Drawbacks to Oversized HVAC Units

STOLISMA:

Several comments are in agreement that you will suffer penalties for installing a 4 ton unit where only a 2.5 ton unit is advised. Specifically; poor humidity control, cycling and poor efficiency are noted repeatedly.

I am in agreement if the unit is installed with no additional changes made. This being an engineering blog, I think you might appreciate some alternative solutions

If the used 4 ton unit you are considering offers a substantial savings and you are willing to make some other modifications, it may be worth considering.

Humidity control is a big deal, especially in Birmingham, Alabama. I'm in Orlando Florida and I know that I can keep my thermostat around 8 to 10 degrees warmer when humidity control works well. While it is an option in drier locations, exchanging with outside air in Birmingham with the hope of improving humidity would be futile pretty much anytime you would need to use AC. I would also ignore the comment below that advises against dehumidifying (and actually humidifying) your air under the auspices of protecting respiratory health. In the South, excess humidity is a more probably route to respiratory threats, like black mold.

The main reason for poor humidity control is a lower change in temperature of the air passing through the handler and the reduced time the air handler is moving air past the coils. While more air passes through cooling the home more quickly, the higher volume, lower change in temp and shorter time on mean less moisture is drawn from the air.

Dealing with humidity control will also help with efficiency since your thermostat won't need to be kept so low. .

Add a recirculation line joining the air handler effluent to influent. You will want to add a throttle to the line. A butterfly valve, perhaps a large throttle body from the junkyard, will allow you to adjust the amount of recirculation, the change in temperature from influent to effluent and therefor the amount of humidity removed. This will also reduce the velocity of the air moving through your ducting and some of the problems associated therewith.

Adding a line or increasing the size of an existing line to an area that could use more cooling can help homogenize the temperature in the house. An extra line or increase in an existing line in your ducting would actually increase the flow and exacerbate humidity control, so it would be prudent to add a recirculation line too. A larger line into the kitchen where cooling loads can be quite high at times, might be an option. A line to directly behind your refrigerator may have an added benefit of improving the efficiency thereof.

Adding a significant thermal mass just after the air handler such as a chilled water tank (I know it doesn't seem that desirable not when it may be snowing in Birmingham, but think back to hot days in the summer when a shower wasn't cold enough to stop the sweating for long... ). Bottles of water and/or paraffin would also be an option. To make full use of this it would be desirable for the fan to continue moving the air for a time after the compressor has stopped.

Significant advantage woulds be available with a thermostat that offers sufficient programming flexibility. Specifically, programming a larger difference between the temperature at which the air conditioner turns on and that which it turns off would reduce cycling and slightly improved humidity control. Programming the air handler fan initiation to lag the compressor initiation by a short period would be another way to improve humidity control. Cycling the air handler fan on and off while keeping the compressor on would help both humidity control and reduce compressor cycling. The obvious thing to avoid is having the compressor on and the air handler fan off for so long that you have things freeze.

Desiccant filter at the main air return would be another way to deal with humidity.

There are probably numerous other ways to improve the efficiency and reduce the negative effects of a mismatched system.

Benbenben

Lower the t-stat if it gets too humid; and put on a sweater.

If it's short term, it probably will work OK. How long is "short term"?

How do you know you need 2.75 tons?

And how did you end up with a 4 ton unit?

Will the fan be able to push the larger CFM through the ductwork? Probably OK, but check duct velocities if ducts are smallish.

There can also be installed, a hot gas by-pass to reduce the capacity of the unit. Once the evaporator tempature drops below set point, no less than 32°F, hot gas will be run thriugh the coil. An energy waste for this system, but for short term or tempory might be okay.

I'm not familiar with the Seer's circuit and compressor engineering. But here's a crazy thought, who knows if a VSD could be hooked up in the circuit if you included an appropriate temp controller with a 4 - 20mA output? Then you may well end up with a more energy efficient set up than a correctly sized 2.75 ton unit.

air flow is gonna be your problem. but, i'd be curious to hear from Engineering dudes on the VSD application possibility

Yes and they are not always right in figuring tonnage. Mine will pull the humidy out and when it is very hot it will not cut off either. That is when we have 100 degree days. And it might still be 80 in the house. I wish I had went with the extra ton just to see how much better it would work.

Hi STOLISMA,

As has already been suggested you would be advised to purchase a new unit sized to the application;but hey we've all been there with a piece of surplus kit ready to use so what are the problems?

1) The fan section will not be operating at the most efficient part of its curve so you will be paying more for energy than otherwise. Static pressure/noise issues may arise but a VSD will make things easier to live with.High pressure drops in domestic ducts will generate whistles and air rush noises which are extremely annoying!

2) The oversized compressor section will be under-loaded and may not appreciate the short cycling with too many starts per hour. Check whether it has a hot gas bye-pass valve fitted 'cos this can be adjusted to compensate for low load conditions. Be aware that the over-sized battery will produce large swings in discharge temperature due to the reduced air flow when your VSD is in use.

You cannot use a VSD on the compressor drive, there are special machines developed for this type of use only.

