2 HVAC units controlled by 1 thermostat?

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  #1  
Old 06-10-19, 09:48 AM
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2 HVAC units controlled by 1 thermostat?

I have a 2 story house, but the upstairs is smaller than the downstairs. The upstairs is one 750 sq ft room that is a kitchen and a living area. The room has a lot of windows, and the sun sets directly in front of the side with the most windows. I currently have 1 HVAC unit for the whole house, and the thermostat is downstairs, and this causes the upstairs to be hotter than downstairs.
I called an HVAC company to give me a quote to disconnect the upstairs from the current HVAC, and install a mini split ductless system for upstairs. His load calculation came to 36,000 BTU. Seems a little high to me, but he's the expert. Anyway, the company is a trane dealer, and the only 36k unit they offer is a wall mount which I'm not crazy about. On to my question:
Trane offers a 18k BTU ceiling mount that I would like much better. Can I get 2 of those ceiling units to run off one multi zone compressor and one thermostat? I want the 2 units to act as one (on/off at the same time).
Are there any better solutions short of spending $20k on a bigger package unit with electronic dampers??
 
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Old 06-10-19, 11:15 AM
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3 tons for 750 sq ft is nuts, what was used to determine that?

Doubt you'll need more than 1.5 tons being provided by one unit.

You can do your own calculation using hvac-calc for $50 or HVAC Load Calculation - Maunualj - Whole House Loadcalc for free.

Are there any better solutions short of spending $20k on a bigger package unit with electronic dampers??
I wouldn't go for zoning, I would get a thermostat that can use a wireless sensor. The stat averages between its reading and the sensor's.

Between that and dampering down downstairs runs, you can probably get the upstairs cooler. The second floor may need more vents.

If required after, you can have a mini-split added for supplemental cooling.

There's no reason to disconnect and cap off the runs going to the second floor, in fact doing so can reduce airflow through the indoor coil and cause a lot of problems, including but not limited to having the coil freeze up.
 
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Old 06-10-19, 11:26 AM
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Some are able to balance their house by running the fan 24x7. Possible that adding a high return or other ductwork modifications could make things better.

Get a few more quotes but don't dictate to them your solution. Instead tell them your issues and let them come up with a solution.
 
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Old 06-10-19, 12:48 PM
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Calculation

User 10, I don't know how they came up with that number. I'm no expert, but I came up with 24k BTU based on an internet formula. Sq footage calls for 18k + 10pct since it's a kitchen +10 pct for all the windows +500 BTU for every occupant over 2 equals 22.6k BTU rounded up to 24k BTU. I also thought 36k was too much, but that's only based off what I've read on the internet.
They weren't going to just cut off the vents and cap them. They were going to cut off the vents and add more supply runs to downstairs rooms to keep the cfm balanced. They were also going to install a high velocity return (whatever that means).
I didn't tell them the solution I wanted. They came out via service call, I told them my problem, and that was their solution.
I did call another company for a quote, but the guy never sent me a plan or a price after coming to my house. I work a lot, so it's difficult for me to coordinate a walkthrough and all that.
 
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Old 06-10-19, 12:57 PM
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Square footage should never be used to size. You need a proper load calculation.

Adding too much for kitchen is a mistake - the unit is sized for the hottest few hours each year and unless you insist on doing a lot of cooking at the same time as you have extreme heat, it won't be an issue.

It makes no sense to divert all the air to the first floor since the unit was sized for the whole house, you can run into issues dehumidifying when the unit is too large.

The second floor needs cooling, so leave the ducts.

Have a mini-split added to supplement the central a/c as a last resort. Balancing and using the right controls can be a huge difference.

Are your air duct in an attic or basement/crawlspace?
 
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Old 06-10-19, 01:12 PM
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The unit is a 3.5 ton, and the ductwork and air handler are in a crawlspace between first and second floor. There are times when we can't use the oven because it will get too hot. There are 2 ceiling fans in the room that we have on 95% of the time we are in the space. The room has 4 regular sized supply vents in the floor.
 
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Old 06-10-19, 03:13 PM
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if the ducts are accessible may be possible to divert more supply to second floor.

what are the duct sizes feeding the second floor? do you have return air up there, near ceiling. if not, that's part of the problem.
 
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Old 06-10-19, 07:36 PM
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All the supply ducts are 6" flex duct coming off a rectangular main trunk line. There is a return upstairs (14x20) in the floor. There is no way to get a return near the ceiling without building a chase. There is a 20x20 return downstairs, and the two returns are interconnected. There is a 18" flex duct connecting the returns to the air handler.
 
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Old 06-11-19, 12:39 PM
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Well, the room will struggle to cool with a floor return.

