Tag: thermostat

carrier_defrost_thermostat

When you work on a heat pump system and you want to test defrost there are many different test procedures to follow to test the board and sensors.

Most involve “forcing” a defrost by shorting out pins on the board or advancing the time of the defrost initiation and installing a factory provided pin jumper.

Lots of pins and jumping involved.

But one thing to need to be able to distinguish is whether the system uses sensors or thermostats to initiate and terminate defrost.

A thermostat is an open and closed switch, they are usually round in shape like the one shown above and they open within a set temp range and they close within a set temp range. The one shown above is a Carrier Defrost Thermostat and it closes at 30 degrees +/- 3 degrees and it opens at 65 degrees +/- 5 degrees. In this case, because this particular sensor closes in colder than 32-degree temps you can’t even use a (freshwater) ice bath to test it.

If it is below 32 outside it is easy to test (duh) otherwise you can just run it in heat mode with the outdoor fan off and see when it closes by using an Ohmmeter and testing against a line temperature clamp in the same location.

On a defrost thermostat you can also easily jump it out to test the board since it is just open and closed.

A defrost “sensor” is generally a thermistor. A thermistor changes resistance based on the temperature it is exposed to. In order to test you can measure the ambient temperature, make the sensor is removed and acclimated, measure the Ohms of resistance and compare to the manufacturer chart.

Thermistor

You CANNOT jump out a thermistor with a typical jumper to test.

— Bryan

P.S. – A podcast about Heat pumps is available HERE

 

Sometimes you find yourself in a position where you are going to replace a fancy thermostat with a simple one. It may be because the customer got fed up with all the options or because you are there on a weekend service call and all you have is a basic stat.

No matter the reason you need to make sure the new thermostat can do the job the old one did before you quote, an option that gets overlooked in matching up is dehumidification.

Most manufacturers of residential variable speed air handlers have a terminal that will drop the blower speed when de-energized. It may be marked DH or D or dehum or something else. From the factory, they generally have that terminal connected to R using a pin or jumper so that the blower will run up to full speed. When one of these special thermostats get installed the tech is supposed to remove that jumper or pin and connect a wire from that terminal to the thermostat dehumidification terminal so that the thermostat can energize the terminal for full blower speed or drop 24v to the terminal to go into dehumidification.

If we install a new thermostat and forget to reconnect that pin or jumper then the system will ALWAYS run in dehumidification mode because there will never be any power on that terminal at the air handler board.

The lesson is to pay attention to whether or not a system is wired for dehumidification. If you do need to replace it with a basic thermostat make sure to replace the pin or jumper (J1 on the board example above).

If you forget to do so the system will run less efficiently with lower airflow, suction pressure and coil temperature.

— Bryan

 

I got this question via email (edited slightly for length)

Some things I’ve done because I’ve been taught to do them yet I don’t know why I do them. One of those things is putting a jumper between w1/e and w2. Sometimes, in the case of a Goodman for example, I’ve been taught to combine the brown wire along all the whites at the air handler. Do you mind just clarifying the whole situation with w1/e jumped to w2? And also maybe x2 on some stats? Thanks for your help.
— J

Back in the early 2000’s when I was the lead trainer for another company some of the most common miswiring issues has to do with electric heat. So much so that I created a bunch of different wiring diagrams with a fancy program called “Microsoft Paint” to illustrate how to wire different combinations of equipment. Here is one of them.

In older thermostats (older than the diagram shown here) there were no installer setup programs in the thermostat where you could designate the type of system the thermostat was connected to. Each terminal performed a particular universal function and you would configure the operation based on how you wired it up. Which terminals you connected where, which you left open and sometimes, which ones you jumpered out.

So first, let’s give a quick look at the meaning of each terminal

W – When you see a W terminal it just means heat. Usually, you will only see W when the control only has one stage of heat

W1 – Means first stage heat. In a heat pump first stage heat is the same as the first stage cool. It just means the contactor/compressor is turning on. Whether that is heat or cool is actually dictated by whether or not the O/B terminal is energized. This is why on many old thermostats you would jumper Y1 and W1 in a heat pump application.

