2 stage split system

ChrisJ

So now that we're on our second season with the 2 stage system I thought I'd share how it handles humidity.

This is 3 tons in a 1600sqft house.

Right now it's 76F @ 69% RH outside. This thermometer is in my bedroom on the second floor which runs around 2 degrees cooler than the first floor. I intentionally sized the duct work to keep the second floor cooler as that's where all of the bedrooms are.


I think it does very well. Though, it does start to struggle when the dew point gets above 75F outside. That's basically where it's in high stage non stop.


Honestly I'm sold on Copeland's 2 stage setup. It's not as good as an inverter but it's so much better than single stage and so much cheaper than an inverter.

SuperJ

What's your indoor temp/humidity control like? Do you have a ECM air handler?

ChrisJ

SuperJ said:

What's your indoor temp/humidity control like? Do you have a ECM air handler?

Indoor control is a Honeywell Prestige THX9421 with 3 additional indoor sensors in our 3 bedrooms on the 2nd floor. The thermostat it self does the first floor.

Air handler is an Goodman / Amana ECM variable speed. I'm doing 400 CFM per ton normally, and around 325 in dehu mode which I have set at 50%. I also have it set to only allow dehu mode in low stage.

ChrisJ

Ran a test last night just to see how things were.

mikeg2015

What air handler size does it have? One challenge with low stage is the indoor coil is effectively almost double the size required. Good for efficiency, but poor latent to sensible ratio and it takes a long time to wet the coil.

The debate, is essentially, does a 3 ton 2 stage perform better than a 2.5 ton single stage that might be just slightly marginal in capacity. OR further, could you install a 2 ton that runs 30% longer and more effecintly because of oversized ductwork.

Low stage with 325 CFM/ton is probably about 2 tons sensible capacity.

With new construction, 2 tons was plenty in SE Michigan in 95F July days.

I wired mine in hat house to slow the fan to dehumidify but it could use either stage to match sensible demand.

ChrisJ

mikeg2015 said:

What air handler size does it have? One challenge with low stage is the indoor coil is effectively almost double the size required. Good for efficiency, but poor latent to sensible ratio and it takes a long time to wet the coil.

The debate, is essentially, does a 3 ton 2 stage perform better than a 2.5 ton single stage that might be just slightly marginal in capacity. OR further, could you install a 2 ton that runs 30% longer and more effecintly because of oversized ductwork.

Low stage with 325 CFM/ton is probably about 2 tons sensible capacity.

With new construction, 2 tons was plenty in SE Michigan in 95F July days.

I wired mine in hat house to slow the fan to dehumidify but it could use either stage to match sensible demand.

It's a 3 ton 2 stage system and the air handler is their *37 model, which is the high efficiency 3 ton.

I seem to recall Copeland saying their 3 ton Ultratech dropped down to 2 tons +- in low stage.

mikeg2015

Copeland want you to think it’s drops 2/3rds capacity. They do not. They drop about 2/3rds of the compression ratio. But you consume about 70% of the energy and get 75-80% the capacity depending on airflow. so a small bump in economy. But they you have ot run low air flows and overcool your home to remove moisture since coil is effectively oversized, so you give up most of the savings unless your ductwork is undersized.

Actual output is around 2.2-2.4 tons depending on conditions. Probably higher since 1st stage is used most in mild weather where capacity exceeds nominal anyway. Rated capacity is at 95F outdoor temp and 80F indoor with 50% humidity which is hotter and more humid than most anyone keeps their home.

But they do work. I have two Copeland 2 stage scrolls on my own home. But hindsight I would have gone single stage units and just saved the money.

