Reduced tonnage recommended -- good idea?

[dcmcm5]

dcmc,

I have been using a hard copy that's several years old,and I see that the on line data from H&C is very different today,I'm not sure why it has changed,as the grilles appear to be the same.

I'll be checking into it.

Dash - Please post back what you find out. Also, I noticed there are grilles made by Titus that appear to have higher flow rates than the Hart and Cooley grilles. An example is in the back of the ACCA manuals (Manual T, I think).

[beenthere]

Wouldn't that give him the best of both?

None of us apparently know the actual heat load but we do know that 3.5 will do what it has always done.

So with a Bristol two-stage compressor he would have 3.5 and also 1.75 for humidity control. A matching variable speed air handler would round it out OK.

Wouldn't it?

They don't make the 2 stage in half ton sizes.

[Poodle Head Mikey]

That would give you four and then a two for most of the time and decent humidity control.

[coolguysfl]

That would give you four and then a two for most of the time and decent humidity control.

OMG Mike --- now you've done it.... employed common sense and no doubt the wrath of the ACCA Gods will be upon you. (Is that sarcastic enough)

BTW - I agree.

On a different note, I've experienced an undesirable side effect w/ two speed systems. Namely duct gain. What I'm finding IE: a 4T 2 speed has for all intents & purposes 4T duct gain... so when we're in the heat of the summer running @ 1/2 speed.. the NET cooling delivered really falls short & results in near 100% run time & as you would guess unhappy consumers.

I believe the true savings w/ 2 speed systems are applications w/ no duct gain.

[Poodle Head Mikey]

That, as with any system - half the air velocity would require twice the duct insulation to equal the same performace.

The better approach would be to triple the dust insulation. <g>

My real concern with a two-stage system is actually duct perfomance - the throws on all the registers would be off spec. The end runs would suffer disproportionately. Etc.

The only Real answer is a two-stage main trunk run and duplicate half sized supplies and registers. Then put a damper in one of the trunks which only opens when the second stage of the compressor kicks in.

The 'cheap and dirty' method of air distribution would be to leave the blower at full power even when the compressor stages down. This of course would reduce the humidity control benefits - but. . . . where does it end? <g>

[coolguysfl]

Your understanding is 100% correct

[beenthere]

The supplies at the end of the main trunk have good air flow with 2 stage units.
Less CFM = lower resistance from the duct. The problem you could run into is that the VS blower (in your 4 ton example)will try to deliver the 1600 CFM in high, when you previously had a 3.5 ton unit, with duct good for 3 tons. And make some noise if you don't alter the duct for the added air flow.

[Poodle Head Mikey]

But if the original was 3.5, the 4 ton is obviously going to be over-sized. So the tendency would be for the system to operate at half speed Most of the time. It would basically just jump into high-speed and then just as quickly cycle back to low speed.

So we can probably live with the air noise.

Of course - The Real Answer is not so much cracker-barrel speculation - it is some simple calculation: heat gain, required cooling capacity, air flows, velocity, and duct sizing.

As it is - I am just guesstimating. Which I don't mind doing but still - it is just an experienced and educated guess. <g>

[dash]

Two stage Bristol is about 60% cpacity on low,two stage Copeland ,about 80 capacity on low.

A two satge Bristol that runs all the time,at 60% cost about the same as high stage running ,60% of the time.

Explaining this to the customer upfront ,avoids compliants abot run time.When they see the kw savings,they realize run time is not and issue.


From Manual D and T we would assume that the "throw" on low stage would be an issue,but we find it's not.You need to remember the "throw" you have been designing for is at design temperature,low stage is running at less then design.

[Poodle Head Mikey]

Dash,

In the Bristol they are stopping and starting one of two equal sized pistons, aren't they?

So why isn't the single stage performance running at 50% of the total capacity? Is there a gain at the evap side of the system that I am missing in my morning fog here?

[dash]

Yes,the larger evap and cond. coil,is what I've been told by factory engineers,is the source of the extra capacity.

[teddy bear]

"Real Humidity Control"-- Is it ever 70^F, 80-90%RH in Houston for a couple days? Might you have 50-100 cfm of infiltration/clothes drier/exhaust fans? You get 500-1,000 btus latent from the occupants. This totals to 4-6,000 btus latent. How will 2 tons of low speed cooling with low air flow keep the home <50%RH without drastic overcooling? 2 tons of cooling is 18-20,000 btus of sensible cooling with 4-6,000 btus of latent.
What is clear is that during a 18-20,000 btus sensible cooling load, 4-6,000 btus of latent heat will be remove. This is better a 50% duty cycle of a conventional 4 ton a/c. Also consider the 6-8 lbs of moisture on the a/c coil that will evaporate back into the home. 1 lb. of moisture re-evaporated off the a/c coil raises the indoor %RH 9%.

Here is ideal humidity control. Start with being able to maintain <50% RH without any cooling load even with extended rainy cool weather and fresh air ventilation. Or if your home is unoccupied, 10 hours a day or a couple weeks, 4,000 btus per hour of latent reat removal will maintain <50%RH. Starting with a 90 pint whole house dehumidifier provides assurance of <50%RH even if the a/c is over-sized to handle peak loads. This is a much more effecient and positive method of maintaining <50%RH throughout your home. Whole house dehus use less than a KW to remove 4,000 btus latent heat. This is a >50% savings. Make your a/c as complicated as you wish, but beable to remove moisture without cooling or over-cooling your home.
Whole house dehumidification makes humidity control a reality not just a hope. We are doing a lot of MS upgrades. Most report improved comfort and less operating cost. See you at Comfort Tech. Regards TB

[pstu]

OMG Mike --- now you've done it.... employed common sense and no doubt the wrath of the ACCA Gods will be upon you. (Is that sarcastic enough)

BTW - I agree.

On a different note, I've experienced an undesirable side effect w/ two speed systems. Namely duct gain. What I'm finding IE: a 4T 2 speed has for all intents & purposes 4T duct gain... so when we're in the heat of the summer running @ 1/2 speed.. the NET cooling delivered really falls short & results in near 100% run time & as you would guess unhappy consumers.

I believe the true savings w/ 2 speed systems are applications w/ no duct gain.

I would like to ask you elaborate on the unhappy customer part. I am a homeowner with a 2-stage, and this is the first report I have heard of customers being unhappy with them. Certainly my experience is the opposite, my 1st stage runtimes have been even longer than I expected, and the energy savings have been more than I dared expect (I think something subtle is going on, such as off-loading the lesser system in the other side of the house).

Your logic about the duct losses sounds excellent. It represents a critique of a 2-stage system exactly where its weakest point might be. However this is a theoretical critique and if there is customer experience to support it, I would like to learn more about it. This would be valuable information and not the kind of thing one expects to hear from anyone in sales<g>.

Thanks in advance -- Pstu