Custom split A/C system

z3bram3n · 07-24-13, 12:41 PM

I have been working on an mildly unconventional system for a little while. I hope to have it operational before summer is over. I am inspired by your project and have used some of the info here.
I started with looking at mini splits and noticed they would spec more evaporative capacity than condensing capacity. I guess they use the piping arrangement and solenoid valves to manage flow.
So I thought I would try using two separate zones on a conventional split system with a an ultra scroll compressor.
I live in a townhouse in DC that is about 38' square maybe 1500 square feet per floor. I have a two ton evaporator for each floor with ducting to 4 BR/2baths on floor two and four rooms on floor one -- very simple layout.
I have a 3.5 ton compressor that can shift to about 2 tons on command.
So generally, I expect to use zone heating concept and cool each floor at differing times. At night the BRs are cooled and during the day the first floor is cooled.
I am using two wifi based thermostats that can signal the condenser to shift capacity. The air handlers have variable speed motors so I can run them at low loads (the evaps have txv's to allow proper subcooling at lower air flow).
So my challenge is to get your type of numbers on my split system.
Oh, the key to making this work is to use liquid line solenoid valves triggered by the thermostat demand. I think the self regulating txv will help with the control but I think I will have to put some logic in between the thermostat and the condenser to keep both units from running on high simultaneously.

Daox · 07-24-13, 12:51 PM

Welcome to the site z3bram3n.

I hope you don't mind, but I moved your post to its own thread. It'll get more attention here. Good luck with the system build!

jeff5may · 07-31-13, 06:47 AM

Quote:

Originally Posted by z3bram3n

I started with looking at mini splits and noticed they would spec more evaporative capacity than condensing capacity. I guess they use the piping arrangement and solenoid valves to manage flow.

The reason the multi-mini-split systems do this is due to the variable speed fans inside the indoor units. Most of them don't run at full speed for very long unless commanded to. This is where the OEM's spend big bucks on control logic research. With the outdoor fan at full throttle and the indoor fans at somewhere less than full throttle, the evaporators are saturated at a lower capacity.

Quote:

Originally Posted by z3bram3n

I am using two wifi based thermostats that can signal the condenser to shift capacity. The air handlers have variable speed motors so I can run them at low loads (the evaps have txv's to allow proper subcooling at lower air flow).
So my challenge is to get your type of numbers on my split system.
Oh, the key to making this work is to use liquid line solenoid valves triggered by the thermostat demand. I think the self regulating txv will help with the control but I think I will have to put some logic in between the thermostat and the condenser to keep both units from running on high simultaneously.

You shouldn't need to add zone control valves. With variable-speed fans on each zone and txv metering, the only thing you should have to worry about is the compressor speed. If you wanted to get fancy, you could make control logic that varied indoor fan speeds to maintain at least a certain delta T at each evaporator coil (at full demand from both evaps, fans could slow to better dehumidify the hot air). Obviously, temperature sensors at each coil would be necessary.

When both units are running full blast, the txv's will maintain pressure drop in the system based on compressor output. Efficiency will soar, since the evaporators are extracting so much heat. As indoor temps drop, the txv's will follow the change, allowing the evaps to provide ever colder indoor air.

At a certain point, either one of the indoor fans will slow or a thermostat will be satisfied, or maybe both. Maybe both of the indoor units can be running at low speed, say to dehumidify the air (control logic needed to prevent evap coils from freezing). At this point, the compressor can be run at low speed to save energy. Again, the txv's will match compressor output to indoor air temp at each evaporator. They are designed to work like magic.

You would think that if one evap fan is running and the other one isn't, there would be problems. But with the txv's controlling flow, the refrigerant only goes to where it's needed. The evap that is satisfied gets its flow choked automatically. The stopped evap coil will not freeze up unless the running evap chokes the low side pressure below a certain value (varies with refrigerant being used - check PT chart). Efficiency will again soar, since the condenser is oversized compared to the compressor running at half speed.

No matter which way you decide to control this setup, it should work very well. I look forward to your progress and success in doing this yourself.

07-24-13, 12:41 PM	#1
z3bram3n Lurking Renovator Join Date: Jul 2013 Location: washington, dc Posts: 1 Thanks: 0 Thanked 0 Times in 0 Posts	Custom split A/C system I have been working on an mildly unconventional system for a little while. I hope to have it operational before summer is over. I am inspired by your project and have used some of the info here. I started with looking at mini splits and noticed they would spec more evaporative capacity than condensing capacity. I guess they use the piping arrangement and solenoid valves to manage flow. So I thought I would try using two separate zones on a conventional split system with a an ultra scroll compressor. I live in a townhouse in DC that is about 38' square maybe 1500 square feet per floor. I have a two ton evaporator for each floor with ducting to 4 BR/2baths on floor two and four rooms on floor one -- very simple layout. I have a 3.5 ton compressor that can shift to about 2 tons on command. So generally, I expect to use zone heating concept and cool each floor at differing times. At night the BRs are cooled and during the day the first floor is cooled. I am using two wifi based thermostats that can signal the condenser to shift capacity. The air handlers have variable speed motors so I can run them at low loads (the evaps have txv's to allow proper subcooling at lower air flow). So my challenge is to get your type of numbers on my split system. Oh, the key to making this work is to use liquid line solenoid valves triggered by the thermostat demand. I think the self regulating txv will help with the control but I think I will have to put some logic in between the thermostat and the condenser to keep both units from running on high simultaneously.

07-24-13, 12:51 PM	#2
Daox Administrator Join Date: Aug 2008 Location: Germantown, WI Posts: 5,525 Thanks: 1,162 Thanked 374 Times in 305 Posts	Welcome to the site z3bram3n. I hope you don't mind, but I moved your post to its own thread. It'll get more attention here. Good luck with the system build! __________________ Current project - To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. & To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.