Guessing vs. Load Calculation

[RyanHughes]

In our tract housing community in Maryland (where today we had a high of around 90 degrees with 50% RH), a neighbor just had a new Evolution 18 heat pump installed today (by a reputable firm in the area), and it's a 3 ton, for probably 2800 sqft. total conditioned space. Another neighbor down the street I already know has a 5 ton single stage (Goodman), and it's zoned for each level . Not sure who's got the better size for the heat gain, but someone's certainly way off with a 2 ton difference in sizing.

Personally I think they're both a bit off for this model home . Nevertheless, the importance of a load calculation is evident here, when you have such drastic sizing discrepancies.

This post has no real purpose but to describe my recent observation, and to add to the importance of getting a load calculation done when the need exists.

[Phasewolf]

When I had my new system installed the contractor that installed it did not do a load calc. I did however do one on my own and wanted to see what he would say I needed before I told him what size the program said was correct. To my surprise he was all most dead on he said to install the only change I made was in the AC unit which he said if I like it cooler he would go 1/2 ton larger but if the Rh was a issue with the old unit then he would go with the 2 ton. I went with the 2 ton unit and it works great so far. 41% Rh and my wife is happy too.

[HVAC9900]

Assume from the post that this is a replacement?

[Phasewolf]

Yes it was. The one that was in the home was over 25000 btu's larger then it should of been and really short cycled a lot in the winter.

[quickcat46]

What the heck maybe that contractor had a extra Evolution 18 laying around. Cause it sure would seem to be undersized to me. Even without being on site to do a heat load calculation. Is this the only heating--cooling source in the house? A split system?
Would be interested in how much the 3 ton runs as compared to the single stage 5 ton. Maybe if it runs all the time but runs EFFICIENTLY they are saving money. LOL.
Thought does come to mind did they change out the coil and air handler also--in either install--both installs?

[RyanHughes]

Split system heat pump, replacement. Both are matched systems. I'm sure both will keep the place reasonably comfortable, but the 3 ton will be running a lot, that's for sure.

[Airmechanical]

and it's a 3 ton, for probably 2800 sqft. total conditioned space.

i think one of our memebers has the record for the least amount of air for the biggest space

sized at 1200 sq.ft./ton

but if i remeber correctly he has whined about it



.

[beenthere]

Paul42 has 2,000 sq ft per ton.(4,000sq ft house)

After switching to the IAQ thermostat. He has no problems with it.

[DPeoples]

In our tract housing community in Maryland (where today we had a high of around 90 degrees with 50% RH), a neighbor just had a new Evolution 18 heat pump installed today (by a reputable firm in the area), and it's a 3 ton, for probably 2800 sqft. total conditioned space. Another neighbor down the street I already know has a 5 ton single stage (Goodman), and it's zoned for each level . Not sure who's got the better size for the heat gain, but someone's certainly way off with a 2 ton difference in sizing.

Personally I think they're both a bit off for this model home . Nevertheless, the importance of a load calculation is evident here, when you have such drastic sizing discrepancies.

This post has no real purpose but to describe my recent observation, and to add to the importance of getting a load calculation done when the need exists.

I say your 3 ton unit is a bit under sized for the total sq ft of the house and would run alot more ergo high electric bill ...i would have gone with the 5 ton unit...more efficient... a proper manual j heat gain/loss would have helped..

[pstu]

I say your 3 ton unit is a bit under sized for the total sq ft of the house and would run alot more ergo high electric bill ...i would have gone with the 5 ton unit...more efficient... a proper manual j heat gain/loss would have helped..

At the same EER for both 3- and 5-ton units, the 3-ton one could run 67% more time and use the same energy (unless I made a math error). But invariably the 5-ton unit will have a lower EER. Does that not argue the 3-ton running longer, will still consume less energy not more?

I completely agree with you about the Manual J.

Best wishes -- Pstu

[valkyrie0002]

The Man J calcs still involve some guess work unless a blower door and duct blaster test were done on that house. You have no idea how loose the house and ductwork are unless you do those tests. You know that manual J adds a 15% fudge factor.

A smaller system running longer will be less expensive to buy, less expensive to operate and will condition better than a large system that short cycles.

[john_ertw]

At the same EER for both 3- and 5-ton units, the 3-ton one could run 67% more time and use the same energy (unless I made a math error). But invariably the 5-ton unit will have a lower EER. Does that not argue the 3-ton running longer, will still consume less energy not more?

A smaller system running longer will be less expensive to buy, less expensive to operate and will condition better than a large system that short cycles.

Both posts suggest that a smaller system will use less electricity to remove the same amount of heat from a house. Is this for a case of a properly sized system versus one that is severly oversized (eg 2 tons too big), or does it also apply to a system that is only 0.5 tons too large?

Does this account for all electrical use including the blower fan which will have to run longer with a smaller unit?

Most (non-informed) people's intuition tells them that a unit that runs non-stop will use more electricity than one that cycles. The thinking is that the smaller unit uses electricity non-stop, while the larger unit takes "breaks". I bet it's a hard sell for a HVAC sales person to tell the average Joe that a smaller unit that runs non-stop will save them in electricity.

Is the only disadvantage of a smaller unit the fact that it will take longer to recover from a temperature set-back and will not be able to achieve lower temperatures than it is designed for?

[motoguy128]

Even a load calculation requires you to make some assumptions. The actual performance of windows and insulation can vary. The affect of wind and trees is vairable... and ultimately, it depends a lot of the climate.


Just curious... but has anyone developed a vairable capacity or vairable surface area coil? The advantages of 2 stage or inverter drive units is always discussed, but even with a passive TXV or electronically controlled TXV, you still end up with slightly lower real laent capacity because on low speed, the coil is oversized.

Has anyone developed a coil with solinoid valves restricting where refrigerant is distributed acorss the coil... or some mthod of reducing the coil surface area?

I know this is a little off topic, but it just had me thinking.