Hi Velocity Needs Bigger Heat Pump?

[Tom9]

I have a proposal to replace window AC units with a high velocity system.

The manual J calcs yielded 39,667 BTUh cooling load, but the proposal is for a 5 ton heat pump and 5 ton blower.

When questioned about the difference, I was told that the lower air flow (250 cfm/ton) of a hi velocity system versus the normal 400 cfm/ton effectively "derates" the cooling capacity and the system had to be larger to compensate.

This does not sound correct to me at all, and I thought I would ask here.


Is there any validity to this at all?

Thanks.

[Keith73]

I'm looking forward to see some answers on this myself. I've never heard of anyone intentionally installing with 250 cfm per ton in mind. Not enough air flow IMO. I would like to se the specs on that equipment. 5 ton is still 5 ton.

[motoguy128]

I think the 250 cfm/ton is measured velocity, not standardized for air pressure. I think when you factor in air pressure, it may have a high effective flow rate.

250cfm on a conventional sytem could nearly freeze the coil in some conditions.

Also, is the 39,667 sensible heat gain, or the total system capacity requried with latent capacity factored in. If you live in a dry climate, sensible however equals total capacity. If you're in a humid climate, a 4 tons unit for 39667 sensible might be just a tad small.

Finally, the actual capacity of a unit at design temprature is less than the rated capacity. I don't know how much less. You might have to use the equipment data sheets from the mfg for that .

[FlyersFan]

Strangely I had the Unico high velocity sizing guide open on my computer when I looked at this thread. What matters most in your load calculation is the sensible gain and the latent gain and the sensible/latent capacity of the equipment he's quoting. From the sizing guide:

Equipment Selection and Size
• Use ARI ratings to select equipment
• Select basic unit size based on cooling load, do
not oversize by more than 1 ton (3.5 kW)

Unico claims a better latent heat removal because of the lower CFM per ton so if you live in a fairly humid climate this will help you out.

[Keith73]

Dang, just did my homework on the high velocity! The cfm ratings are quite different from what I am used to. Rare to see one in my area.

[Tom9]

Sensible Load = 36,299

Latent Load = 3,368

These might help one determine the correct unit to install, but really I'm looking for comment on the idea that high velocity air handlers are "mis-rated". If you read my original post, you'll see that the the explanation for installing a much larger unit than called for by the Manual J calcs is basically: "a 3 ton hi velocity system will not really deliver 3 tons of cooling -- you need 5 tons of cooling to make up for that lower cfm."

That sounds really bogus to me. If a hi velocity air handler is rated 3 tons, then I'm sure it has the fan/coils combination to really deliver 3 tons, under rated conditions of course.

Any comments?

[beenthere]

High velocity systems don't cool a house by air exchange like a conventional system does. they cool it by aspiration. So they don't need to move as much air, they just need to move it much faster.

Find another contractor.

[FlyersFan]

As weird as it seems, the answer to your question varies based on the outdoor equipment you select. Not all 3 ton units deliver 3 tons of cooling, depending on the indoor unit matchups. Upsizing by 1.5 tons from the load calc seems a little CYA to me, but depending on your contractor's experience in the area of the country you live it may be perfectly OK.

[tedkidd]

Do the window units cool the home? Sort of, ok, good, great?

How many btu are they?

[motoguy128]

Do the window units cool the home? Sort of, ok, good, great?

How many btu are they?

Are the combined tons of window units a reasonable measure of cooling demand? It just seems like there's a whole lot of vairability there.

I suspect the combined capacity of the window unit that were onc installed in my home was around 7-8 tons. Based on what I observed this last month with current split systems, I need somewhere bewteen 4-5 tons.

[tedkidd]

I suspect the combined capacity of the window unit that were onc installed in my home was around 7-8 tons.

How many window units, somewhere between 12 & 18?

Wow, that's a lot of window units you've got!

Based on what I observed this last month with current split systems, I need somewhere bewteen 4-5 tons.

How do you arrive at that conclusion? You have twice as many window units as you think you need?

[gregp]

with BT. I would be looking for another contractor with HV experience. Maybe go to the website and find a local contractor/dealer. Wonder where is going to put all those extra vents?