Help with Tonnage

Inspecting a 2 yr old house today with 1892 sq ft. My question is this Comfortmaker heat pump is a 5 ton correct ? I just thought it was a little over sized for the sq footage, but was working great just trying to nail down the tonnage the 460 is throwing me off some. Am I correct in saying 5 ton.DSCN5516

What is the model number?

Oh, now the picture pops up. 5 ton is probably a good guess. :face_with_monocle:

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5 Ton it is
The 060 as in 60,000 BTU cooling capacity

60,000 BTU/ 5 Ton seems like way too much for something 1800 SF

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460 indicates 14 SEER, 60,000 BTU. 5 tons. Don’t quote 14 SEER though because SEER is determined by the combination of components.

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Yes it is a 5 ton…something is wrong there. No a/c contractor would have installed a 5 ton in a 1,900 sq ft. home.

My home was originally built with 3 ton condenser (with a 3.5 ton coil) for main body of the home and 1.5 ton for the 800 sq ft great room. Main unit replaced 5 years ago with a 4 ton system, 1.5 ton unit will be replaced in next year or so. I regret not stepping up the main unit to 5 ton when it was replaced. Getting to the point I don’t think you can have enough AC capacity at peak of the summer anymore.

As to the point of installing 5 tons AC in a 1892 sqft home, who knows why they did it, as long as the supply and return was sized to move enough air for that capacity.

You can also make an educated guess using the RLA number. Yes, 5 ton.

60 divided by 12= What ? You got it.
It’s in multiples of 12 unless it part of a 1/2 ton and that would be 6…

Totally oversized IMO, but as always, I would suggest Further review by qualified licensed HVAC contractor.
The problem with oversizing the tonnage is removal of moisture. When a unit is grossly oversized like this, yes it will cool like polar ice cap. However, it will cool the space too rapidly and shut off before adequately removing moisture. You will get a very damp home.


You are absolutely right.


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Well, that thinking is totally wrong…

I am not even going to list all the reasons why. Most of you already know, the rest should not be making these calls.

A 3 ton with a 3.5 ton coil is a “High Efficiency” unit. This causes lower head pressure and saves energy. So long at the metering device is correct for this coil, it’s fine.

Let me give you an example outside the general way of thinking. 26 years years ago I built an addition on my house. I got a used 3.5 ton heat pump from the company I was working for, for the price of scrap. It ran without incident except for two motor capacitors. The unit was over sized for the addition, but the addition had a west exposure (major solar load) with a large amount of glass on all walls (floor to ceiling).

Well, this month it finally failed during a significant heat wave and I replaced it with a 2.5 ton unit (which is handling the cooling load perfectly). When I cut up the old unit for scrap I found excessive oil (quarts) in the low pressure refrigerant lines. The compressor was too hot to touch when it failed and was blown electrically.

So, what happened to this unit?
For all these years the unit was operated only under a high cooling load (primarily to deal with the solar load). At other times the main house unit shared the load and worked fine controlling moisture. The 3.5 ton handled sensible heat quickly as desired.

What changed?
I now use that room for dialysis 4-5 days a week. My wife, going through those change of life hot flashes and would jack up the a/c at a moments notice (any time of the day or night). The system was being operated under little or no load and it would short cycle as a result. This prevented the return of compressor oil due to low head and back pressures. It failed!

Not often do we get to live through the failure of equipment and see the effect of what simple “change of use” can do to any system (electrical, sewer, landscaping, HVAC). Text books say not to oversize and gives reasons. This is a real life example that they are right.

I could have prevented this by installing a hot gas bypass valve or control the condenser fan. But this was a $200 unit that worked fine, far longer than than most inspectors would allow to exist without mention in a report. 99.9% of “Investigate Further by HVAC contractor” would have just assumed “It’s old, change it”.

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Once again I learned something from David!

AHRI’s text, Fundamentals of HVAC/R, p. 902, mentions in passing that, because the required heating capacity for heat pumps may be much greater than the required cooling capacity, “engineers will select a unit with a cooling capacity of a half ton larger than necessary to help increase the heating capacity without causing significant problems in cooling operation.” You could note that this may be the case and that it might cause this heat pump to run at some indeterminate level of inefficiency in cooling mode to cover yourself. This is a single stage system, right? I believe if the compressor was 2 stage it would say “2 Stage” on the label.

And by the way, there is no friggen way a 5 ton is needed on that sized house…

David, should inspectors be on the lookout for 2 stage-compressor heat pumps, operating at the lower capacity in cooling mode, so they don’t make the mistake of suggesting or reporting over capacity?

I’m also thinking that the wide range in the installer’s choice of electric supplemental heat capacity and calculations regarding trade off with larger heat pump capacity, makes it more complicated to call out overcapacity than it might seem.

