New unit performance/design temperature

[roadking321]

Just had a Rheem 12 SEER, RPMD 3 ton heatpump with Rheem air handler RBHC17J11SHH, RCHA36A2G coil installed. This repalced a Heil 3 ton unit manufactured in 1993. Previous unit was able to maintain 73 degree setpoint on 95 degree day and occaisionally cycle off. New unit ran for 8 hours straight yesterday maintaing house temp. of 75 degrees(set point 73)before it finally managed to attain set point and cycle off. I'm located in central South Carolina and temperature yesterday was 95 degrees and humid.

I'm calling the contractor to check out the unit but would appreciate some insight. By reading one of the previous posters who has a similar problem I understand some of the things the contractor should be looking for. I am however curious to get your opinions on the following:

During the morning when the outside temperature is 80 degrees or so the temperature from the register nearest the airhandler is 56-57 degrees or so. Measuring my return temperature nearest the central return grill the return air is about 74-75 degrees. The delta seems very good to me.

During the afternoon when the unit cant meet set point(73) and is keeping the house at 75 degrees the return temperature is about 76-77 but that same register is only providing 68 degree air.

The house has a west orientation and therefore gets the direct blast of the sun in the afternoon. Attic temperature hasn't been measured but its hot as hell up there. There have been no changes to the house at all other than the new system.

Hopefully the contractor will have a fix. I'm assuming the best time to have him check the units would be mid afternoon when they are unable to keep up.

Lastly, it was my understanding that in this area design temperature was 75 degrees on a 95 degree day. Is that correct. Again unit I replaced easily maintained set point of 73 on 95+ degree day and did cycle off and on. Is there a difference between 3 tons in 1993 and 3 tons today?

[dash]

Tons is "nominal tons",either system could have been 32,500 to 36,000,btus ,with a ton being 12,000 btus.

Outdoor design is ,in Man. J,indoor is whatever is agreed upon,usually 75 to 78.


I'd have them check the return temperature entering the indoor unit and at the return grille,the difference,indicates return ducts leaking,if it is more then a couple of degrees during the hot part of the day.Depends on length of return and insulation on the ducts.

[theredneckrev]

Originally posted by dash
Tons is "nominal tons",either system could have been 32,500 to 36,000,btus ,with a ton being 12,000 btus.

Outdoor design is ,in Man. J,indoor is whatever is agreed upon,usually 75 to 78.


I'd have them check the return temperature entering the indoor unit and at the return grille,the difference,indicates return ducts leaking,if it is more then a couple of degrees during the hot part of the day.Depends on length of return and insulation on the ducts.

Good point. Could also be overcharged or TXV could be "sticking"

[roadking321]

I just checked air handler. Found that portion of the mastic used to join airhandler to aquecoil heating coil(hot water loop for heat) is loose and probably leaking 150-200 cfm's or so(wild guess). Obviously this has a detramental effect on cooling capacity and may be the straw that's breaking the camel's back.

I'll make sure they fix this and check for possible intrusion of warm air from return lines as well as checking charge on condensor.

Appreciate all your input.

[docholiday]

Yeah it all adds up, however, I doubt your losing 150 CFM from a mastic leak. However if you feel it, its wrong.

That unit, the 3 ton RPMD, has a lower sensible capacity than the old unit did in all likelyhood, its got about 24,000 sensible, good latent at around 9,100 with a total capacity of around 33,100.

The airhandler is probably struggling to get full airflow with that hot water coil in the duct further reducing the sensible capacity. (I hope this doesnt come back to haunt you come winter) Additional losses with the mastic arent helping matters as you pointed out.

My guess is a ton for ton swap was done with nobody consulting the spec sheets or doing a manual J. In reality, you may have installed a 3 ton nominal system in place of a 3 ton nominal system and lowered your sensible capacity by as much as 5000 btuhs. In the heating mode you have more than nominal heating btuhs at around 37,400.

[roadking321]

Doc:

What your saying is that my new system might have about 10% less cooling capacity then my old one just looking at it on a straight swap out.

The new Rheem system replaced an Apollo system that consisted or a straight 3 ton Heil air conditioner,Apollo air handler with a heat exchanger coil. After consulting this forum and doing some research I decided to go with a heat pump and new air handler system. The hot water loop will be my auxiliary, defrost, and emergency heat instead of the heat strips.

The Apollo had the hot water coil within the Apollo air handler so with the new Rheem air handler they needed to attach the new aquecoil to the discharge end of the airhandler...much the same as the original Apollo system was designed just not all in "one box".

Looking at the aquecoil and comparing it to the heat exchanger supplied by Apollo in the old unit, the new heating coil appears to be somewhat less restrictive then the old one...hope it heats as well during the winter!

[Panama]

Get your contractor to record (with a copy to you) all his temperature and pressure readings. Then you can post some meaningful information here if your system still does not cool well.

[docholiday]

Well yes, but no, it isnt a straight swap out. For a layman, yes it might be considered one but there is some science to all this. In your case, you could have a 15-20% difference.

My boss dscribed nominal best. It means not true. We say nominal because we like rounded numbers and when we talk tonnage, rounding numbers is rounding by as much as 3,000 btuhs with no regard for details on latent sensible ratios.

This rounding can mean as much as a 6,000 btuh differnece in one unit to the next and there are really no limits to the difference in sensible only values.

