Oversizing with 2 stage equipment to save energy/run less

[ga-hvac-tech]

We install a lot of 2 stage units, and some inverters... note that inverter split systems (conventional design equipment) should NOT be put on a house unless the furnace/coil is centrally located and the duct runs are short... as air will not get out to the far ends of the duct runs most of the time. And you will have an un-happy customer!

With the 2 stage units:
Design temp in Atlanta is 92*F... however that can be anywhere from 60-90% RH... (ooops)...
So the load is not really constant.

AND... I can tell you this from experience: If we have a HOT summer (days in the mid 90's or upper 90's)... and the system runs full time and cannot get the house cooled down until after dark...
YOU are gonna get lots of BAD reviews!
Customers do NOT care about what the ACCA says... they want to be comfortable... which means they want to FEEL comfortable... and it is YOUR job to deliver that!

Herein, lies the advantage of a 2 stage unit:
Lets say we did a Load Calc on the house... and it says right at 3 tons (lets say 35K BTU's total load, because we know most 3T units do not actually do 36K BTU's).
SOOO... a 4T 2-stage unit will almost make design most of the time in low stage (remember, a 2 stage unit is about 70% in low, which is about 33,500 BTU's.
So we have the low stage running most of the time, it is about 1500 BTU's under-sized... which is great for longer run times and good de-humidification (we have humid summers).
Now look at those HOT days... which are above design temp... the unit can go to high (always use a 2 stage stat)... and cover the load!
And as noted in a previous post: Say the customer has a party on a summer day, with 20 guests and cooking... the added capacity will go a LONG ways to holding temp... where the barely capable single stage will just loose ground!

Now one needs to remember: Match the equipment properly:
4T 2-stage outdoor unit
Matching 4T VS furnace
Proper coil with proper TXV valve
Large enough media filter
Added return so the system can suck 1600 CFM's of air when asked to do so...
Yeah, the supply will be a bit low for the full CFM's... however the system will not run there much and that is what a VS motor does well!
We have dozens and dozens... probably over a hundred... installs of systems similar to this... and ZERO VS blower issues!

And of course, a quality stat (we like to use the factory communicating stats, or high end Honeywells).

We install anywhere from half a dozen to over a dozen of these high end systems a season... and we have literally NEVER had a customer complain about how comfortable their home feels...
NEVER!

We do, however, get a lot of referrals from these jobs...

[markbrown1]

Are you a technican? or why are you interested in knowledge of energy effiicency and usage.
usually the calculations and the best use come from experience. once you will fix the systems, you will become expert in knowledge.

[TMJB]

As some others have mentioned, the key question is: is your duct work system capable of handling the additional airflow needed to accommodate the larger equipment? In 90+% of residential systems the answer is NO! National Comfort Institute has an on-going study in which they have data on 20,000+ homes in North America. The study has shown that the average heating & cooling system is performing at 57% of its rated capacity! By far, the largest cause of this is undersized duct systems. The other major factor is systems not being set up properly. If your system has not had the static pressure measured, that is the first step. If your system is 'average' you could increase effective performance by up to 20% by simply having the system set up properly!

[Poodle Head Mikey]

You could install two entirely separate independent systems. With HV systems (like Unico) the dual ducts systems and registers would be easy accomplish.

What would be nice would be to make them 33% & 66% of total heat gain. And alter the staging sequence based on outdoor ambient temperature. Below the change-over point the smaller system would be staged on first - and above it the larger system would be staged on first.

That would the best way that I can think of to have two stage cooling.

[shellkamp]

You could install two entirely separate independent systems. With HV systems (like Unico) the dual ducts systems and registers would be easy accomplish.

What would be nice would be to make them 33% & 66% of total heat gain. And alter the staging sequence based on outdoor ambient temperature. Below the change-over point the smaller system would be staged on first - and above it the larger system would be staged on first.

That would the best way that I can think of to have two stage cooling.

Why wouldn't you simply use a 2 or more degree of disparity between temperature setpoint and actual temperature to determine staging?

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[Poodle Head Mikey]

I would use the method you mention to do the stage cycling. But I would also want to alter the lead/lag order as well / switch the stages: one for the other.

Because if the 33% was always stage one - at some point the larger 66% would be cycling on and off to maintain the set point.

So, to minimize that, above some 'to be determined' outdoor ambient, I would want the 66% to lead - with the 33% cycling to maintain the setpoint.

PHM
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Why wouldn't you simply use a 2 or more degree of disparity between temperature setpoint and actual temperature to determine staging?

Sent from my Moto Z (2) using Tapatalk

[shellkamp]

I would use the method you mention to do the stage cycling. But I would also want to alter the lead/lag order as well / switch the stages: one for the other.

Because if the 33% was always stage one - at some point the larger 66% would be cycling on and off to maintain the set point.

So, to minimize that, above some 'to be determined' outdoor ambient, I would want the 66% to lead - with the 33% cycling to maintain the setpoint.

PHM
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How about:

3 stage cooling.

1st stage is the smaller, 33 percent compressor.

2nd stage activates larger 66 percent compressor as well as a relay where the small compressor contactor control voltage is run through the N/C contacts of it, meaning on a call for second stage cooling only the 66 percent compressor will run.

3rd stage will activate both compressors by using a relay that bypasses the 2nd stage " lockout" relay, which would still be

You'd have to get a Honeywell T350 or some other more-than-two-cooling-stages thermostat but it sounds potentially simpler than involving an outdoor reset that swaps your lead/lag stages.

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[Poodle Head Mikey]

That's a good idea - I hadn't considered using a three stage thermostat.

