"A" coil size

[deejoe]

Heat gain for 1400 sq ft house is 18-20 K,so we are going to have a 1.5 ton condenser installed outside.
The inside (evaporator will be installed within an oil furnace plenum (approx 10" above exchanger) Plenum size is 20x19- 40" high and we would like to take full advantage of this large plenum size and install an oversized "A" coil rather than a matching 1.5 ton that will restrict the air flow in the heating season. The heat "rise" is nearing the high end now(85)but is still quite satisfactory with the present Med blower heat speed in the heating season.

Is it feasible to have a 1.5 condenser matched with an oversized evaporator(2 or 2.5, or larger) in order to lessen any restriction that would occur thru a smaller coils opening aperature?
Thanks for any help.

[casturbo]

Many 1.5 ton systems use a 2 ton a-coil. Even air handlers do too, as the supply house I get equipment from said many manufactures don't even make a 1.5 ton A/H, they say to use the 2 ton instead and change out the piston or use an TXV.

A 1400 sq. ft house that only needs 1.5 ton of cooling?...sounds too low to me. Unless of course this house in the shade and new and up north. Is it?

[Irishmist]

There are a number of considerations to be aware of here:

1- The system, i.e. your central air conditioning system should be sized and the indoor and outdoor components 'matched' to meet the requirements of the space.

2- If a manual J load calculation was not performed, be sure that this is the basis for determining the capacity requirements.

3- You need to be sure (have your contractor verify) that the airflow associated with the furnace meets the requirements associated with the central air conditioning. Depending on the furnace, it may or may not be set up for air conditioning airflow requirements. Again, your contractor can verify this.

4- If a smooth transition is fabricated and installed in conjunction with the evaporator coil, then no airflow restirctions should occur. If, on the other hand, someone is simply trying to physically 'fit' an evaporator into the existing plenum, then the proper consideration to capacity is not being given. In other words, if some sheet metal work needs to take place to minimize any airflow restrictions then now is the time to do it. Your contractor should also be able to address this for you.

5- It is feasible to match a system as you indicated, but a knowledgable individual that is aware of the exact results of such a match needs to be making that determination.

6- Sounds like from your comment about the 'medium blower speed,' that this is a direct drive multiple speed blower. If that is the case, chances are the furnace is already set up and capable of handling the air flow requirements of adding on central air. Your contractor can verify this as well.

All of these areas can and should be addressed by your contractor. Bottom line, a (your) contractor is the person trained to take all the above into consideration when he makes his proposal for the scope of work to be performed. Actual physical dimensions of equipment and components are just a small part of the overall picture. Don't have something installed simply because it 'fits.' It may not be properly 'sized,' even though it fits.

[deejoe]

Thanks for the replys. The heat gain was recently done using HRACI.. Ontario,Canada,Electrical league) the home is very well insulated, tight construction, well shaded,etc ,thus the seemingly low 'Gain'.
The furnace is rated to accept a/c, and has 4 speeds available for any required cfm that is needed.
I am aware that the speed for the 'air' may have to be increased to M/high, or even High, that is fine, but I do not want to have to increase the heat speed up any higher than it is now (m/low)because of any restriction that a evap coil will create. This is WHY I want to go with a LARGER coil inlet AREA.
I also plan to fab a metal transition at the base of the plenum and extend it up into the entrance of the evap coil inlet opening.....this transition would be approx 8" long(high) and would be 20x19 to whatever size the coil entrance would be. This should help "streamline" the entry of air and help to eliminate any significant increase in "heat rise" in the heating season.
what do you think, or suggest? I also plan to have a TVX installed with this larger 2.5 coil.
I also want to keep the condenser as small(1.5-2.0) as possible as I want the unit to run and not short cycle,thus eliminating more humidity.


[Edited by deejoe on 09-23-2005 at 03:09 PM]

[beenthere]

Whats the brand and model # of your furnace.

What size is the supply and return ducts.

[deejoe]

Hi, beenthere. The furnace is a Brock, oil fired, 90k input,74k output.The supply consists of 2 main plenum takeoffs; an 18x8, and a 12x8.... The main Return drop duct is 25x10 into the 25x16 filter and into the fans'compartment.The existing ductwork has been properly sized using the appropiate Manuals, etc.the system is operating (heating) at 875 cfm, .05 sp. Heat rise is approx 80 degrees, just within the furnace' specs.
There are NO past or present problems with the HEATING system .This home is only 6 years old.
Normally in the past years, the summers heat has not risen much over 75 in the home, but with the constant heat days of this summer, we are now considering adding a A/C split system. The main issue here is we do not want to have to increase the Fan 's heating speed (because of rise) in the winter season just because of a too small or restricted evap coil.
Any further suggestions or advice on this would be very much appreciated.

[beenthere]

Many 1.5 ton units are matches to 2 ton e vap coils.

At 74,000btu output, with your temp rise I come up with a 856 cfm.

The evap coil will lower this.

Your return duct should be checked to see what its sp is.
You may be able to add a return, to keep the same blower speed.

Keep in mind though, that just be cause you raise the blower speed, it doesn't mean you will can a large increase in cfm.
The coil will still restrict the air flow alittle.

