Thread: Mismatched Lennox heat pump and high temp diffs

Maybe one of you HVAC guys can chime in on this...

From today's inspection, a 2005 Lennox heat pump. 2 1/2 ton outside condenser with 3 ton inside air handler. Pleated box filter was clean. Coils were clean. All ductwork seems in order. Temperature differential of 28 degrees taken just above and below the unit.

Should I be concerned with the different ratings on the condenser and air handler? Would this cause the high differentials? Or do pleated filters restrict the airflow enough to raise the differentials.

I plan to recommend servicing by one of you qualified techs.

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2.5t condenser unit and a 3t evaporator coil is not a problem. This helps in the efficiency of the unit.

Without knowing the outside temps and humidity, the TD is almost useless. It can help but many factors can also impact the TD.

Originally Posted by Scott Patterson

2.5t condenser unit and a 3t evaporator coil is not a problem. This helps in the efficiency of the unit.

Without knowing the outside temps and humidity, the TD is almost useless. It can help but many factors can also impact the TD.

Thanks, Scott. Outside temperature 84 degrees, humidity probably in the 30-40% range.

I know it is fashionable here to discount temperature differentials, but most of the techs I see around here take them. They even have a place on their checklist to write them down. I usually take differentials as one more tool to evaluate the system.

Maybe some of our recent HVAC techs can fully explain where they stand on taking differentials.

Oversized coils may work in some parts of the country, but along the hot humid gulf coast, undersizing will help with humidity removal.

Central City Air-Coil Sizing...the Full Story

Originally Posted by imported_John Smith

Oversized coils may work in some parts of the country, but along the hot humid gulf coast, undersizing will help with humidity removal.

That is incorrect.

Reducing "the coil size" is not the answer they are seeking.

Not (assuming here, like we all are) when they are referring to reducing the size of the evaporator coil.

There is nothing wrong with a small condenser coil (smaller tonnage coil and unit) and a larger evaporator coil (large tonnage coil and unit) to increase the overall efficiency "of the system", then (*THEN*) reducing the size of the system so it matches the load in the house.

This means replacing both units with smaller tonnage units, but the AHU with the evaporator coil could still be larger than the condenser unit and coil. Provided, of course, that the heat load was properly assessed and calculated, and the new unit was sized to that heat load.

Just arbitrarily reducing the size of a coil, or of both coils, could have a very costly and negative effect on the house and its occupants.

You mean Dave Lennox is wrong?

Originally Posted by imported_John Smith

Oversized coils may work in some parts of the country, but along the hot humid gulf coast, undersizing will help with humidity removal.

Central City Air-Coil Sizing...the Full Story

This is absolutely dead on, if anybody knew how to achieve sensible latent ratios that were needed for that type of area it was David Debien.

Davids designs were built by him or an outside vendor to his specs, nothing cookie cutter about them.

I have seen more detrimental effects from oversizing the evaporator, no latent removal almost all sensible.

Originally Posted by Lou Collier

Maybe one of you HVAC guys can chime in on this...

From today's inspection, a 2005 Lennox heat pump. 2 1/2 ton outside condenser with 3 ton inside air handler. Pleated box filter was clean. Coils were clean. All ductwork seems in order. Temperature differential of 28 degrees taken just above and below the unit.

Should I be concerned with the different ratings on the condenser and air handler? Would this cause the high differentials? Or do pleated filters restrict the airflow enough to raise the differentials.

I plan to recommend servicing by one of you qualified techs.

Taking the dry bulb temperature drop alone can be very deceptive at first glance.

It would be nice if you could have taken some wet bulb readings in addition, the two readings combined will give you a much better idea of the equipments performance.

That being said everytime I have seen a dry bulb drop that high there are airflow issues involved.

The pleated filter would be a good place to start, more than likely it is compounding the problem.

Originally Posted by Lou Collier

Should I be concerned with the different ratings on the condenser and air handler? Would this cause the high differentials? Or do pleated filters restrict the airflow enough to raise the differentials. I plan to recommend servicing by one of you qualified techs.

Lou,
Good idea to recommend one of those guys.

When you say pleated, are you talking about the deep 3 or 4 inches thick types or one of the one inch pleated?

Also, when you say coils are you talking about the evaporator or condensing unit fins? I ask because of the number of times I have gotten way out of line TD's and discovered that the original filter was never removed from the AHU when the installer placed the wall return grill and filter. The response I usually get is "Where did you get that? I just changed the filter last week." Then I know that they never knew it was there and it has been there since the system was new.