I used to set up boilers the way I described. On larger houses where there was enough load to make two boilers worthwhile.

Although, on anything above a very basic level I expect that 'the next guy' just jumps everything out to make it all run together anyway.

PHM
--------

How about:

3 stage cooling.

1st stage is the smaller, 33 percent compressor.

2nd stage activates larger 66 percent compressor as well as a relay where the small compressor contactor control voltage is run through the N/C contacts of it, meaning on a call for second stage cooling only the 66 percent compressor will run.

3rd stage will activate both compressors by using a relay that bypasses the 2nd stage " lockout" relay, which would still be

You'd have to get a Honeywell T350 or some other more-than-two-cooling-stages thermostat but it sounds potentially simpler than involving an outdoor reset that swaps your lead/lag stages.

Sent from my Moto Z (2) using Tapatalk

[Unlimited1]

i did work for a place that used the 2 stage but has the refrigerant solar panels coming off the condenser then into the AH coil... very expensive. it was supposed to run making it have a seer rating of 45. the math was never provided to me. i never saw it run in 2nd stage..but thats what they said made it efficient..
i was not at a tech level yet...only install

[Unlimited1]

i remember as a child, a relatives home had a huge 3' fan in the attic, if it got too hot you open the bottom floor windows opened the attic door and hit the switch...that was "real" air conditioning...i still remember the roar of it...massive airflow, lol

[ga-hvac-tech]

i remember as a child, a relatives home had a huge 3' fan in the attic, if it got too hot you open the bottom floor windows opened the attic door and hit the switch...that was "real" air conditioning...i still remember the roar of it...massive airflow, lol

Yeah... those were called 'attic fans' when I was a kid (back in the dark ages)...
Ran it in the early morning and as the sun went down.

Now they are called 'whole house fans'... are 24-30" dia... move about 1/3 to 1/2 the air... and builders do not include them in a house.

I have taken many an old fan, built a box around it, and used it to keep the air stirred up where I was working.

Picked up one from the side of the road a month ago... gonna build that one for my shop...

[Poodle Head Mikey]

One time I was doing some work for the US Navy in a gigantic (former) airplane hanger / test area. While on site another company was tearing down some offices in the same space. The offices had been heated with a large steam coil up hear the ceiling with a 12" turbo-axial fan blowing air down to the offices.

The fan was laying in a pile pf debris and they said I would have it. I only wanted because it was so industrial-cool looking. <g>

My house had no AC at the time and one hot day I happened to see that axial fan and thought: Whole House Fan!

It had come home with a short piece of really heavy gauge round duct flanged onto it. I hauled it up to the attic and hung it up under the ridge beam - slung from ropes. <g> The discharge end of the round duct was flush against the screen on the gable-end-vent.

It vaguely strikes me now that it was 460V or maybe 3 phase and I had to install a 120V motor but I'm not sure. Anyway; I get it hung and set the belt tension and so forth. Then I start it going. It's like a friggin Wind Tunnel in the attic and when I get downstairs it is making a pretty stiff breeze though the house. It was loud in the attic but can only hear the slightest hum from it running downstairs.

It's been a hot sweaty bastard in the attic so I go outside - the sun is setting and it's cooling off outside. That house is on a corner. Out front I see my oil burner guy neighbor Mike - he smoked a pipe and would often walk around with it.

Hey Mike! Just trying to wear those shoes out or what?

Yeah I guess so. Can you hear that noise?

No; what noise?

So Mike comes across the street and in front of my house we listen. He says; Damn - I don't hear it now.

What did it sound like? I've been here all day - I didn't hear anything.

I don't know . . . maybe like a helicopter or something? Like a weird howling noise almost? I can't describe it. At first I thought it Was a helicopter hovering - that's why I came out to look.

So Mike and I walk down to the corner and he says: Now I hear it - where the hell is that coming from?

I have already heard the weird noise and I'm trying to get a bearing on it. Mike and I walk around and listen and the sound seems to be coming from everywhere - from every direction. The neighbor from right across the street from the end of the house comes out: What are you guys doing?

Can't you hear that noise? Where is that coming from?

I hear it now - what is it?

Damn if I know; we're trying to figure it out - I never heard it before.

In a minute or so his wife comes out to see what we're all doing. And then joins us: "What is That???"

After several minutes the light goes on in my tired head. "I'm thirsty - I'll be right back . . . you all want anything?" and I head inside. I race upstairs and pull the plug. Downstairs, grab some iced tea, and head back out.

Figure it out yet?

No; but we can't hear it now. It just stopped.

Stopped? How can that be? Where was it coming from anyway?

Of course nobody knows, we all puzzle over it for a bit, and then each wander away.

So the 'whole house' fan worked great - just couldn't use it. <g> Inside the house it was fine - but it kind of 'projected' it's operating noise out with the airflow. I resolved to change the sheaves (the blade ran at motor speed or maybe slightly higher - I forget which sheave was slightly larger than the other) to reduce the noise but pretty soon after that I got pissed off at the oil furnace clogging nozzles every three days and ripped it all out to install new Coleman 'super high efficiency' (13.1 SEER <g>) cooling and a 'super high efficiency' (91% <g>) gas furnace.

And that was the end of my Whole House Fan use. <g>

PHM
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Yeah... those were called 'attic fans' when I was a kid (back in the dark ages)...
Ran it in the early morning and as the sun went down.

Now they are called 'whole house fans'... are 24-30" dia... move about 1/3 to 1/2 the air... and builders do not include them in a house.

I have taken many an old fan, built a box around it, and used it to keep the air stirred up where I was working.

Picked up one from the side of the road a month ago... gonna build that one for my shop...