Single vs. Two Stage Heat Pump Condenser

[hblock72]

Any opinions on single vs two stage heat pump condensers? We're about to undertake a big backyard renovation which will require me to move my 2 outdoor heat pump condenser units. Since they're 25 years old, I figured this would be a good time to replace them as well. I live in central NC, which is a mild climate and the house is two stories, ~2400 sf. The current units are Trane's with a 2 ton upstairs and a 1.5 ton downstairs. I'm planning to go with variable speed air handlers, but I'm torn on which condenser unit. The price difference is $1000 ($500/unit). The installer who seems knowledgeable is recommending the single stage, saying the return on investment would be long to recover the upcharge with energy savings. That makes sense, but I'm wondering if there is enough of a "felt" difference in comfort and humidity control to justify going with the two stage system. Not really considering variable speed do to the cost increase. Anyone have any experience with this that could provide some input? Thanks in advance.

 

[ericm]

Just A/C but I wish I'd gone with the two stage compressor. With a single stage it cycles on and off. I'd rather it run more consistently. It's more about comfort and keeping a consistent temperature than energy savings.

 

[chinboys]

There is also a third option... variable speed compressors.

You might get away with one larger unit that connects to both of your evaporators and talks to each zone separately or together.
The compressor will speed up based on the demand or slow down when less demand is sought.

Many of these variable speed compressors are located in mini-split systems dedicated to cooling or dual function as a heat pump.
Look for your utility for rebates based on SEER or COP figures.

 

[pcmeiners]

"The installer who seems knowledgeable is recommending the single stage, saying the return on investment would be long to recover the upcharge with energy savings. "

I figure the installer is proposing a single stage as it is old tech by which he can get a higher profit margin.

"It's more about comfort and keeping a consistent temperature than energy savings."

it more about consistent temperature and energy savings, than a single stage unit offers. Wait till the OP gets his energy bill in a few years from now.

 

[theoldwizard1]

There is also a third option... variable speed compressors.

A. K. A. inverter ! Worth the additional cost !

 

[u3b3rg33k]

A. K. A. inverter ! Worth the additional cost !

definitely 2 stage, single stage is LOL at this point. but also, just go inverter. almost all the higher efficiency stuff is inverter now, even with standard 24V controls. you don't need communicating equipment to get the benefits!

inverter also almost always means you get an intelligent defrost and solid diagnostic & data output WITHOUT needing to put gauges on the system. the Bosch BOVA/BOVB systems will give you pressure data via the built in LED display.

 

[azmodela]

I've been considering the same thing. I'm probably staying with a single stage, maybe I'll go two stage on the compressor. With the tech being put at these things these days, a lot of parts are proprietary, with long lead times on some parts. Then once you get the parts, the part cost is high for the circuit boards, motors and similar. It's a matter of when, not if things break down these days.

 

[danski0224]

Any opinions on single vs two stage heat pump condensers? We're about to undertake a big backyard renovation which will require me to move my 2 outdoor heat pump condenser units. Since they're 25 years old, I figured this would be a good time to replace them as well. I live in central NC, which is a mild climate and the house is two stories, ~2400 sf. The current units are Trane's with a 2 ton upstairs and a 1.5 ton downstairs. I'm planning to go with variable speed air handlers, but I'm torn on which condenser unit. The price difference is $1000 ($500/unit). The installer who seems knowledgeable is recommending the single stage, saying the return on investment would be long to recover the upcharge with energy savings. That makes sense, but I'm wondering if there is enough of a "felt" difference in comfort and humidity control to justify going with the two stage system. Not really considering variable speed do to the cost increase. Anyone have any experience with this that could provide some input? Thanks in advance.

Generally speaking, two stage or multi-stage/inverter is not selected to "save money", but rather to improve comfort.

You will get a longer run time in cooling mode to help reduce humidity in stage 1. Yes, amp draw is less, but it is on for longer. The VS air handler should have a dehumidify mode.

I have a standard 2 stage heat pump, and it has greatly increased comfort in the cooling mode. Heating is only good down to 35*F, and it will run in stage 1 much of the time. It can heat to a lower temperature, but the economic balance point is even higher than 35* at this time.

You really need to have a load calculation performed and look at the thermal balance point for the equipment you are looking at. There is usually a difference in heating capacity between 1 and 2 stage, non inverter units, with the single stage units having a bit more. There is also a wide range of heating capacity based on indoor evaporator coil selection. One really has to go into the manufacturer specs to choose the right stuff.

There is much more flexibility in running an inverter outdoor unit, and you will be able to get more heating capacity without any detriment to cooling capacity or functionality. This will be much less expensive than running on resistance backup heat.

The inverter units, at least the Bosch one, would be able to run off of a generator compared to a regular non-inverter unit.

Yes, the inverter control board is expensive. No way around that. Definitely need some surge protection and monitoring.

