I'm currently using the Thermostat Scheduler to set temps throughout the day. However, I don't like when a temperature change occurs and then the system runs continuously for a long period of time trying to make the temp change. We've been told the builder undersized the system for our new house so just trying to figure out the best way to optimize it. So now that I have Hubitat, maybe there is a way I can control it more than just by temp? I'm a novice at Rule Machine so I'd appreciate any guidance or input.
So my idea is for certain times of the day, I want the AC to run for set time periods and then turn off. Perhaps throughout the day I run for 15-20 minutes every hour or two as an experiment, to start with - means my inside temp varies depending on outside temp, how often the kids leave the door open, etc. I would probably want a contingency if the temp gets above a certain point to add an extra 15-minute run, but only after the system has been idle for some amount of time. I may never achieve a certain set temp, hoping just giving little injections of cool will help throughout the day. Then as the day cools down, maybe I can lengthen the cool times or shorten the idle time to try to bring it down further before my husband gets home...
Is this possible to do somehow? Or is there a better way someone else has thought of?
How new is this house? If it is new, you have a legitimate gripe with the builder who should fix it. Get with a heating and air person to evaluate your system and if that evaluator agrees that the system was undersized, it is further evidence to give to the builder. If the builder does not want to fix it, then you may need some legal advice which I am not qualified to give.
2015 build. We were told by an hvac guy it is within spec for the square footage but only just barely so it isn't technically/legally undersized, but for practical purposes it has to work really hard/long. Hence trying to help it be more optimized....
You can't go just by square footage. That calculation is done using ideal conditions for your duct work. And I guarantee your ducts and return ducts are not perfect. You never get the performance out of your HVAC system that you are supposed to because as soon as the thing is installed it's no longer optimized. I would definitely look at getting a bigger unit. If you try to control it by runtime, you're not going to get consistent temps. If the temp is just above turning on the AC, it's going to get too cold. And if it is WAY above the setpoint, it will never fully cool down and just continue to get hotter and hotter and hotter.
For example, you have it set to 70 and the temp outside is 95. At 72 it kicks on and runs for 10 mins till it cools it off to 71, then it shuts off for its off duty cycle, since it can't turn back on again for so long. This time it turns on at 73 and after 5 mins shuts off without cooling the house at all. You can see where I'm going with this. There's a reason that thermostats are used to control HVAC equipment instead of timers. Timers don't really work.
It isn't bad for the A/C to run continuously. It is worse to stop and start all the time. On the hottest day of the year (the ASHRAE design temperature for your area) it should theoretically run 100% of the time.
If you run for too short, you will never dehumidify which will make things feel sticky in the house. The system efficiency drops on short runs too. It takes a while to get the evaporator down to temperature and the system pressures stabilized.
I think this is a bad idea, but admittedly I am not an HVAC professional, but I do have lots of training in another field working with refrigeration.
I think you should look into how big of a setback you are using as well as things like thermostat cycles per hour, thermostat swing, and other thermostat settings.
Thanks for the input everyone! Clearly I need to do some more research before I mess with trying to write a bunch of rules that may not work like I hope anyway!
@neonturbo's advice is on the money. Given that your house is a new construction, it is extremely likely that your contractor's HVAC installer did Manual J and Manual D calculations to ensure that the system and duct-work were sized correctly for the design temperature in your geographical area, the specific construction and insulation in your house, the number/size of windows, the direction they face, the insulation used etc.
Oversized systems are horrible - they leave your house with hot and cold pockets and do not dehumidify. I knew nothing of ACs when my house was redone after Katrina; my neighbors and I went along with a fly-by-night AC installer (the only one we could find at the time) who replaced our single-stage 3-ton units with 4-ton units. This was a horrible decision - the house was clammy and very uncomfortable. But I had to live with it until I could afford to replace the AC.
When I replaced the AC, I had a contractor do Manual J and Manual D calculations. He recommended a 3-ton variable speed unit, or a 3-ton 2-stage unit. I went with the latter. It mostly runs in a the first stage (i.e. like a 2-ton unit). There are long runs - providing effective dehumidification (as @neonturbo indicated).
On one of the many occasions when I rued the decision to go with a 4-ton unit after Katrina, I came across a really good article on AC sizing, run times, and dehumidification written by an HVAC engineer. I have linked to it below:
I prefer 2-stage over variable speed. While variable speed are more efficient, they are more expensive and usually require proprietary thermostats (which I don't like).
With a 2 stage you can use pretty much any thermostat you want, which is a big plus to me.
EDIT: Also, I read the article you linked to. Interesting!
