Wow, you guys are in the mid-high 80's and we are at freezing or below back east.

Typical high temps in summer are mid-high 80s but often low-mid 90s, fairly low humidity; winter temps can get down to 30s-40s overnight but tend to stay in 50-60s during the day (but only for 2-3 months tops; even less this year).

Replacing a 3.5 ton furnace/AC so not sure if we need to go up to 4 tons. A contractor did a Manual J that he said called for 4.25 tons; I have the paperwork if you tell me what numbers to check and share. We asked about whether the ductwork can handle increased airflow. According to contractor, the extra .5 ton will only increase by 200 CFM and should be fine. (We have a load calc and duct system summary that I'm not really sure how to interpret.)

In winter, heating has been fine; we keep thermostat close to 68 and it's a bit colder downstairs but the living room has cathedral ceiling which partly explains (and we have a gas fireplace that has glass doors but is a little drafty). We tend to keep the thermostat at 78 degrees in summer and the downstairs stays fairly comfortable but, again, living room cathedral ceiling comes into play.

Upstairs is usually about 2 degrees different than downstairs, but the biggest temperature range (2-3 degrees) is actually between the upstairs bedrooms that have southern exposure. Smallest one is hottest late afternoon/evening and coldest overnight, but it gets the most direct sunlight than the other and lacks the thermal curtains the other bedroom has. The contractor has also suggested that the master needs at least 220 CFM (currently calculated at 157 CFM). We think that the upstairs ductwork leading to these two bedrooms needs to be replaced because the temps at the vents are 10-15 degrees warmer than at the trunk, but there is no attic or crawl space so we can't examine very well until we pull out the old equipment.. We've tentatively rejected zoning the upstairs/downstairs because we think that issues are better addressed by ductwork/insulation.

Maybe I am rusty at this but I would think that 2 degrees difference between your upstairs and downstairs is not a problem that you are likely to solve in a single zone system. Physics being what it is and all.

No, the upstairs/downstairs temperature difference is likely related to hot air rising, which a single zone system can't solve. But if the upstairs rooms are also warmer because duct issues mean they are getting less air delivered to them than they need and that air is warmer than it should be, a two zone system can't rectify that. And if there is a difference among the three upstairs bedrooms, being on their own zone won't fix that either and zoning all the upstairs rooms separately is cost prohibitive. Fixing the ductwork and even possibly adding a vent is more affordable than zoning. and definitely necessary. If it reduces the temperature difference even incrementally that would help. But we haven't completely ruled out a two zone system.

The load calculation for 4.25 tons seems high. What are the indoor and outdoor design temperatures used in the calculation? What are the sensible, latent, and total cooling BTU numbers?

The 2-stage condensers are only available in whole tons, so this may force you to go to a 4 ton condenser. I would think a single stage 3.5 condenser would be sufficient. The 110K BTU furnace is very oversized. I assume the Carrier contractor did not do a load calculation.

A temperature drop of 10-15 degrees on the bedroom supply is significant. Does the duct work travel through unconditioned space?

How much insulation is in the attic? You want at least R30. Increasing the attic insulation is going to help the temperature differential.

Let me clarify a couple of things. You state that you have a 2 - 3 degree difference between your upper floor and lower floor. I don't care if the temperature difference at the register between the upstairs and downstairs is 40 degrees. If your house can get to the desired temperature and your upstairs and downstairs are 3 degrees apart you don't have a problem. Actually, you may have several problems that are currently canceling each other out, and putting better ducts will just make the system worse.

Next, you insinuate room differences are caused by sunlight in the windows . By getting bigger ducts, the room will get cooler in summer because you increase the airflow relative to the airflow near the thermostat, but it will also get hotter in the winter. If your house is insulated well then the upstairs rooms should already be warmer in the winter so they are going to be saunas.

Finally, you can fix a temperature difference at the register by zoning the system, but why even bother. If the air coming out of the register is closer to the desired temperature than the room temperature it will cool or heat to the desired temperature at the thermostat.

You have lots of symptoms but you are missing a problem, if your temperature readings are correct that is.