Also be informed about any rebates and other incentives.

 

[pcmeiners]

Second the need for surge protection.....

not only from surges but line power abnormalities, lightning strikes and EMP pulses from lightning, near and far. Years back I was setting up long distance modems on ships ($600 each). Every couple of weeks we would lose one . Was on one of the ships when one was destroyed during a thunder storm 25 miles away from an EM pulse. A simple gas protector in line of the coax cable stopped the damage to the modems. Another instance occurred in an alarm system in a building when an electric freight train passed a school building at 3 am, a few MOV solved that issue. I installed MOVs and transorbs in each of my mini split disconnect boxes and have an industrial sized unit on my breaker box.

 

[P0234]

definitely 2 stage, single stage is LOL at this point. but also, just go inverter. almost all the higher efficiency stuff is inverter now, even with standard 24V controls. you don't need communicating equipment to get the benefits!

inverter also almost always means you get an intelligent defrost and solid diagnostic & data output WITHOUT needing to put gauges on the system. the Bosch BOVA/BOVB systems will give you pressure data via the built in LED display.

As does Rheem. We're on a 7yo Rheem 20 Seer Inverter, our hot water heater uses more electricity than the 5 ton unit does to heat a house.

 

[fitter30]

Any brand of equipment get over 17 seer it's getting into more training and sharper service people. Get into variable speed compressors/ systems the contractor has to make a commitment to training, laptops, software and the service people. Larger contractor will have a better relationship with the distributor than a smaller one.

 

[Hobby_Man22]

Generally speaking, two stage or multi-stage/inverter is not selected to "save money", but rather to improve comfort.

You will get a longer run time in cooling mode to help reduce humidity in stage 1. Yes, amp draw is less, but it is on for longer. The VS air handler should have a dehumidify mode.

I have a standard 2 stage heat pump, and it has greatly increased comfort in the cooling mode. Heating is only good down to 35*F, and it will run in stage 1 much of the time. It can heat to a lower temperature, but the economic balance point is even higher than 35* at this time.

You really need to have a load calculation performed and look at the thermal balance point for the equipment you are looking at. There is usually a difference in heating capacity between 1 and 2 stage, non inverter units, with the single stage units having a bit more. There is also a wide range of heating capacity based on indoor evaporator coil selection. One really has to go into the manufacturer specs to choose the right stuff.

There is much more flexibility in running an inverter outdoor unit, and you will be able to get more heating capacity without any detriment to cooling capacity or functionality. This will be much less expensive than running on resistance backup heat.

The inverter units, at least the Bosch one, would be able to run off of a generator compared to a regular non-inverter unit.

Yes, the inverter control board is expensive. No way around that. Definitely need some surge protection and monitoring.

Also be informed about any rebates and other incentives.

Probably the same as the control boards in the inverter welding machines that go bad, then it's like $1200 for a new one.

 

[TRITOON]

I put in a single stage heat pump at the lake, over the years I've gotten real big on the beauty of simplicity after having a home that has a complex system and required many service calls basically each year. Easily ate up 5x and savings I was getting.

I also hate the, is it running, why is it not running as hard since it's cold or hot out? Is it broken? Do I need to call someone or wait until tomorrow? Is the freon low? Is some safety thing kicking in?

If its properly sized for the house...If its on, I want it on all the way.

Since it's your primary, I would go single or all the way with variable.

 

[danski0224]

a complex system and required many service calls basically each year

Sounds like an install with issues... Something isn't right.

 

[TRITOON]

Sounds like an install with issues... Something isn't right.

This was around 15 years ago and the unit was on a house I only lived in for 3 years. It was a rheem/rudd heat pump. It was installed correctly but maybe some of the issues where related to the dual stage technology being newish at that point.

I just remember a lot of headaches and wishing I could just lock it in one mode so it wouldn't screw itself up.

 

[bonneyman]

I didn't work on many two-stage refrigeration systems. Locally they didn't sell well, as they were pricey.
One I did work on had a cooling issue - the thing wouldn't cool well then it would cool great, over and over. His electric bills were higher, too.
Found the first stage compressor (3 ton) almost totally wore out and not pumping to capacity, and the second stage compressor (5 ton ) was like barely broken in. So you see the problem right off. Only thing was, the two compressors shared an oil line. Their sumps were connected. If the low speed comp burned out it would contaminate the (good) high side comp.

I don't know if all two-stage units are the same but it's something to watch out for.

 

[danski0224]

Mine has 1 scroll compressor, not 2 individual compressors.

 

[P0234]

I also hate the, is it running, why is it not running as hard since it's cold or hot out? Is it broken? Do I need to call someone or wait until tomorrow? Is the freon low? Is some safety thing kicking in?

Glad I'm not the only one that thinks that way lol. Don't forget the why is the defrost cycle running so often....

