I get emails from time to time with questions that stem from the articles or the podcast. This was a great question, but I was not the best person to answer it.

I reached out to Jeff Neiman, our resident HVAC School chiller tech and he answered it. Here is the question

Hello Bryan,

Thanks for all the good material you provide. I mostly work on the commerical building side of HVAC where chilled water is used as cooling medium and cooling towers provide condenser water. We have chillers as well as heat pump and air cool splits throughout facilities. Most of your diagnostics and troubleshooting methods are for air cooled units. Can they be applied to water cooled evaporators and water cooled condensers? My thinking is yes and no, because with cooling tower 85 supply and return 95 is maintained and 45 supply and 55 return chilled water is provided. Since there is not much change in these temps as opposed to outdoor ambient temperature there won’t be much pressure change in condenser. And as long water is regulated at proper flow to evaporators and condenser then all should hold steady. Do you have any input on this? I’m in NYC. Went to 2 year hvac school and worked almost 3 years in field starting out as a helper in service van as experience and learned as much then got into the building side for about 8 years now. I like listening to your podcast and reading your material as it keeps me refresh with field work as the building side is a little different but the basics and fundamentals are the same. Thanks!
–Anand

Hey Anand,

The answer is yes.

Some of the measurements can be applied to chillers as well. Just some of the verbiage is different and the values differ.
The numbers for chilled water (44 out 54 in) and condenser water (85 out 95 in) are industry standard values at full load conditions. Most chillers regardless of manufacturer will have a 10*F delta T on the cond and evap. Machines that operate outside of those ranges are chillers that were ordered specifically to provide a lower temp or larger delta T.

Many people look at the compressor motor RLA% as the chiller capacity, which is not accurate. Chiller capacity is measured by the evap delta T. If the chiller is designed for 10* delta, and is currently providing 44* water and the return water is at 49*, the delta T is 5*. So that chiller is currently running at 50% of its total capacity.

Subcooling is still measured the same, although the reading that you get will change as chiller capacity changes. At low loads your subcooling will be lower and will increase as capacity increases.

Suction superheat is a value that I really don’t look at because the reading on a flooded type of system will usually be very low or even 0. Rather discharge superheat (discharge temp – cond sat temp) is a more accurate reading and will be a direct result of you suction superheat. High suct SH, there will be high dis SH and vice versa. Again, this value will change as chiller capacity changes.
However if the chiller is a DX type, the suction superheat is just as valid as on a residential system

One of the values that was described in the podcast was temperature difference (supply air temp – coil temp).
In regards to air handlers with chilled water coils you can do the same thing. Measure your supply air temp minus the coil leaving water temp. This will tell you how well the heat is transferring to the water from the air going across the coil.

In chiller lingo this measurement is called approach

There are two different approach temps that i look at on a chiller:

Condenser approach (cond sat temp – lvg cond water temp)
Evaporator approach (lvg water temp – evap sat temp)

Approach values should range in 0 – 3*f, given that your flows are correct.
Just like on air cooled units where proper airflow is needed across the evaporator and condenser, you need to verify that you have proper water flows.

In air to air applications you are measuring static to identify airflow issues. In water applications, I’m measuring pressure differential across each barrel. If I know my design pressure drop on the evap and cond, I can compare to my actual to know if my flows are proper. Keep in mind though that most chiller manufacturers will give the the design pressure drop in ft/hd. You will need to convert your real time reading to ft/hd to have an accurate comparison if you are using a gauge with a psi scale.

Even if your water temps stay pretty constant while in operation, your pressures will veer off as problems arise and your approach values will increase.
The chiller will always try to maintain at 44*f chilled water out (or whatever the setpoint is) as long as it can do so.

The refrigeration cycle doesn’t change, stick to the basics and don’t over think it

When running building, try to get your condenser water as low as possible when running. But stay above 65*.

Anytime you can provide condenser water lower than the design of 85* you will lower your condenser pressure and lower the lift (cond pressure – evap pressure). This will result in less work the compressor has to do and lower KW. This is a common method called condenser relief.

— Jeff Neiman

Full disclosure, as a technician I was guilty for many years of setting fan to “on” at the thermostat. I never really thought of any of the negative impacts that could happen.
I wanted to circulate the air, and to keep air moving through the high efficiency air filter that most of our houses had. Later I learned that in many scenarios fan “on” is not a good idea.

For this discussion I will be talking about the cooling season in a humid climate. Many adverse impacts may occur in the heating season, depending on the region.

Things to be aware of when running the fan “on”.

Condensate on the coil after a cooling call with the fan running will evaporate back into the living space. Some thermostats combat this by having a fan off period at the end of a cooling call to let the coil drain.