The outlet of that (or any air handler) is very small compaired to the overall dimentions of the unit itself. I hope this was accounted for when the coil was added, and I hope the tabs were folded up. You may need to have your contractor find out what pressure drop for that coil he used and did he compensate for that in the ductwork. The original air handler had that included in the airhandler so as far as the new system is concerned its an added restriction. The same duct work will likley be too small. Airflow is probably jepordized but I am just speaking from experience. Come winter, this will be a problem as he needs to adjust the refrigerant charge to keep from tripping the head pressure. Then defrosts will be a problem because then it will not be properly charged for cooling (which is defrost).

I tell you, the proper thing to do is a load calcualtion. Select the equipment based on actaul numbers. Design the duct with that added coil in mind and consider a variable speed air handler to overcome the added restriction. Many air handlers dont have the added drive for add on coils since they often already have a coil internal.

[roadking321]

Repairman/installer came and here's what he told me: ( I didn't see the post asking for pressure readings before he left so I'm sorry I can't report those).

1. There was indeed a farily large air leak where aquecoil joined the airhandler. He also reported some leaks at the new airhandler cabinet itself and sealed those as well. No obvious air intrusion at the two return ducts.

2. He checked charge at condensing unit and added a few ounces of refrigerant...but felt that it wasn't off by very much.

He tried to explain away the performance during humid weather by saying that with the temperature in the mid 90's and with high humidity(heat indexes in the 105 range) that all you sgould expect is 78 degrees house temperature at 95 degrees. I disagreed with him based on my previous experience with the house maintaining 73 degrees at 95 degrees and high humidity.

I am retired from a position in commercial construction management and the mechanical engineers in this area used 95 degrees outside temperature and 75 degrees inside as their design parameters for AC. I would think that residential design should be similar but don't know that for sure.

Also, I'm wondering about the comment about heat pump operation during the winter using my hot water heat exchanger in place of the heat strips. Is the implication that every system that uses a dual fuel option instead of heat strips requires that the charge level on the condensor be regulated for each season to perform correctly?

[dash]

I'd say the implication of adding the coil is,it may restrict air flow,both in heating and cooling.

Dual fuelsystems,are more commonly a gas furnace with one coil,for heat pump and cooling,not two coils .

[royc]

Something else that has to be looked at is the Rheem unit suppose to have a supply duct the size of the flanges on the rheem airhandler. The manual states 3 feet but can be less. Is the water coil mounted directly to the airhandler?? If it is you going to have an airflow problem.


Roy

[docholiday]

Dash got what I meant. The implication is airflow. Airflow affects the charge, the unit will surely be overcharged as a result of low airflow. No biggie in cooling, but when switched to heat, it will be. Adjusting the charge in winter makes cooling not work right. Defrost is cooling so defrost doesnt work right.

Your duct system was originally designed for say 1200 CFM at a certain External Static Pressure and took into acount maybe the filter, duct length, equivenelnt lengths of fittings etc. The air handler you had, had a much larger motor because it had to blow through the water coil and cooling coil. The air handler you selected was never intended to blow through a an additional coil. In other words, the original coil was not external pressure drop, the new coil is because, well its external to the air handlr. However, with a duct system designed to keep the friction rates to a bare minimum, it may be do-able. But from what I can tell, the exsisintg duct was never changed. In other words, you bought a new car and decided that the tires and seats be reused from your old one. The system includes the duct as much as it includes the units. You start off handicapping a system with an addtional coil and dont upgrade the duct to boot, you might find it to be a problem.

Today, lets hope the reair to the leaky duct and the few ounces of refrigerant solved your problem. I'm always up for hoping the "simple repair" will do it, just not always convinced its nesscessarily the right thing.

[roadking321]

Doc and Dash: Thanks for your insight on this issue. Today it hit 94 degrees with high humidity and the inside temperature held at the set point of 73...although the AC has run continuously for a few hours. Looks like the air leak "fixes" and slight addition of R22 have helped.

I checked the specifications on the Apollo website for their current model airhandler(which probably hasn't changed much from the 12 year old one I had). Their specs indeed show 1475 cfm @ .5 ESP vs. the Rheem air handler's specs of "meet or exceed" 400 cfm/ton at .5 static pressure, therefore 1200 cfms in this case. So we are starting out with 275 less cfms(18% less air)assuming that the static pressure is the same. Not sure what 1475 cfms vs. 1200 cfms would do regarding cooling performance as its an 18% reduction in air flow. I agree that ideally a variable speed fan would have helped mitigate this issue.

The new aquecoil has a large diameter and it was attached to the end of the Rheem airhandler in such a way that it "appears" (I cant see inside)to be of the same diameter as the supply end of the Rheem... it butts up to the Rheem which has its supply side panel removed. As I noted before it actually seemed to be of a less restrictive design than the Apollo coil. I don't see any reduction in size just some modest increase in total airhandler/heating coil length compared to the previous Apollo air handler.

Checking the specs on the Apollo is seems like the higher cfms for a 3 ton unit are a slight bias towards heating vs. cooling as more heating BTU's are supplied with the increased air flow according to their specifications.

Again thanks for everybody's thoughts.

Roadking321

[dash]

1475 @ .5ESP might deliver 1200 cfms at .65 ,1200 cfm @ .5ESP is just that,so if the ESP is more than .5 ,the new air hanlder might not deliver the proper cfms.

Hot water air handlers that I've look into,are designed for the north.Meaning 60,000 btus of heat has enough air flow for 2 tons of cooling,where in Florida ,it might need a 3 ton system.

Large developement east of of was convinced by the the gas company to go that way,heard the ripped about about a hundred of them when summer time rolled aroud.large builder of ours ,got the same pitch,we said the gas company or the builder needed to sign as being responsible for the air flow,end of story.


Hoping the OP's air handler has the sir flow needed,maybe it does ,just be aware.