new system with Aspen coil, low head pressure

Chuck78

I installed a system for my neighbor in a rental yesterday. I went through a lot of trouble finding a coil that would fit below the floor joists in the basement (3.5 tons with 24" tall plenum sitting on the joists, and a tall ICP 92% furnace under it). I settled on an Aspen Coil that was 20" tall, 4 row coil with the higher fin count of the two they offered. It's just a rental, so we stayed with the fixed orifice to save cost. It's got a .080 piston in it. The 3.5 ton we took out was a .078 but it was 10 seer. Now on startup, my head pressure at 87 degrees outside was at 225 or so. Suction was around 81 with the indoor temp at 78. After the rainstorm, 77 outdoor, I was at 75/175 with with maybe 74 or 75 indoor temp. Getting tons of condensation and 20* temp drop. Everything seemed good to me except that i thought the head was a little low. I considered installing a Parker adjustable TXV, as the next size orifice down is a .074 for 3 ton. Should I be alarmed at this head pressure? Superheat was within the condensor manufacturer's recommendations. It's a Goodman GSC unit. I know, I know... It's got a Copeland scroll in it (so I was feeling better about using a Goodman), and they wanted to save as much money as they could on a rental.
I felt like the head should have been slightly higher, around 260 or so, which made me wonder if it would work better with a slightly smaller orifice or (obviously better off) with TXV. If you run too small of an orifice, or the TXV isn't backing up enough pressure, what are the consequences?

I'm new to this forum since hvacmechanic.com now seems defunct. HVAC tech in Columbus, OH w/ 6-1/2 yrs exp. Always looking to learn more, which brings me here.

Yuri

I am finding LOTS of the 13 SEER units running lower head pressures and higher suction and that is not a problem. Just the way they are designed. All your specs are fine and I would not worry about it. 20 deg delta T is great.

nicktech

well after the rainstorm the air temp was probably lower and the condenser was wet and running cooler. the units are designed for 95 outdoor ambient temp, if your in a situation where its lower than that then wrapping the condenser to build heat/ pressure up to what you want it to be can give you a better lowside reading. but there doesnt sound like anything is outta wack. look for about 10 deg of superheat +/- 2 deg its a more accurate test! goodluck

RoBoTeq

Our industry depends on temperature/pressure relationship. Neither temperature or pressure alone is really relative if the system superheat and subcooling are proper.

That coil will give you better dehumidification at the same nominal 400 cfm per ton then a Goodman coil will give you. Just an FYI, Goodman coils are getting shorter as we type. The new rated coil for that application from Goodman I believe is 22" tall and if I'm not mistaken, is all aluminum; fin and tube.

Chuck78

Wow, that's news to me about the Goodman coil. I remember hearing a while back when was putting in my Nordyne stuff in my house (Westinghouse), that you could bump up the SEER rating of your unit by running an Aspen coil vs the Nordyne "X" coil I think it was.
Now I talked to Aspen today, and they basically told me that with the 4 row coil with the higher fin count (19?), that it's capable to run a much lower head pressure and get a greater temp drop vs the OEM offerings in the way of coils.
The only thing that I didn't like is that Johnstone only carries the ones with the steel drain pans. Those are great if you have a blast furnace, er, um, I mean, Fuel Oil furnace, but I definitely prefer plastic after having to deal with some really rusted out drain pans on otherwise still in good service systems. Aspen makes them in plastic.

As for the dehumidification, you're not joking, they tell you to slope the coil up 1" higher in the rear for drainage due to this. I swear when I was doing some sheetmetal work after I started the unit up, that the condensate removal pump ran every 10 minutes! Wish I would have gotten an Aspen coil for my Nordyne heat pump now. I had pretty close clearance to my joists from the top of the X coil case, and had to "T-bone" the case straight into the trunk line with a few salvaged turning vanes overhead. Not what I wanted, but had no choice.

I think Aspen is going to get a lot of my business from now on, with a lot of these retrofits on taller furnaces wtih height issues.

I was more suprised by the suction pressure not dropping more after the rain/cooldown than I was that my head was only 225 @ 87* outdoor temp. I haven't done a whole lot of 13 SEER residential split system stuff this year, but now that you mention the pressure differences, I do recall the first few I did running a lot more efficiently (lower) than I was used to.

RoBoTeq

Just an FYI, several equipment manufacturer's are going after Aspen and other aftermarket coil manufacturer's for their claimed efficiency ratings with ARI.

Keep in mind that ARI does not do any rating. The manufactures of products do the ratings and submit those ratings to ARI. Once in a while, ARI will have a random rating retested by an independant lab just to keep manufacturer's honest.

Manufacturer's of refrigerant bearing equipment must actually test system components in a real life bench test using actual system components. They are allowed a margin of error by 5%. Manufacturer's of aftermarket coils need only test their coils using computer simulations and are allowed a margin of error by 10%.

The bottom line is that aftermarket coils are most likely not really more efficient, but rather are taking advantage of that 10% margin of error to boost their rating numbers. Since efficiency ratings are basically a lot of smoke and mirror number crunching anyway, much like gas mileage ratings on vehicles are, it does not matter to me how a rating is achieved as long as a sale can made because of the ratings.

Another factor is that many manufacturer's of outdoor equipment are requiring their matched indoor units to be installed in order to take advantage of extended warranties being offered.

mechanicalDvr

Putting a new system in a rental property with an orifice instead of a txv sounds like a good way to bang up a compressor or at least make it over amp from slugging back liquid in low temp operation.

nicktech

actually piston systems have an easier time equallizing off cycle than a txv. none the less, delay on make timers and hard start kits and LPC's should be utilized to prevent short cycling.

RoBoTeq

TXV's give more control but piston's don't fail.

beenthere

In low temp operation. Weather a Piston or TXV. You can have liquid slug back.

A lot of systems out there with pistons, that are operated at low temps(mines one of them) and have no troubles at all(and you would fall over if you knew how low of an air flow I run).

__________________
How many times must one fix something before it is fixed?

mechanicalDvr

A piston knows nothing of what a system is doing a txv uses suction line temp to meter the liquid. Do you have a fan cycling control on your condenser?

mechanicalDvr

News flash, everything can fail. Never seen a dot of solder in an orifice, that woukld have been caught by the screen in a txv?

nicktech

the big difference between the two is that txv can handle a greater variety of pressure variances or swing from the liquid line. the piston is dependent upon a constant pressure and mass flow rate at its inlet to maintain the proper evap. pressure/temp/boiling point. they both can handle normal summer swing, one better than the other. but in enviroments where the swing is too great it won't matter what metering device is used, you'll still have problems. this is where a LPC, condenser fan cycler or RHPC's as well as blower speed control would help.

beenthere

Nope.
And I run it when its lower then 64°F outside at times, not too often.
But when I do, I usually have my stat set to 68.

Piston systems run fine at temp on the 50's, if the system has proper air flow, for the load it has to work against.

A piston, feeds X amount of refrigerant based on bore diameter, and of course the liquid line and evap coil pressure.

__________________
How many times must one fix something before it is fixed?

RoBoTeq

That is not a failed piston, that is a contaminate blockage which would have had the same result with a TXV eventually. Also, not all TXVs have prescreens.