DIYbanter - View Single Post - How to test a wall thermostat to see if it's actually working?

The Daring Dufas[_8_]

On 12/14/2013 8:55 AM, wrote:
On Friday, December 13, 2013 11:12:19 PM UTC-5, The Daring Dufas
wrote:
Danny you can switch to an electronic thermostat for very little
money and use the mercury switches from your old one for the next
bomb you build. The electronic thermostats have a built in time
delay for their operation which really protects your system by
preventing "hammering" of the control circuit. A time delay is
really more important for air conditioning since turning it on and
off repeatedly will cause a great deal of stress on the compressor.
I install a timer in AC condensers for customers who have an older
mechanical thermostat to prevent the compressor being turned on and
off repeatedly, I use a 3 minute delay.

That's a good point that I never thought of before. And now that I
think about it, you would think they would just have put the delay
into the HVAC eqpt to begin with. I guess the other side of the
argument is that if it was so bad for the compressors, you would
think a lot of them would have failed in the days of old. I wonder
what they actually did? Did they manage to start anyway or did they
have an overload that kicked out and recycled like refrigerators
had?

The contractor grade HVAC systems are made as cheaply as possible and
often lack all the bells and whistles of the more expensive units. I've
added anti short cycle timers, surge arresters, low pressure switches
and high pressure manual reset switches. The low pressure switch cuts
the control circuit power if the freon leaks out to protect the
compressor and the high pressure switch cuts control voltage if the high
side pressure goes too high like when the condenser fan fails or if the
condenser coil becomes obstructed because it's clogged with dirt, animal
hair or trash that gets sucked up against it. The expensive units
usually have everything except the surge arresters which I add to the AC
systems in rural areas because they are more likely to get power surges.
The AC system compressors have an internal automatic overload which pops
if the the compressor gets too hot or draws too much current because of
a locked rotor due to high pressure from the compressor being stopped or
slammed on and off by power blinks or someone playing with the
thermostat. With the anti short cycle timer, the system won't cut on and
off with power blinks or thermostat fiddling because every time the
control voltages goes off, the timer keeps the contactor from pulling in
until 3 to 5 minutes pass. I use the adjustable timers but potted preset
timer modules are also available with a 5 minute delay. On all of the
commercial AC condensers, I will also add a fan cycle control because
the AC systems are often run in the middle of winter. When the high side
pressure drops too low, the system will not operate properly because
there is no proper pressure differential. The fan control will not turn
on the condenser fan until the high side comes up to a proper pressure.
You will see the condenser fan turn on and off as the high side pressure
goes up and down. In very cold weather and a slight breeze, the
compressor is happy without the condenser fan ever coming on. ^_^

TDD