How's the flow at the vents? The 6" flex is only as good as a 5" metal and when the vents are large relative to flow, the velocity isn't high enough for the supply air to be blown up to the ceiling which is essential with floor level return.

What are the vent sizes?

In the end, you can probably benefit from some duct alterations, balancing and using a stat with wireless averaging sensor.

You'll probably never get it perfect without adding a mini-split.

Just get (or do) a load calculation on the second floor only, no need to divert the supplies to the main floor. You're probably looking at 1.5 tons max, but it's a guess without the calculation.
 
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Old 06-11-19, 04:19 PM
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The flow at the vents is fine I guess. Doesn't feel underwhelming, but won't blow your hair like a rock star either. Floor vents are standard 4"x10".
How is it possible for me to have a heathy balanced system with a 3.5 ton unit for 1500 sq ft?
 
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Old 06-11-19, 04:41 PM
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How is it possible for me to have a heathy balanced system with a 3.5 ton unit for 1500 sq ft?
That's a big unit for that size of house.

You need a good duct system to get balanced cooling and the loads between floors have to be similar.

For example, if the first floor windows are more shaded than upstairs, there will always be a difference.

But the temp difference can be reduced, following the suggestions I gave.

You would need a lot of vents if they're all 6" flex for 3.5 ton.

Floor vents are standard 4"x10"
3x10 is better for 6" flex, nets higher velocity to push the air up to the ceiling.

I wouldn't put smaller vents - you may benefit from upping the ducts to 7" though.

If you want, post a diagram with full duct layout including all sizes.
 
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Old 06-11-19, 04:56 PM
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My house is approx 2200 sq ft total. Only 750 sq ft is upstairs, with obviously the rest being 1st floor. My house has 12 supply vents, all 4"x10" and all are 6" flex. 4 of the supply vents are upstairs. There are two returns. One is 20"x20" downstairs, approx 18" off the floor. The other is upstairs in the floor and is 14"x20". The returns are run from the same duct, and connect to the ahu via a 18" flex duct.
​​​​​​Part of the offer of the HVAC guy was to add a 2nd supply in the master bedroom and one in a hallway to try to keep proper balance of the unit. I called a second contractor, but he was a dud and never even called me back after doing a walkthrough. I have a third contractor coming Friday. Just trying to be as knowledgeable as I can to make an informed decision.
 
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Old 06-11-19, 07:05 PM
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I have a Nest thermostat. I see they have remote sensors, but they don't have an averaging functionality yet. A quick search hasn't yielded any 3rd party sensors that will work to average temps with nest.
what duct modifications do you suggest? Should I switch to something like an ecobee thermostat?
 
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Old 06-11-19, 09:01 PM
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The ecobee can use sensors, as well as honeywell redlink.

the ecobee i believe can be set to average or cycle based on the sensors that detect the room is occupied.

My house is approx 2200 sq ft total. Only 750 sq ft is upstairs, with obviously the rest being 1st floor. My house has 12 supply vents, all 4"x10" and all are 6" flex. 4 of the supply vents are upstairs. There are two returns. One is 20"x20" downstairs, approx 18" off the floor. The other is upstairs in the floor and is 14"x20". The returns are run from the same duct, and connect to the ahu via a 18" flex duct.
​​​​​​Part of the offer of the HVAC guy was to add a 2nd supply in the master bedroom and one in a hallway to try to keep proper balance of the unit. I called a second contractor, but he was a dud and never even called me back after doing a walkthrough. I have a third contractor coming Friday. Just trying to be as knowledgeable as I can to make an informed decision.
12 6" pipes is not enough for a 3.5 ton.

Each one is like 60 to 70 cfm max and the 3.5 ton needs 1400 cfm at 400 cfm per ton or 1225 cfm at 350 cfm per ton.

12 * 70 = 780 cfm.

Now, you're likely getting more than 780 cfm but probably falling short.

A static pressure test can be done to see how much air the air handler is moving based on the blower performance data. Useful for duct diagnostics too. There are other methods, one involves very expensive tools and the other you need electric heat in the air handler for.

In all likelyhood, you can benefit by increasing supply to the second floor if you can do so without major demolition. You said the ducts are in a crawl space between the floors - is it accessible?



The trunk lines need to be sized to handle the extra airflow, otherwise adding vents will starve other rooms. Extra vents to second floor will need good manual dampers, to damper them down in the winter for balancing purposes.

Need a room by room load calculation to see how the cooling load is distributed vs how ductwork was done.

Also need to look at trunk line sizes.

hvac-calc is good for the room by room calc.

I'm not familiar with sizing return air grills - usually its done to get a certain velocity.
 
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