Of course the 5-ton system is too big for this house, but how do you say it without getting burned in view of the above points? If I’m off the mark, I hope you’ll take the time to explain why, which may be of interest to others, too.

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Regardless of the defect I am reporting, I always document what is happening as a result of the observed issue. Thus I am a fan of narrative reporting. If the unit appears to be too big for the house, what is happening as a result of my observation that the nameplate indicates the Btu/hr is excessive. As someone stated above, duct sizing, filter sizing, (excessive static pressure in the duct system), refrigeration pressures, psycrometric properties of supply/return air etc.

Yes, this is all beyond the scope and capabilities of most HI’s. Therefore reading nameplates and trying to find something wrong should also be beyond what a HI takes on in their reporting. I have never responded to a request on this board about dates or sizes from data plates. That is because the HI has no idea what this information indicates one way or the other and should not be just stacking crap up in the report.

Does the unit operate as intended? Home Inspections are “observations”. What can you observe about an over sized unit? Excessive air flow, filter gets sucked into the duct, noisy airflow, frost on the suction refrigerant line, little or no condensate discharge etc. If you do not observe an associated condition like in this case where you do not see this sized unit on a house like this, simply state this in the report along with explaining that this inspection is not intended to analyze the system design. You have no idea how or why someone designed a system you are operating. Don’t second guess them, or your ability to break the nameplate code without your secrete decoder ring if you are not prepared to conduct the tests to make such a determination!

As for your question on the two stage units:
There are two speed (or variable speed) compressors and there are two stage compressors (two separate compressors in series with each other). They have separate purposes. You must understand the difference and purpose.

Two speed systems are sized for maximum design conditions at maximum capacity determined by ASHRAE for your city. So you should never see a five ton unit on a three ton house with the intention to have more capacity if/when outdoor conditions exceed design temperatures.

A HI should never try to analyze a system operating at reduced capacity unless you have the data on what the unit is operating at in that mode (sensible heat ratio). You just observe that it is responding to operational controls.

As far as the supplemental heat thing, a heat pump is designed for maximum load. That would only be in the summer. There is no load in the heating mode (it always operates below capacity in the heating mode, that is what an accumulator in a heat pump is designed for). The refrigeration circuit should never be sized for heating loads or they will be too big in the cooling mode. That is what supplemental heaters are about. If you need more heat pump capacity in cold climate regions you upgrade to a cascading compressor system (two compressors in tandem). Refrigerant is temperature/pressure dependent. No load, no heat, no pressure. Without pressure there is no heat for the house. Compressor #1 gets what heat it can from outdoor air and discharges it into compressor #2 which collects and creates even higher pressure, which produces the required heat for the house. When you are designing a system up in Canada where high heating loads exist, your cooling load is also significantly lower. So the bigger unit for heating ends up being way too big for the lower cooling load and all kinds of problems will arise.

Remember capacity and efficiency are not related. Capacity is Btu/hr. Efficiency is the range of temperature (enthalpy) in which the unit is designed to work in. Increasing efficiency, does not change the Btu/hr capacity.

You never install a 150 watt halogen light and dim it down to 40 watts for every day use in case someday you loose a contact lens and need more light to find it. Everything in the lighting circuit is bigger and costs more. And it is less efficient under low load than if properly designed for the average daily needs. Same with HVAC design.

An undersized heat pump in extreme weather will still maintain “comfort” in the cooling mode. Even if the indoor temp comes up to 80F and can not achieve thermostat set point, you will still be cool because of de-humidification through the removal of latent heat by the undersized for existing conditions unit. So the need for increasing equipment size even less of an issue.

Hope this covers what you were asking…

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David, I like your analogy to a halogen bulb. As for what HI’s can check beyond checking to see if the system is running, I believe it is reasonable for them to check the Delta T between the return and supply if they can get a reading close to the air handler.

A popular tech blog claims rule-of-thumb Delta T is too often misleading to be of any value unless worked out using psychrometric formulas. AHRI on the other hand thinks otherwise and has a straightforward, simple graph that adjusts Delta T for various levels of RH. Honeywell even has an educational piece titled “Why Delta T is the magic number for proactive diagnostics and problem-solving.”

I’m in no way am intending to give the impression you and I are on equal footing when it comes to HVAC, just raising the possibility for HI’s using an RH-adjusted Delta T, while noting its limitations in boilerplate, to say more than that the a/c or heat pump appears to be running.

The “gurus” love to assume that home inspectors are a bunch of dimwits that don’t know anything beyond plate reading and that everything in a specific trade, such as HVAC, is rocket science. These guys crack me up, pumping their ego each & every time :slight_smile: Funny thing is they get things wrong quite often, go figure.