[GravityAlwaysWins]

I've noticed some seriously knowledgeable HVAC folks here, so I'm looking for advice.

We have renters in the walkout basement in our 4yo home. The basement is on the same HVAC unit, different zone (via electronic baffling) as our main floor. They like it cold. So it's 40 degrees outside and they're still running A/C! It's a ground floor space so I can understand why they wouldn't want to sleep with the windows open (I don't think they use the windows regardless, tho ; convo hasn't happened yet).

So what happens is the A/C runs to cool their space down, turns off, then the furnace turns on immediately to heat the space upstairs.

First question, is this bad for the system? I've always been under the impression it's really bad for the A/C compressor to be trying to cool 70 degree air indoors when it's 20-30 degrees colder than that outside. It also feels counterintuitive that, even with the baffling, the system would just be fighting itself. But maybe the baffling makes it OK?

Second, anyone have solutions that aren't obvious to this problem, of keeping 2 zones on the same units very different temperatures? The obvious one is them opening windows when they're home, and if it comes to me asking them to do that b/c they're damaging the A/C, it has to be done. But I want to be sensitive about it -- they're awesome renters otherwise.

Or are today's systems advanced enough that I have nothing to worry about?!

Thank you!

[jhsmith]

The more modern systems monitor pressure/Temp in the compressor and automatically cutoff the compressor before any damage would occur. The carrier system installed in my house in 2011 has that feature. Not sure about other brands and older systems.

[Vicegrip]

Can't give a good answer without far more info in the system. OTOH install security grates on a window or 2 and add in a fan for the $ win. Outside air is far better then recirculated too. In home air in modern tight homes often exceeds occupational regulations for certain contaminants.

[Don Wohlfarth]

Good luck with this problem. Most zoning systems have a master thermostat that controls whether the equipment will operate the heating or cooling. The zoned area will have a slave sensor that will raise or lower the temperature in that zone but will not change the system from heat to cool.
Suggest having some one that knows zoning systems check it out. The damper to this zone may be broken or only closing 50% instead of 100%. Try setting master thermostat to 80, go to area with slave thermostat and set to 50. Check the air flow from the registers in zone area. If there is not a noticeable difference in air flow please stand by with your checkbook.

[Dr K]

Lots of people know a little about HVAC, esp. engineering types like on this board. Some people are experts, function as contractors or even work at this full-time. There there's the person who not only works full time, but has figured out ways to improve the commercial systems with which he deals, and I'm not talking about just changing the ducts; I'm talking about reworking and building new boards to run the systems more efficiently than the manufacturer designed, make them more reliable and reduce maintenance. If he suggests security grates and a fan, that is likely better for multiple reasons and not just money.

PS: I don't want to tie up Kurt's time, but I have to say the day I visited him at work and he gave me the grand tour was truly eye-opening. I knew he was smart, creative, and hard working, but I thought that was just with cars...

[BlackTalon]

The nose browning is strong with this one

[GravityAlwaysWins]

Thanks for the feedback, Guys. This is why other opinions and expertise are useful. I hadn't thought about the possibility of just putting bars on a couple of windows so it's safer for them to open at night. And the added bonus is fresher air indoors. Along the way, I will investigate if the compressor has a safety auto-shutoff.

Now, on to more interesting thoughts. Bolts for my calipers and wheels, a third front rad, power window regulators, and the kitchen rewiring I'm doing...

[AznDrgn]

Definitely need more information on layouts and equipment specs but in general unless the outdoor unit is equipped with a low ambient controller which will allow for the condensing unit fan to vary speed / turn off you don't want to be running cooling mode when the outside air temperature is much less than 50F. The best way to solve this issue is to split the systems. The only other thing I can think of is do it the way commercial buildings do, cool all the air to ~55F, distribute that cold air to the entire building, and then reheat it for use in areas that need heating instead of cooling. Neither option is cheap or fun to deal with.