Originally Posted by DavidR

This is absolutely dead on, if anybody knew how to achieve sensible latent ratios that were needed for that type of area it was David Debien.

DavidR,

Please address this in 'sizing the system properly' versus just installing smaller coils.

The thread started out referring to, as one example, a 2-1/2 condenser unit and a 3 ton evaporator unit. Reducing the 3 ton evaporator coil to a 2-1/2 ton coil will not have the effect stated, correct?

However, 'sizing the system properly' might indicate that the condenser unit should be a 2 ton unit and the evaporator could be a 2-1/2 ton unit, correct?

It is too simplistic to just state 'oversize coils' and 'smaller coils' ... or am I way off (I don't think so, I think you are referring to properly sizing both coils, not just making the evaporator coil smaller).

I am awaiting a hand slap or (hopefully) a high five. I injure easily, so hit me lightly.

Jerry, as I have mentioned before, it has been quite a while since A/C school and the application of the theories learned there, but in general reducing the evaporator coil size or airflow in relation the "correct" size condensing unit will increase the humidity removal since you will be lowering the temperature of the evaporator coil and thus the dew-point of the air. The penalty is a slight reduction in overall tonnage and system energy efficiency (SEER).
Also, running the colder evaporator coil increases the chance of icing the coil.

The calculations can be done to achieve the desired results by the qualified tech, but may not be "approved" by the manufacturer as far as the warranty, etc.
Back in the day, all refrigeration systems were designed by the installing company without the aid of an engineer. Components were listed with the ratings, charts, etc. and you could mix and match to achieve the desired results. For example, a walking cooler for produce would use a very large evaporator coil in relation to the Hp (tonnage) of the condensing unit in order to maintain high humidity levels, while a beer cooler would not require the high humidity so a smaller evaporator coil could be selected while delivering close to the same sensible temperature in the box.

Oversizing the tonnage of an A/C unit in relation to the total load of the house will result in higher humidity as mentioned since the sensible load will be satisfied before the latent load is controlled.
You are correct that a smaller unit running longer will result in better humidity control, but it is not the only answer.
Jim

It would be helpful to me in this thread if all responants will say evaporator coil or condensing coil when stating your points.

David
imported_John SmithOversized coils may work in some parts of the country, but along the hot humid gulf coast, undersizing will help with humidity removal.

In what area of the country would it be a good thing to mismatch the sizes? Wouldn't adding humidifiers in arid areas and dehumidifiers in humid areas be more efficient and economical? Does the undersized condensing unit really do a better job here in the gulf coast or does it just run longer to get to the desired temperature and thereby dehumidify better? What effect, if any does that have on the life of the system and energy usage and costs?

It seem like the biggest problem I see is that few HVAC installers calculate anything. The rest "know from experience" what to install.

Originally Posted by Thom Walker

In what area of the country would it be a good thing to mismatch the sizes?

Well, just because two coils are not the same size/capacity does not make them "mismatched". In fact, that could be the ideal "match".

"Wouldn't adding humidifiers in arid areas"

That is already done.

"and dehumidifiers in humid areas be more efficient and economical?"

That's what the a/c does, why go to the expense to purchase something else too, and then have to pay to operate it too?

"Does the undersized condensing unit really do a better job here in the gulf coast or does it just run longer to get to the desired temperature and thereby dehumidify better?"

Yes.

I would not call it "undersizing" if the system were designed to operate that way, but, yes, it does a better job *because* it runs longer.

A larger capacity system would not run as long, thereby removing less moisture from the air.

"What effect, if any does that have on the life of the system and energy usage and costs?"

The longer it runs, the more it costs, however, it draws less power while running, but, it removes more moisture from the air, making the conditioned space more comfortable, which ultimately (I would guess) reduces energy costs - unless we just go back to the days of running a fan and blowing air through the house.

Anyway, I'm not an HVAC tech, so I'll let them go into more depth.

Jim, "You are correct that a smaller unit running longer will result in better humidity control, but it is not the only answer.", I know, my point was that it also *was not the wrong answer*.

I knew my reply was toooooo simple!!

Originally Posted by Scott Patterson

I knew my reply was toooooo simple!!

It wasn't too simple. In the situation you gave where 1/2 ton difference was present, it could be the exactly right solution or may be the only choice. My point with you and Jerry, is that unless we are certified or licensed, HI's should not make the determination that it's okay or not okay when we encounter systems with coils of different sizing. In this case we report what we see and express our concerns.

