Year: 2019

In residential air handler/fan coils it is common to use a high voltage interlock between the blower and the electric heat strips to ensure that the blower comes on whenever the heat is on.

The problem is that it CANNOT work the other way around where the heat comes on with the blower.

Heat strips are generally going to draw 20+ amps depending on the voltage and KW rating which means you CANNOT power them through a typical relay like a blower relay or board which are generally rated for 15 amps or less.

The way the interlock is wired is really quite obvious but is easy to forget because it’s the reverse of what we are used to with a relay.

In short, we connect the blower to the “common” terminal on the relay, L1 power to the normally open (n.o.) terminal and the load (out) side of the heat strip contactor /relay/sequencer to the normally closed (n.c.) terminal.

This diagram from Carrier shows the blower connected on the common terminal and constant power coming in on black to the normally open terminal from the right side of the transformer primary.

Using this 90-340 relay as an example, the blower would connect to 1, power to 3 and the heat strips to 2.

I made a video on it as well if you need it. The result is that the blower runs with the heat but the heat doesn’t run with the blower.

— Bryan


In order to maintain combustion (burning) you need three things, fuel, heat and oxygen. If you have all three in the proper proportion you can maintain a continuous state of combustion.

Remove one (or reduce one sufficiently) and the triangle of combustion can collapse.

In a common NG gas furnace the heat is the igniter, the fuel is Natural Gas and the oxygen is provided by combustion air.

Combustion air is literally just the air needed to provide a continuous supply of air for proper combustion (burning). In the case of burning fuels like natural gas our goal is to achieve complete combustion where the end products being vented are CO2 and H2O and this requires the right mix of air and fuel.

For perfect combustion you need about a 10:1 ratio of air to fuel with safe levels of extra air or “excess air” putting us more into the 13.5:1 to 15:1 range.

All gas fired appliances must have both a flue / chimney to exhaust the leftover products of combustion (outlet) as well as combustion air to provide the oxygen for burning (inlet).

In high efficiency furnaces the combustion air is generally piped in, directly from the outside straight into the combustion chamber. This creates a dedicated source of oxygen and also a cleaner install as no other provisions need to be make for combustion air.

In 80% furnaces the burners usually have “open” combustion and they rely on air being drawn into louvers on the furnace cabinet. In this design the space on which the furnace resides must have open communication to the outdoors or other “uncontained” space.

The International Fuel Gas Code requires the following combustion air openings for a room containing combustion appliances:

Vertical opening – One-inch free area for each 4,000 Btu/hr. input of gas burning appliances in the room.

Horizontal duct opening – One-inch free area for each 2,000 Btu/hr. input of gas burning appliances in the room.

Mechanical fan – One CFM of air for each 2,400 Btu/hr. input of gas burning appliances in the room.

Indoor air –  50 cu. ft. of area for each 1,000 Btu/hr. of the appliances.

Not to get into the specifics of code becasue there are lots of specifics that you need to pursue beyond a tip like this, but you must have a dedicated method to get significant air to the furnace to ensure safe and complete combustion.

If you do not, the real possibility exists that the furnace could begin burning improperly creating an unsafe condition for the occupants due to Carbon-monoxide.

Different parts of the country provide combustion air in different ways, but you MUST have some method of providing unlimited fresh air to a furnace or to the room in which the furnace is located. This means when a furnace is in a tight space, ensure you have some sort of significant combustion air.

— Bryan

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

belly band crankcase heater

When I first started in the trade as an apprentice we worked on a lot of Trane heat pumps that used crankcase heaters that slid into the compressor sump on the big orange Tyler reciprocating compressors like the one below.

It was very common for these heaters to break off where the wire entered the rod and short against the bottom of the condensing unit. Some of the old timers I worked with would say “This is Florida, we don’t need those things here”, disconnect it and move on.

I later learned that isn’t the correct approach

Systems that have crankcase heaters, have them for a reason and while outdoor ambient temperature is one factor it isn’t the REASON crankcase heaters exist. Refrigerant is attracted to the refrigerant oil in the compressor when the system goes into the off cycle, the amount of refrigerant in the oil and the rate at which it moves into the oil depends on the type of refrigerant and oil and the temperature of the compressor.

When the compressor is off for a while a significant quantity of refrigerant can migrate to the compressor and condense. When the compressor comes on the refrigerant rapidly expands and foams the oil, forcing it out of the compressor and into the system. This is called a “flooded start” and will eventually result in compressor damage due to lack of lubrication, it also decreases system efficiency due to the oil in the system inhibiting the transfer of heat.

