A/C

Mark J Strawcutter

masterplumb

Johnson control 66

I was repairing an a/c condensing unit the other day, and there was a control that was inside the unit that Ive never seen. It was a blue box and it said Johnson controls "66". I have decent a/c knowledge but I am by no means an expert, but have serviced quite a bit. There is a capilary tube that comes out of this box and ties into the valve on the high side on the condensing unit. My guess is that this is some kind of fan control? but I am not sure. What does this control do and what is its purpose. There is also a dehumidifier system that the boiler heats into a coil in the duct.I know it isnt hydro air because there is a stat for this and there is baseboard heat in this zone and im not sure if this has anything to do with that Johnson control. Any input or direction would help.Chris

Don_4

Fan Speed?

Check this link. It indicates the control is on a three phase system and controls condenser fan speed. http://www.johnsoncontrols.com/cg/html/refrigeration_controls.htm

Bob_19

Lets see

not to sure on the model number use, but if it's on the high side there is a very good chance it's for head pressure control. Could be condenser fan control, or HPCO for compressor.
What area of the country are you in? Is this a residential split system?

masterplumb

Info

I am in Westchester County, New York. It is a residential home with three seperate systems. As stated this unit takes care of the finished basement and the Johnson 66 control is inside the access panel on the condensing unit. Why would you modify the speed on a condensing unit, if this is what it does? Also there is a coil in the duct near the air handler,on its own zone from the boiler, that is some kind of de/humidification system, (not hydro-air) my guess is it takes care of mold. From the zone on the boiler it goes into some kind of very large (honeywell)mixing valve with line voltage going to it,(cant see any numbers) and into the ductwork. Thanks for the help with this guys. Chris

joe_17

I would suspect you would have a control to maintain proper head pressure when unit is running and outside temps are low.High inside heat load but low outdoor temp.,therefore cond. fan is cycled to maintain proper head press. Sounds like you are talking about a large residential or small commercial unit.Opps you did say residential home.It must be large.

Wayco Wayne

With low outside temps

and the A/C running you could have too low of a head pressure. Look in the Sporlan catalog, it recommends you never go below a pressure difference of 100 psi for expansion valves. The evap coils could begin icing up. WW

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Tom Anderson

What you have is a Penn P-66 head pressure controller. Penn was purchased by Johnson Controls a few years ago.

The P-66 senses condensing pressure, and modulates the condenser fan speed to maintain head pressure within a defined range. As the head pressure increases, the fan kisks on at about 35% speed. Upon further pressure increase, the fan speed increases. Sometimes refered to as floating head pressure control.

The purpose of the P-66 is to maintain appropriate head (condenser) pressure under a variety of load conditions as well as variable ambient conditions, thus keeping the compressor "happy".

It reduces the operating cost of the condensing unit by keeping the head pressure low when suitable ambient conditions occur, reducing the AC compressor lift, thus enhancing the compressor efficiency (EER). It also reduces condenser fan power... which is small, but can ad up in a years' time.

Hope this helps.....

Dale

The other guys

Other posts are probably correct about keeping up the head pressure. We see these controls on smaller eateries where you still need AC when it's 60 or so outside or even colded, first try is to drop the condenser fan heat exchange as the outside temp drops, if theres cold wind which acts like a fan sometimes a shield is built arround the condenser.

masterplumb

Thanks for your help guys (nm)

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flange

as all have stated this is a fan control. it is installed to allow operation in low ambient conditions. this is a very common application anymore.

Anthony D.

Your probably right - it sounds like someone modified the unit so it could run during low ambient conditions. Most residential and many commercial systems dont run during cold weather, but if theyre using this system as a large central dehumidifier, it will run regardless of indoor or outdoor temperature. These systems usually have a dehumidifier to start the air conditioner, and a heating thermostat to reheat the air temperature so the space doesnt get uncomfortably cold while removing the moisture from the air. This operation is very common in commercial bldgs where there are large main frame computers, which require the temperature and humidity to stay within close limits.

FYI - From what Ive read, the first air conditioner was develop by an engineer named Willis Carrier. Carrier was hired by a company to solve a humidity problem they were having with a paper plant. The system he designed to solve this problem developed as a by-product - cold air. Hence, air conditioning was born.

Hope this helps.