What temp do you keep your house in the winter time.

[deejoe]

Beenthere, thanks for the reply.
We keep the house T stat set at 72F for heat. The outdoor design temp for Heat loss in our area is calculated at -4 F
If the evap coil restricts in the cooling mode I can up the fan to either Med/high, or if necessary to High.

In the heating mode I do not want to have to increase the speed any more than it already is set at which is Med/Low.
That is why I want a coil that has a large inlet area that will not add too much restriction and cause me to go to a higher speed in the heat season.
Are there any special type uncased "A" coils (2 ton) on the market with a larger inlet aperature than the normal type.? the normal coils i havelooked at are approx 16x12 o.d., but as my plenum is 20x19 that is quite a restriction. A coil size would be best at around 19x19 o.d. or so.(or larger)....I can easily fab a flare fitting for the plenum if necessary to accomodate it.

[beenthere]

Finding a 1.5, or 2 ton coil to fit that opening may not be possible.

One thing you might want to consider is leaving an opening to beside the coil.

And make a damper that is open in winter, and closed in the summer.

That way your air flow is not slowed down in winter by the coil.

[deejoe]

Is there any mathematical way to estimate how much a coil would actually hinder the Heating air flow? I would think that knowing the coils "pressure drop" one could get a fairly close idea of what will occur before this coil is actually installed. This way, we would know in advance how much we will have to 'up' the fans' speed in order to keep the winter 'temp rise' within the furnace specs.
Tthe present heating speed (without the evap coil)is M/L,(quiet & comfy) but if we have to up it to a Med thats ok too. But if we have up it any higher(High) then the air noise will be much too noisy and we will scrap this idea of adding a/c and do with out it.
Thanks for any suggestions

[dash]

Test the ESP (external static pressure),look up the PD for the coil,at the requred cfms,then look at the fan chart for the furncace,and you'll see what speed will be required.

[deejoe]

Thanks Dash for your help. I will figure the necessary cfm to overcome any added restriction that this coil will create.Hopfully, for the heating mode , we won't have to go up more than one speed level. we can live with that,but not two levels up.

[twcpipes]

Do you feel you will have sufficient cooling air coverage using 600cfm for 1400sq.ft.? This is open to all.

[deejoe]

twcpipes; you ask if I feel if i would have sufficient cooling air coverage using 600cfm for 1400 sq ft.
The answer would be probably not, but then it will not be at 600 cfm because we plan on upping the fan speed when in cooling mode.
The COOLING cfm adjustment will be no problem making it adequate,.....It's the HEATING TEMP RISE we are concerned with.... that could be a problem come heating season.
Read the previous posts in full and you will know the whole story.
Thanks anyways.
Btw,Twcpipes where did you get this "600 cfm" figure from?

[beenthere]

The base air flow for a 1.5 ton a/c is 600 cfm.

[deejoe]

The furnace blower and ductwork, etc is more than capable in delivering 600-800-1000 cfm by merely using the proper motor taps. As I've been saying all along, the cooling will be no problem. The main idea is to select a evap coil that will not cause (restrict) the "heat rise" to go above the furnaces specs. It's at the high end now (but still within) even without a coil installed in the plenum.
It was earliersuggested to fab a plenum ducted dampered by-pass around the coil for the winter operation in case the rise did go too high. but there must be a better way around it than this by-pass. Trouble is ,I, (and the contractor ) are at a loss as to how to acccomplish this.

Any body out there got any more ideas?
I sure hope so. Thanks in advance

[Edited by deejoe on 09-24-2005 at 06:16 PM]

[beenthere]

The easest way, is to just hook up the next higher speed for heat. check the temp rise, and if its too high, go up to high speed, the coil isn't that restrictive.

Your contractor is going to use a fan center right.

[deejoe]

I take it from most (not all) of the replys that the one way is a "trial and error" approach.Which we want to avoid.
I know there must be a mathematical solution in finding the actual cfm required with a known pressure drop of the coil, opening size,etc. The only problem is we can't find any data as to the p.d. on any uncased coils.

[dash]

Originally posted by dash
Test the ESP (external static pressure),look up the PD for the coil,at the requred cfms,then look at the fan chart for the furncace,and you'll see what speed will be required.


This is the way to avoid trail and error,it's fool proof.

[beenthere]

This link is for Lennox coils, has pressure drops.


http://pirl.lennox.com/C03e7o14l/UFnCIDK4mO/ehb_c33_0506.pdf

[twcpipes]

>>Read the previous posts in full and you will know the whole story.
Thanks anyways.<<

Before signing off this thread I have to ask:

1.What makes you think you can just arbitrarily run up the evaporator CFM with an oversized coil and not overload your 1.5 ton compressor or, if required, address humidity.

2. What also makes you think you can stay around a minimum of 700CFM-800CFM and not have hot and cold spots in your 1400 sq.st. envelope?

I read all the posts and unfortunately you may not be looking at the whole picture of the system in operation, but then again, what the hell do I know?
The drawback placed on air conditioning requirements for hi-eff construction design is that the need for adequate air flow throughout the envelope is jeopardized. That holds true with clean filter and coil.

Good luck with your project.