W2 – Means second stage heat. This could be the first stage of heat strips in a common southern heat pump, the gas furnace backing up the heat pump in a modern “hybrid heat” application or just a second heat strip bank in the case of a straight electric system. W2 is generally called on based on a temperature differential between setpoint and space, outdoor temperature and/or run time.

W3 – Is just the next stage of heat after W2

E – Is emergency heat, usually just a way to manually drive on what would normally be the secondary form of heat without stage 1 heating.

Emergency heat only makes sense when there is some sort of secondary heat source and really even then, it only helps if the secondary heat source is sufficient to heat the space as in the case when the secondary is a furnace, Hydronics or a large heat kit. In Florida, most of our units have 5KW auxiliary heat which will never be sufficient to heat a home in an “emergency”.

Many of these other terminal designations are a holdover from a time when all the controls in the thermostat and defrost board were electromechanical, and much of it was for indication/trouble lights and some of it was for the thermostat to be able to perform staging based on outdoor temperature due to the fact that run time logic was not available. So for your X2 question, have a look at the thermostat and diagram below.

In the modern thermostat, they have usually relegated these staging configurations and terminal designations into the installer setup and every thermostat is a little different. In general, in the south we jumper w2 to E because they truly are the same, in some cases, this does nothing, in others, it just ensures that the aux heat comes on quicker if the user chooses emergency heat.

Are there some cases where emergency heat could be totally different than aux heat? I’m sure… I have just never seen one personally. Like usually, it all comes down to knowing your particular piece of equipment and your controls, reading the installation instruction is a good first step.

— Bryan

 

 

Does setting a thermostat too low directly cause an air conditioning system set in cool mode to freeze?

The answer is, no, at least not directly.

However, low evaporator load (low return temperature or low airflow) and low outdoor ambient temperature can both lead to evaporator coil freezing. Low indoor set-point can lead to low return air temperature which is a form of low load condition that can lead to coil frost accumulation on air conditioning systems… but it is actually pretty rare.

It isn’t the act of setting the thermostat too low that causes the freezing, it is only when the return temperature drops below the acceptable limit that the freezing can occur.

There is an important distinction we need to make before you go any further. Just because low indoor temperature or outdoor temperature CAN cause freezing, that doesn’t mean that it is the actual or only cause of freezing.

Many units are misdiagnosed as freezing because of these two causes when the actual cause (low airflow, metering device issues, drier restriction etc..) are left undiagnosed.

When a system is found frozen, it must be fully defrosted and tested for other issues before a conclusion is made that low set-point or low outdoor ambient were the root causes.

Most air conditioning systems are set up for around 400 CFM per ton of Cooling (but not always).

This will generally result in a 32° to 38° DTD (design temperature difference) on the evaporator coil with 35° being the typical standard. This means that the evaporator coil will be 38° to 32° colder than the return air that passes over it and ice will begin to form at 32°.

Based on this, return temperatures of below 70° begin to enter a zone where freezing becomes possible (on systems set up for 350 CFM per ton for example). At a return temperature of 64° frost formation on the evaporator coil becomes likely.

Again, this is due to the return air temperature not just the thermostat setting.

Just because the thermostat is set to 67° doesn’t mean the return temperature will achieve 67°. In the case of a supply to return bypass damper as is common in some residential zoning systems, you may see a low return air temperature even when the thermostat is set normally.

Systems that have long run times and high humidity return air are the most likely to freeze the coil due to low return temperature caused by low thermostat set point. For example in Florida, I have seen vacation homes on a rainy Spring day where a vacationer sets the thermostat to 50° in cool and leaves for the day… that makes for a nice frozen coil when they return because the combination of low sensible load (outdoor temp), lots of moisture to freeze and an all-day continuous run time.

A typical residential system with no special low ambient controls or freeze protection shouldn’t be set below 70° indoors or run for a significant amount of time when it’s below 65° outdoor.

This isn’t to say that they WILL freeze if you set it colder or run when it’s colder than that, but given enough runtime it is possible.

It isn’t a system “running too hard” that causes it. It isn’t the set-point itself, it is the heat load on the evaporator coil that can cause the suction saturation to drop below 32°.

Once again, as a technician, never blame set-point until you have exhausted all other possibilities.

— Bryan

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