2.5 Condensor with a 5 ton coil

NJ, Designer

I was asked to try to salvage a central hvac job on a new construction summer home in upstate NY, The installer never heard of manual J, so he put in a 5 ton air handler and coil, I want to suggest that he only install a 2.5 ton condensor, That's all the house needs. I did review the duct design, and for this job it should work ok if I run the furnace on low speed 915 CFM @ .7 IWC from the manufactures data, never did anything like this before, What problems can I run in to doing this on the refrigeration side?



Thanks all.

Kakashi

uh...

Some brands do this to make them more efficient. Without knowing all the info and the installer didn't know what a Manual J, I would ask did you pay in full yet?

NJ, Designer

I'm a designer

I got paid in full to review this job and see if there is anything I can do to make it work a little. I did the manual J and went over the duct sizing with the installer, his sizing for 2000 cfm should work for my 1000 cfm, it won't be balanced but it will be better, and of course I am suggesting some changes for better air flow in each room.

meplumber

It should work, but...

In our somewhat cool, yet humid climate here on the coast of Maine, I frequently use an oversized evap coil and a properly or slightly undersized condenser.  It allows me to lower the wet bulb temp of the space, without dropping the dry bulb temp too far.  This is a little more of a gap than I normally use.  But it should work.



I would not trust a guy that didn't know what a Manual J was to charge the refrigerant side of this combination.  The risk is going to gas too quickly in the evap coil or carrying the liquid through too far.  When you charge a system like this, you are going to have to play with the TXV to get your superheat set and then play with the fan speed and charge to get your subcool correct.



You are also going to have to replace the lineset.  The pump on a 2 1/2 ton condenser will not be able to maintain any kind of refrigerant velocity through 7/8" or 1 1/8" suction line that is normally installed for a 5 ton system.  Max suction line size on a 2 1/2 ton should be 3/4" unless you do some creative engineering to create some false backpressure.



It will work, but this doesn't sound like the right guy to hack it up trying to make it work.



Good luck.

Techman

Mis match equipment

Is the 5t supply duct in place already? With a twice as large evap as needed for the total load(sensible/ latent), would that cause the evap low side pressure to rise? Raising the lo side press affecting the dew point? The SuperHeat will stay the same for both the 21/2 t txv and the 5t txv, just a lot more txv hunting if using the 5t txv . You can maintain a .7" static on lo speed in pre existing duct?

meplumber

Evap low side

Techman, it will cause an the evap pressure to slowly rise during a run cycle.  That's why I suggested he look for someone else to trim the charge.  It is tricky to get it just right.



I normally limit my difference gap to 1 or 1 1/2 tons.  This is a 2 1/2 ton difference.  We did this once in a manufacturing facility's small test lab.  The engineer was given some pretty odd conditions to operate in.  His solution was a 4 ton evap and a 2 ton condenser.  We used the 2 ton TXV and ran a lineset to match the 2 ton condenser.  When we got it dialed in, we were able to get an almost constant condenser run cycle.  The AHU fan was already set to run constantly and we used a return air sensor to cycle the condenser call.



As far as space comfort goes, it worked awesome.  No "On/Off" cycling, just a constant balance point between space DB and WB.

Spence

Equipment Matching

First check AHRI to see if your OD unit and ID coil have been certified. If not you may have a humidity issue. Then compare the SH and LH capacities of your OD unit at your design DB and WB (not AHRI conditions; yours!) to your load requirements. Remember, you can take "credit" for half of your excess latent should you have any to apply to your sensible, which always allows for smaller OD units if yoi can neutralize both loads.

Eugene_Silberstein

System MisMatch

You will provide some degree of cooling to the space but, as Techman mentioned, your low side pressure will be high and this will cause the temperature of the evaporator coil to rise as well. This may sacrifice the coil's ability to dehumidify the space. If such is the case, be prepared to deal with a cool, clammy space... Not very comfortable.



Reducing the blower speed will help alleviate this problem, but evaporator coil freeze-up may very well become a problem.



I would personally steer clear and have the correctly sized equipment installed.

NJ, Designer

Freeze up

I can lower the cfm to 1000 cfm by running on low speed, will that help?

Techman

Miss Match

Not if you are putting 1000cfm into a duct designed for 2000cfm

NJ, Designer

Air flow

Thank you all so much for your input, I  hope to be able to give back too.

Now for the air flow, on the manufactures specs for the furnace installed, on low speed @ .1 IWC it will give  me 1200 cfm @ .7 it will give you 915, so the coil will take at least .2 iwc @ 900 cfm the filter will add another .1 iwc the balancing dampers .03 + the register .03 and the flex not fully stretched .? ETC. The point is there no way this system will be running bellow .5 iwc, even though this guy thought he was sizing for a 5 ton system, I will deal with the balancing when I get there, My question for all of you is should it be charged differently from a typical system when the coil and the condenser are closely matched?

Techman

2.5 w/ 5

The air side not considered here. Upon start up of this MissMatch the 5t TXV will try to obtain and maintain a preset and non adjustable factory setpoint SuperHeat of (lets say )12*f . The TXV will be wide open for startup, "stealing" Freon from the hiside and putting the Freon into the loside & evap coil, the now hot Freon( as compared to a normal 2.5 t TXV ) will keep the TXV wide open for a long long long time. With very little dehumidifying taking place. Then the "hunting" will begin, opening, closing ,opening, closing , on & on & on.Each time the TVX opens you will have to check the SH AT THE COMPRESSOR to check for flooding of liquid Freon into that poor unsuspecting compressor. There will be flooding. So then you might consider a Suction Accumulator to protect the comp. I would definately put a SightGlass in just before the TXV to verify a "full liquid line" feeding the TXV under some kind of stabilized operating conditions. I do not know what "trimming the charge"means or how to do it.I would shy away from this setup . Now , if there was a 21/2 t TXV on the 5t evap ,that would help a little. Someone had to do some fancy math to figure out the Sensible to Latent heat ratio with this setup and how to obtain /maintain a balanced Freon system with this setup

Techman

Air side

Flapping jaws. You got real lucky to find a supposedly 2000cfm duct system that can now handle 1000cfm without any duct changes and do it properly. Let's go to the track and bet on some horses.lol

Techman

Mr. meplumber

I got involved with a "trimming the charge " system .So, I take back what I said the other day about "not knowing how to/about" "trimming the charge". I have added to my vocabulary and knowledge, Sincerely, Terry.

meplumber

Trimming the charge

Terry, that was a term that my dad and grandad used to describe a situation like this.  Back then, we worked on a lot of poorly designed industrial and heavy commercial systems. 



For those who haven't yet had the pleasure.....Trimming the charge is when you have to play with all the factors to find the happy medium.  You can't charge off some chart pasted to the inside of the condenser.  It requires a really deep understanding of the science of the refrigeration cycle and psychometric chart, and knowing how to use your tools to their fullest.  It involves tweaks of the TXV and adjusting of the airflow across the evap and etc....etc....  It can take hours or days to get it right depending on the size of the system.  There is no magic SH or SC number.  But when you get it right, you just know it when you see it.



Not for the uninitiated or the inexperienced.



Good Luck,

Chris