The thing I don't like about the 20 seer inverter we have is how is ramps up the blower speed. On defrost and with the coils on its like someone with a backpack leaf blower is in the vents.

It does do crazy well on efficiency and it keeps the temperature within half a degree. Also in lower speed modes the outside unit is extremely quiet. My hot tub is louder.

 

[bonneyman]

Mine has 1 scroll compressor, not 2 individual compressors.

I think the 2 comp design was Trane's first attempt at the multi-stage system. Now with newer variable speed technology one compressor will do.
And being a scroll is a big plus!

 

[danski0224]

I think the 2 comp design was Trane's first attempt at the multi-stage system.

I remember those. They were crazy expensive.

 

[u3b3rg33k]

I put in a single stage heat pump at the lake, over the years I've gotten real big on the beauty of simplicity after having a home that has a complex system and required many service calls basically each year. Easily ate up 5x and savings I was getting.

I also hate the, is it running, why is it not running as hard since it's cold or hot out? Is it broken? Do I need to call someone or wait until tomorrow? Is the freon low? Is some safety thing kicking in?

If its properly sized for the house...If its on, I want it on all the way.

Since it's your primary, I would go single or all the way with variable.

that's kinda the point of inverters tho. they size themselves to the house. "properly sized" means a design day, which means that 99.998% of the time the unit is too big (for AC).

on "all the way" means lower SEER, higher costs. you can have that if you want (just buy single stage), but most people want the savings and comfort.

Glad I'm not the only one that thinks that way lol. Don't forget the why is the defrost cycle running so often....

The thing I don't like about the 20 seer inverter we have is how is ramps up the blower speed. On defrost and with the coils on its like someone with a backpack leaf blower is in the vents.

It does do crazy well on efficiency and it keeps the temperature within half a degree. Also in lower speed modes the outside unit is extremely quiet. My hot tub is louder.

that's normal behavior for heat strips - full fan so they don't slag the strips. there ARE fancy units that are either communicating and stage the strips, or they can be PWM modulated for a discharge temp, so full fan isn't necessary.

 

[u3b3rg33k]

I think the 2 comp design was Trane's first attempt at the multi-stage system. Now with newer variable speed technology one compressor will do.
And being a scroll is a big plus!

it's extremely common in industrial/commercial systems (and RTUs) to have 2-4 compressors. if it's really fancy you'll have 1 inverter compressor and a a handful of standard ones to stage (no point in them all being inverter).

 

[bonneyman]

it's extremely common in industrial/commercial systems (and RTUs) to have 2-4 compressors. if it's really fancy you'll have 1 inverter compressor and a a handful of standard ones to stage (no point in them all being inverter).

The industrial compressors I worked on (very few) had what I think were called unloaders. They were like 4 cylinder compressors, and the second, third, and fourth comps had mechanical (then electronic) controlled relief valves that wouldn't allow them to pump until the load hit a certain point. Then they'd activate the valves in each stage as necessary to move more freon to meet the load.
But I admit that was like 20-25 years ago now.

 

[u3b3rg33k]

The industrial compressors I worked on (very few) had what I think were called unloaders. They were like 4 cylinder compressors, and the second, third, and fourth comps had mechanical (then electronic) controlled relief valves that wouldn't allow them to pump until the load hit a certain point. Then they'd activate the valves in each stage as necessary to move more freon to meet the load.
But I admit that was like 20-25 years ago now.

those are still a thing, but for smaller scale HVAC it's usually simpler (and more reliable) to have multiple circuits with hermetic compressors (sealed inside the metal dome/tube). if one circuit fails, you often can still have the other circuits running, just minus the capacity of the failed circuits.

otherwise you end up with a single 15 ton compressor, and you just can't get good comfort, nevermind good humidity/temperature control out of cycling that on/off. 3x 5 ton circuits that add up to 15 tons? much easier. 2x 5 ton single stage and 1x 5 ton inverter? that likely gives you capacity from 1 ton up to the full 15 tons. there's also utility billing implications for running one big motor vs smaller motors.

 

[albertdyan]

Mine has 1 scroll compressor, not 2 individual compressors.

Yes, I have also the same 1 scroll compressor, not 2.

 

[bonneyman]

those are still a thing, but for smaller scale HVAC it's usually simpler (and more reliable) to have multiple circuits with hermetic compressors (sealed inside the metal dome/tube). if one circuit fails, you often can still have the other circuits running, just minus the capacity of the failed circuits.

otherwise you end up with a single 15 ton compressor, and you just can't get good comfort, nevermind good humidity/temperature control out of cycling that on/off. 3x 5 ton circuits that add up to 15 tons? much easier. 2x 5 ton single stage and 1x 5 ton inverter? that likely gives you capacity from 1 ton up to the full 15 tons. there's also utility billing implications for running one big motor vs smaller motors.

Yeah, I figured if they still were in use it'd be a capacity thing that determined when to use them