**What are the indoor and outdoor design temperatures used in the calculation?**

Winter: outside 42 F/inside 70 F

Summer: outside 97 F/inside 75 F

**What are the sensible, latent, and total cooling BTU numbers?**

I'm trying to attach a picture on the calculation that was done, but this is what I see

Sensible Cooling Equipment Load Sizing: structure 48078 BTU, ducts 11989 BTU, equipment sensible load 61268 BTU

Latent Cooling Equipment Load Sizing: Structure 494 BTU, ducts 314 BTU, equipment latent load 808BTU, equipment total load 62076 BTU

Cooling Equipment Summary: sensible cooling 0 BTU, latent cooling, 0 BTU, total cooling, 0 BTU

(The calculations actually WERE done by the Carrier contractor, but I'm feeling a little concerned now about them. I noticed the footnotes say the front door faces N, when it is NE. Not sure if that's a negligible difference or indicative of sloppy work.)

**A temperature drop of 10-15 degrees on the bedroom supply is significant. Does the duct work travel through unconditioned space?**

Yes

**How much insulation is in the attic? You want at least R30. Increasing the attic insulation is going to help the temperature differential.**

There is no attic space. The roof is basically right above the upstairs ceiling with maybe a foot of space between the two. There is no crawl space or access to this area unless you open up the ceiling drywall. This is where the upstairs ductwork is located. It is probably the original 1976 ductwork, 5" diameter. A camera inserted in one of the grills for a very rudimentary visual inspection showed that this main upstairs line is not in the best shape. When we settle on a contractor we were planning to try to get a more thorough inspection and expect that this ductwork should probably be replaced. I'm not really sure what our insulation options are, but the Carrier contractor (who is an approved Energy Upgrade CA contractor) seemed to be hinting that our options are expensive and limited.

The load calculations the Carrier dealer don't match his proposed equipment sizes. He calculated a total cooling load of 5 tons, and total load of 7.3 tons for a SHR ratio of 0.70. Yet he proposes a condenser of 4 tons. He must believe the calculation is wrong and is using the 500 sq. foot per ton rule of thumb. He is also over sizing the furnace. I think you could find a smaller furnace that can accommodate a 3.5 or 4 ton condenser, but the options could be limited.

It is going to be difficult to have consistent first and second floor temperatures if the duct work in the upstairs ceiling is not insulated. It sounds like the only fix is to open up the ceiling and install insulated ducts.

I appreciate the help interpreting the load calc and I agree that the ductwork needs to be addressed, which is something that is currently under discussion. I could still use some further guidance in reference to my original questions about 1 vs. 2 stage cooling and furnace efficiency.

At this point, it seems as though a 3.5 ton 1 stage system has been suggested as an option and I've requested a quote from the American Standard contractor for that equipment. But I'm not clear about the pros/cons of the 1 stage vs. 2 stage or whether the degree of furnace efficiency matters.

The main reason for getting a 2-stage AC is to increase comfort and humidity control. There is a small increase in efficiency but in most cases not enough to spend the additional money. If the unit can operate in the low stage for longer periods of time, then there is less short cycling which allow the system to run more efficiently. In addition a 2-stage will help lower the humidity which may allow you to raise the thermostat temperature a degree or two. This would help reduce operating costs.

The negative is the extra cost. You need a furnace which is variable speed. The 2-stage equipment only comes in whole tons. You will have to buy a 4 ton 2-stage unit instead of a 3.5 ton single stage model.

mike_home, thanks. We got an estimate for the 3.5 ton 1-stage unit for American Standard.

$8190: 3.5 Tons condenser 4A7A6042H1000, 208/230 volt. with Puron R-410A refrigerant; 80% 3.5 Tons furnace AUD2C080A9VA; 3.5 Tons evaporator coil 4XADUC62/C

(The estimate for the 4 ton 2-stage system was $11,900: Gold 17 2 stage condenser 4A7A7048A1, Gold 80 2 stage furnace *UD2C080A9V4)

At this point, I think we're leaning toward going with the 1-stage 3.5 ton option. We've been closely monitoring temperatures and comfort levels for the past three weeks and our current 3.5 ton unit seems to be handling our needs, so upgrading to a 4 ton seems unnecessary.

The $3700 price difference is hard to justify overall, and the 3.5 ton unit actually has lower estimated operating costs when I checked the AHRI directory: $314 for the 3.5 ton 1-stage, $351 for the 4 ton 2-stage. (I understand that these are only national averages, but I'm assuming that the AHRI numbers considered reliable starting points for comparing efficiency.)

Make sure you get a 2 stage thermostat for the furnace...if not you will always be in high stage whether needed or not. Plus a pleated filter media cabinet. No Nest!

IMO