Strategies like hard shut off expansion valves, liquid line solenoids help to keep liquid refrigerant out of the compressor and oil separators help to keep the oil in the compressor and out of the systems but the trusty old crankcase heater is still a simple and commonly used strategy to prevent flooded start. If you find one that is failed you would be better off replacing it instead of taking the word of techs who tell you just to cut it out, like I once did.

— Bryan

 

 

 

Take a look at the specs from this Copeland scroll compressor pulled from the Copeland Mobile App (which is an incredible app by the way).

This is a single-phase compressor so the amperages listed are based on an amperage reading from the wire connected to the common terminal.

LRA is locked rotor amperage which is the expected measurable starting amperage and RLA is rated load amps, meaning the amperage it will draw when running normally at its rated load. You may wonder why there are two different RLA ratings here… that’s not what this tech tip is about but if you get the app and click the i with the circle around it you can find out.

The point is we are always taught to measure amperage on common with single-phase motors, but do you know why?

A single-phase motor like the one shown above has three terminals (Common, Start and Run) but only two actual windings (Start and Run). The common terminal is just the “common” point between both of the windings so when we measure the amperage on common we see the total current of both windings.

In tradeschool we learn Ohms law which teaches us

VOLTS = AMPS X OHMS

However, when we try to apply that in the field we realize some things pretty quick that get in the way of applying that neat little formula

Namely –

  • Voltage (and therefore amperage) isn’t fixed in an alternating current so we measure RMS values not ACTUAL peak values
  • The total resistance (impedance) in an inductive (magnetic) load isn’t fixed and is a combination of the static resistance of the windings and the inductive reactance that builds as the magnetic fields expand and collapse and as back EMF is generated when motors spin.
  • Even in a simple DC light bulb circuit we cannot simply measure the resistance of the bulb with a meter and apply ohms law because the resistance of the filament increases as the filament heats up (try it sometime).

So to summarize….

YOU AREN’T GOING TO BE ABLE TO ACCURATELY APPLY OHMS LAW IN THE HVAC/R FIELD

When we measure the ohms of windings from terminal to terminal it is mostly meaningless because the readings are often very low anyway… sometimes so low that your meter becomes inaccurate.

Notice how low the resistances are of this same compressor.

The real resistance of the motor only shows up when it is energized with alternating current and the magnetic fields begin to interact, this total resistance when energized is called impedance.

We do know that the start winding has a higher static ohm value than the run winding and that when we add start to common and run to common together that it will equal run to start (which is a fairly obvious statement since common is just a center point) and that if the thermal overload is open we will measure OL between C-R and C-S but will read the combined value R-S.

These are all true and are reasons to pull out the meter but this still doesn’t tell us anything about the title of this article and you are probably wondering what the heck I’m driving at.

I’m making sure we are all on the same page before I drop a start winding fact bomb on you…

But one more thing we need to come to an agreement on.

The run winding is connected “Across The Line”, in other words with one leg of split phase power connected to Common and the other to run. The current that travels through that run winding is completely a function of the total impedance of that winding which has several factors including the static winding resistance, the inductive reactance of the windings and the back EMF that builds as the motor starts running.

In other words… the amperage starts high because the resistance starts low in the run winding and amperage goes down as the motor gets up to speed because the total impedance increases.

Remember, ohms law teaches us as resistance goes up, amperage goes down if the voltage stays the same.

The start winding is connected through a run capacitor and potentially some other start gear and not connected “across the line” like the run winding. This means that the current that moves through the start winding is limited by BOTH the total impedance of the winding AND the capacitance of the run capacitor and any other start gear.

 

Here is an image from an oscilloscope on this very same compressor referred to above with 197V applied, a proper run capacitor and no hard start kit…

Take a long close look.

Notice the blue line is the RUN WINDING CURRENT and the red line is the START WINDING CURRENT.

Notice that ALL of the true inrush current occurs on the run winding and the start winding current doesn’t go up until the run winding current starts to go down?

That’s because unless the start winding has some form of start capacitor it cannot draw any amperage higher than what the run capacitor will allow. In essence, the run capacitor becomes a ceiling or current limiter that allows only so much stored current per cycle and no more.

Try it sometime.

Measure the running amperage on the start winding with a capacitor slightly larger, slightly smaller and then with none at all. You will see higher amps, lower amps and then (obviously) no amps.

Try taking an inrush reading on the start wire of a compressor with no hard start and see what you get.

Then try it with a hard start.

Notice anything different on the start winding amps? Can you see the moment the back EMF removed the hard start from the circuit? Was the TOTAL amperage actually lower with a hard start or was the time to start decreased and more current shifted to the start winding?

— Bryan

 

 

 

 

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