3)I would disagree with the comment about the need not to introduce a degree of fresh air as this assists with the humidity issue you should not dry out the air as this will increase the risk od exacerbating respiratory problems. Rather than squiring steam into the airstream I would adopt a passive form of humidification through a combination of 'water features' and evapotranspiration from large broad leafed plants. They are extremely effective in the likes of your climate.

Hope this helps,

Good luck

Massey.

Hey HOSS,

IF this units free and it's going in a hunting cabin .........then I say yeah it's rigged up but OK.

I installed for 3 years and serviced for 2 years after that and have seen this several times........LISTEN......do not put this unit in your house.

Get a 3 ton 15 SEER Goodman heat pump. It's generic (owned by Amana) but has the same components as the brand names without the advertising cost plus if has a 10 year parts and a lifetime compressor warranty.

ALSO....It will cost you more than you save if the duct work is not sized correctly.

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NOTE:Don't condition outside air.....dump some supply back into the return if humidity is too high (short cycling)......... yet another rigged up quick fix.

In answer to your question, Tonado gave you all the correct answers. I would like to expand on them a little. If installed it will short cycle colling the area faster but not spending enough time running to remove the humidly in the air. Letting in fresh, unconditioned air in from the outside only makes it worse. If you are reusing the old duct work system, the air will cavitate and when you are running your heat. It will not allow enough air to pass through the heating coils and it will shut the unit down on high heat which is an emergency shut down not an operational shut down. This also causes damage to your unit and is a slight fire hazard.

So we have a low cost temporary repair which is going to do damage and shorten the life of the 4 ton unit, not be able to satisfy the occupants, and probably double the cost of operation. On the other hand it has a cheap initial installation cost.

Personally I don't like temporary repairs since they tend to become permanent repairs. Right? We spent the money to repair the system, its running, so we need to find something else not running or in need of repair and fix it. In the meantime our budget has increased due to utility costs and so we have less money to spend on an actual repair.

Grey Haired Old Goat:

You wrote: '... If you are reusing the old duct work system, the air will cavitate...'

I am unfamiliar with cavitation as it applies to a gas filled system. I am only familiar with cavitation that occurs in a system where pressure drops below vapor pressure causing a phase change. As there should be no liquid air in a residential HVAC system, I am curious if there is another type of cavitation of which I am unfamiliar.

I learn something new each day, and if you respond quickly perhaps it will be two today.

Benbenben

Cavitate can be used with air flow also as it has the same principles and physics as liquid except it is able to be compressed.

Point being if you call it cavitation, stacking air, static pressure, or whatever the heating unit will still go off on high heat.

Grey Haired Old Goat:

We seem to speak completely different languages. In my language, 'cavitation, stacking air' and 'static pressure' do not share a common meaning and are not interchangeable.

'Cavitation' occurs in a liquid system when pressure drops below vapor pressure causing a phase change. It causes pitting, erosion, noise, and pressure shocks to the system. Since the Air in a residential air handler is a gas and will not undergo a phase change it is difficult to understand why anyone would call this 'cavitation'.

'Stacking Air' occurs when numerous aircraft are in flight in the same area, as in a aerial assault or when many planes are circling an airport waiting to land.

'Static Pressure' is the pressure a fluid exerts at rest with respect to it. It is also used referring to the amount of pressure needed to overcome the resistance in a distributions system at a sufficient rate of flow.

To me, these things are not just different, but belong to vastly different categories.

As long as I am taking issue, I might as well put it all on the table....

You wrote, '...Cavitate can be used with air flow also as it has the same principles and physics as liquid except it is able to be compressed....'

All fluids (including liquids) will experience an increase in density with application of sufficient pressure. There are no truly incompressible fluids.

At relatively low mach numbers we can assume that fluids flow like a theoretical fluid which is incompressible. Mach numbers <0.3 are typically considered a safe threshold below which the assumption can safely be made.

In residential conditions, air would have to be traveling in excess of approximately 225 MPH to have a mach number in excess of 0.3. I am unaware of air handlers creating speeds like these which necessitate modeling flow as anything other than incompressible.

Am I off base here?

Benbenben

If using the 4 ton is your only option, I would at least put it on low speed. If you end up seeing high humidity, you can do a couple of things in the short term. The best choice would be to run a dehumidifier in the house. Then you would have a de-coupled system. The 4 ton would handle the sensible load, and the dehumidifier would handle the latent load. If you can't do that, the other choice (not recommended because it is wasteful) is to increase the sensible load on your house. You could run an electric heater while air conditioning - this would end up reducing the humidity. This is basically the same thing as reheat humidity control (but again, it is wasteful). Be advised that you will waste the cost of a new dehumidifier in a couple of months on your power bill.

First thing comes to mind is how did you size the 2.75 tons is this on a total house load with an upstair and a basement. what I'm trying to say is a 4 ton unit replacing a 2.75 ton unit, will really not be a terrible problem. you should figure 400 cfm per ton and you can go as low as 300 but, it's not the best economical idea although it still will work . you can dump the extra 500 cfm worth of air in say a basement or just add extra supply air outlets. so if it's free great! use it .