of the return either a little less than 3.5 or 5.5" deep, that is the duct capacity area.

If it's 2X4 studs they're normally spaced 16" apart, 2X6 maximum spacing allowed by code is 24" apart. Figure sq.ft area of return ducting accordingly; CFM / sf of duct area = fpm velocity.

If there is one filter placed near the furnace in the RA duct it will NOT be large enough to get the air flow through the filter near the required 300-fpm required by manual D for the cheap fiber glass through-away filters.

A cow is out running in my yard, gotta go & put it back where it belongs!

[llskywalker]

Air conditioning requires proper airflow across the coil in order to not freeze and work properly. If your return is too small and you put a 3ton ac on the furnace it will freeze. Have your contractor perform a load calc to properly size the new furnace and ac. You can look at adding turning vanes to allow the air to flow easier. Your contractor will need to set up the furnace and ac to match your zoning. In other words you might have more than one zone that would need to call for heat or air for the minimum run.