ChrisJ

> @mikeg2015 said:
> Copeland want you to think it’s drops 2/3rds capacity. They do not. They drop about 2/3rds of the compression ratio. But you consume about 70% of the energy and get 75-80% the capacity depending on airflow. so a small bump in economy. But they you have ot run low air flows and overcool your home to remove moisture since coil is effectively oversized, so you give up most of the savings unless your ductwork is undersized.
>
> Actual output is around 2.2-2.4 tons depending on conditions. Probably higher since 1st stage is used most in mild weather where capacity exceeds nominal anyway. Rated capacity is at 95F outdoor temp and 80F indoor with 50% humidity which is hotter and more humid than most anyone keeps their home.
>
> But they do work. I have two Copeland 2 stage scrolls on my own home. But hindsight I would have gone single stage units and just saved the money.

Id never go back to single just because of the fact it's almost always quieter. Also in hotter weather it goes in between low and high rather than off and high.


I also wonder if my added outdoor fan controller buys me some efficiency because it allows the condenser fan to run full speed with the compressor in low stage. This does happen when it's in the low 90s out. There original 2 speed fan didn't do that.


However, speaking of things that piss me off....
I knew I didn't trust SEER ratings.

I wish I could have a on delay but no off delay.

mikeg2015

Is the outdoor fan an ecm or psc motor? The fan at full speed probably reduces efficiency below 90f. It also increases capacity slightly.

What delay? Bower Fan delay?

ChrisJ

> @mikeg2015 said:
> Is the outdoor fan an ecm or psc motor? The fan at full speed probably reduces efficiency below 90f. It also increases capacity slightly.
>
> What delay? Bower Fan delay?

The indoor fans built in 1 minute off delay is what I don't like.

I have an ICM fan controller outside controlling a single speed ball bearing fan motor. In controls the speed by condenser temp so the speed drops below 90. It barely runs at 50 and is usually off for a bit after the unit starts.

I have it setup for low ambient operation.

mikeg2015

If it’s a spit capacitor motor, it will
Consume nearly As much electricity at low speed as high speed. Those controllers are mainly for low ambient operation to raise condenser pressure in cold weather.

ChrisJ

> @mikeg2015 said:
> If it’s a spit capacitor motor, it will
> Consume nearly As much electricity at low speed as high speed. Those controllers are mainly for low ambient operation to raise condenser pressure in cold weather.

Understood.
I run it a lot when it's in the 50s out and I didn't like it starving the txv. I've also been known to fire it up when it's in the high 40s out.

I'm just thinking as and added bonus perhaps I gain a little when it's in the low 90s and in low stage.

I also run a CCH and evap freeze stat just in case.

mikeg2015

With 325/ton shouldn’t need a freeze stat
If RH is over 50%. But not bad insurance.

ChrisJ

> @mikeg2015 said:
> With 325/ton shouldn’t need a freeze stat
> If RH is over 50%. But not bad insurance.

Due to a continuing condensate issue....I lowered my blower speed today. Now running 380 per ton normally and 313 in dehu mode. I'm hoping dehu mode doesn't cause any issues. It should only do it when RH is above 50%

ratio

ChrisJ, have you looked into reprogramming the blower motor? It may be possible to remove the off delays from the OEM motor. Aftermarket ECM motors are definitely programmable, see e.g. https://www.regalbeloit.com/About-Regal/Sustainability/Motor-Programming-Made-Easy. Might take some fiddling to get the speeds best, but that the fun part!

ChrisJ

> @ratio said:
> ChrisJ, have you looked into reprogramming the blower motor? It may be possible to remove the off delays from the OEM motor. Aftermarket ECM motors are definitely programmable, see e.g. https://www.regalbeloit.com/About-Regal/Sustainability/Motor-Programming-Made-Easy. Might take some fiddling to get the speeds best, but that the fun part!

Will that work with the OEM ECM motor in my AHU?

Wait why would you show me this! No good can come of that.

ratio

mikeg2015

Fan delays might be saved in hte board, not hte motor. Motor would just have ramp rates and the fan curves (RPM vs torque for that given blower.)

Remember “variable speed” is really a constant airflow motor. There are also constant torque ECM motors. PSC motors are constant RPM.