If the ducts are in unconditioned spaces, outside the thermal envelope of the house, the sensible heat will be added back to the space. If the ducts are warmer than the air traveling through them there will be a transfer of heat. If there is any duct leakage latent heat (moisture) will be added as well. Latent heat gains do not only apply to return duct leaks. Supply air leakage can also contribute to this.

It is common that the HVAC system can cause the house to go into a negative pressure. When this happens sensible and latent heat will be added. A common cause of the pressure imbalance is when the duct system is in an attic or crawl space, and the return duct has fewer connections than the supply ducts.
Since the supply has more connections than the return there is more of a potential to leak air. If the supply air leaks into the attic or crawl space this can cause the living space to go into a negative pressure. The leaked air is replaced with either attic, crawl space, or outside air. One CFM in = one CFM out.

Ducts in conditioned spaces with panned returns can add latent and sensible heat as well. This happens when the panned joists and studs are not sealed by the HVAC contractor on all six sides. Joists and studs are part of the building network that when not air sealed during construction, by the builder, they will communicate with air outside the building envelope. With blower door testing, and air changes per hour requirements now code in many jurisdictions houses are being built much tighter. In an older home, with panned returns, expect to be bringing in some outside air.

Even if the ducts are sealed, and 100% in the conditioned space it still costs money to run the fan. Running a PSC motor 24/7 can be costly. (ECM motors on a property sized duct system do have considerably lower operating cost when compared to PSC motors.)

The duct leakages and pressure imbalances mentioned above will also occur during a cooling call. Most of the time these issues can go unnoticed because of the ability of the HVAC system to overcome or mask them.
The goal is to get maximum customer comfort with minimum power usage and maximum system longevity. In many cases the fan being left in the ON position detracts from these goals.

Hopefully this Tech Tip will make you think twice about running the fan “on”. Every situation is different. I encourage you to think outside the box, if you are not already.

— Neil Comparetto

We’ve seen it before.

A tech diagnoses a failed blower relay or board so they leave the blower jumped out by putting a terminal multiplier on the common terminal of the relay / board and connecting the fan speed tap right to power.

There can be an issue with that.

Some electric heat fan coils have a heat / blower interlock where the heat relay / sequencer back feeds and brings on the blower across normally closed (NC) contacts. The purpose of this is to ensure the blower comes on with a heat call without the need for a G call.

In some circumstances when you put in a terminal multiplier and constant power the blower, the G call sends that constant power back to the Heat strips and brings them on.

Not good… high power bills, melted wires, fire, death and stale doughnuts.

So, if you are leaving a blower jumpered out to run constantly I advise doing it separate from the board completely.

Coincidentally the photo at the top is a setup that will not back feed because it uses two isolated circuits on the Heat sequencer for fan and heat…. so the photo I chose wasn’t the best. Cut me some slack, the blower assembly was sitting right behind my office.

— Bryan

See the photo above? This is a unit we (my company) recently serviced for a commercial customer. 

It doesn’t matter if we aligned the belt, dialed in the charge, cleaned the condenser and got the drain pan clean. We look like dummies because the panel fell off.

It doesn’t matter that we’ve had some crazy storms or that some of the screws were stripped out long before we got there. What matters is that we serviced it and the panel fell off.

Some of you will roll your eyes that this is even a tech tip. But if you are honest, how many units have you left that didn’t have ALL the screws properly in place. How many times have you left a unit where one of them are so stripped out that the screw was doing nothing?

So, the primary message is

Don’t leave unless all designed fastening points are secured

This occasionally means tapping in a new screw, sometimes in a new location. Sometimes it may mean running to hardware store to get a fastener thats lost. Just take care of it properly and take pride in the finished product of your service. 

While on the topic keep in mind that screws left on a roof or in the grass can cause roof damage or get picked up by a lawn mower and thrown into a car or another person. It isn’t just the panel that comes off that lead to property damage and a safety hazard, it is also the screw itself.

Now… there is something else to consider. The use of impact drivers and drills with no clutch or the clutch set too high has resulted in a big increase in stripped out fasteners. 

An impact driver (like shown above) is meant to DRIVE screws either in repetitive or high torque applications. Impact drivers are designed with a “percussion” action that drives screws quickly and forcefully into the base material. That high torque action also does a great job of stripping out screws.

A driver like the one shown above does not have the TORQUE of an impact but it turns screws and fasteners with a smooth motion without the percussion of an impact. It also has a clutch that should be set as low as possible to get a snug fastener without the risk of striping out.

For the average HVAC/R technician I would advising using a clutched driver as your primary “go bag” tool and only reach for an impact or larger drill if you are driving screws repeatedly into new material.

Using the right tool consistently can make keeping panels firmly in place an easier task and avoid embarrassing situations like the one at the top.

— Bryan

P.S. – you can get a great discount on the Milwaukee driver shown above by clicking HERE and using the offer code getschooled at checkout