Jerry, I know I'm not you, but I am smart enough to know that "matched" does not always mean "same size." I also know that a lot more guessing than calculating goes on in my neck of the woods. I'm also just barely smart enough to understand the concept of short cycling.

And I'm still interested in hearing from the HVAC guys regarding trying to make the A/C do all the work when a dehumidifier MAY be the better or more cost efficient means. If I'm wrong I can handle it. I'd rather learn here than from a lawyer, Client, or HVAC specialist AFTER having said something stupid in a report.

Thom, a dehumidifier is the correct choice in situations where there is high humidity and little sensible load.
Say 75 degree days with 80% humidity, the A/C will never control the humidity no matter what the size, since at 75 degree design, there is no sensible load, only latent.


A dehumidifier will never be as efficient as an A/C when cooling and dehumidifying are needed.
But depending on the conditions, the stand alone dehumidifier can be the right choice.

Dr. Joe has some studies on this if you can find it on the new site.

Thanks. I'll go there.

The circumstance I was thinking of is where the humidity is so high and consistent that the A/C does not dehumidify adequately by the time a comfortable temperature is reached, so a dehumidifier is present as an adjunct.

I would agree that a stand alone dehumidifier would have very few practical application locations. I never intended to indicate they should be used in lieu of A/Cs for dehumidification.

What the designs David used did was to alter the sensible latent ratio to match that of the building.

The systems were setup in a way that they did more latent removal than sensible thus dropping the RH%.

The manufacturers are trying to squeeze every theoretical BTU they can out of the equipment many times doing this runs the sensible heat ratios of evaporator up around a .85 or even higher leaving little to no room for latent removal.

By the time the equipment satisfies the sensible load very little latent removal has even taken place.

I personally like dehumidifiers that are independent of the HVAC equipment as many times there will be a large latent load on the building while no sensible load is present.

David addressed the needs of the enviroment his equipment was being installed in, it might not work as well in a dry arid climate where sensible load is king.

Originally Posted by Thom Walker

I'm also just barely smart enough to understand the concept of short cycling.

Isn't that what those kids today do when they ride those tiny bikes instead of the regular one like we used to ride?

Thank you David and Jim. I think I've got it. Your patience is appreciated.

Among the reading I've been doing was this:
Cooling Loads - Latent and Sensible Heat

Many links off of it are also helpful to those of us trying to get a better understanding of proper sizing and dehumidification.

I use to install what is called a latent load discriminator kit in houses with high humidity problems. This is a set of controls that includes a humidistat and some additional blower controls. The humidistat would sense the RH in the conditioned space and would cause the A/C system to run on a lower blower speed when the humidity was high.

Operating at a lower blower speeds optimizes the system for latent heat removal. When the humidity constraints are satisfied, the system opimizes for sensible heat removal by kicking the blower speed back up.

These always worked very well every time I tried them to solve a humidity problem. If you have a house with a humidity problem I would recommend this kind of control configuration over using a stand alone dehumidifier.

Phillip,

Kind of like what the Thermidistat now does, right?

Originally Posted by Jerry Peck

Phillip,

Originally Posted by Jerry Peck

Kind of like what the Thermidistat now does, right?

Not really.

The "Thermidistat" just runs a cycle of the A/C system in order to remove more humidity from the air and will drop the indoor temperature setting as much as 3 degrees below the thermostat setting to do so.

The Thermidistat does not "optimise" the AC system for humidity removal while its running so much as it makes the system run a longer cycle in order to remove more humidity/latent heat.

In this regard, the "Thermidistat" less energy efficient and causes longer equipment run times when humidity constraints have not been satisfied.

On the other hand, a latient load descriminator kit does not require the running of additional (or prolonged) cycles in order to remove more latent heat.

Admittedly it a subtle difference in the grand scheme of things.

Originally Posted by Phillip Stojanik

The "Thermidistat" just runs a cycle of the A/C system in order to remove more humidity from the air and will drop the indoor temperature setting as much as 3 degrees below the thermostat setting to do so.

The way thermidistats were explained to me and the way I've seen them operate (to the extent I've been able to tell), they reduce the fan speed and remove humidity from the air while not making the air noticeably cooler (because of the much slower fan speed).

Then, after the humidistat part of the thermidistat is satisfied, the fan is allowed to wind down and stop.