Category: HVAC


I have spent most of my career being afraid of hard start kits, I heard too many horror stories of start caps exploding and sales technicians telling every customer they need one.

It dawned on me recently that it may be time for me to take a more mature look at start capacitors, potential relays and hard start kits and find some best practices.

Let’s start with how they work.

When a compressor first starts up, it requires a lot of torque to get from 0% up to 75% of running speed, especially when it has to start under pressure load (unequalized pressures). A start capacitor is designed to create the optimal phase shift for that first 75% of synchronous speed. A run capacitor is sized to create an optimal phase shift for a compressor that is running at full speed and at full design load because the run capacitor never comes out of the circuit.

While a run capacitor has heat dissipation capability for constant duty a start capacitor MUST be taken out of the circuit VERY quickly to avoid melting down as well as causing compressor damage.

The start capacitor is REMOVED from the circuit by a relay called the potential relay. The potential relay is normally closed and it OPENS when a sufficient PICKUP voltage is present between the 5 and 2 terminals on the relay. This pickup voltage is potential (voltage) that exists in the start winding when a motor gets above about 75% running speed and it is GENERATED in the start winding by the motor itself NOT the capacitor. A capacitor DOES NOT boost the voltage, when you see that increased voltage across the capacitor that is back EMF being generated by the motor, just like in a generator (pretty cool huh?).

Some hard start manufactures wire the coil on the potential between start and common and some wire it between start and run. You will find that most OEM’s wire between start and common but this does not mean that wiring between start and run is bad… it just needs to be designed correctly for that purpose (Kickstart does it this way for example).

A properly sized start capacitor and potential relay are not BAD for a compressor, they just must be sized and installed correctly and there are some cases where they are more likely to be useful that others.

Cases where they may be very useful useful

  • Long line set applications
  • Hard shut off valves
  • More often on reciprocating compressors than scroll or rotary (but still OK on scroll and rotary when beneficial)
  • on 208V single phase applications

Things to consider

  • Mount the relay properly, there is a proper UP configuration on most potential relays
  • Use hard starts with REAL potential relays not timers, solid state or other relay types (in my experience)
  • Size the relay and capacitor according to manufacturers specs
  • Ensure that you have a good quality, properly sized run capacitor on any system with a hard start

For a complete write up on potential relays you can read these articles HERE and HERE


17 Mar

Is it possible to prevent smoke coming into the house and how much would it cost?

Findingmorty asks:

My neighbors smoke like fiends and I’m having breathing difficulties. As best as I can tell, they smoke outside and it drifts up and into the attic via a permanent vent on the side of the attic. Then somehow it makes it’s way into my room and the ventilation system. The A/C unit is on top of the house and come summer it will suck it in as well and blow it all throughout the house. Is there anything an HVAC professional can do about this and how much would it cost?

Can this issue be fixed? Yes, Will it be expensive? Yes. Your house is allowing outdoor air, which is contaminated with things such as your neighbors smoke, because your home is under a negative pressure and/or extremely leaky. The first thing I would do is find a reputable HVAC contractor who specializes in indoor air quality. They can examine your homes envelope and HVAC duct system and other venting systems like bathroom and stove exhaust to try to determine exactly how the smoke is making it into your living space. From there they should be able to recommend steps that can be taken to correct the home envelope. Another benefit to hiring a professional such as this is that many of the repairs needed to solve problems such this also benefit you by preventing the outdoor heat from entering your space in the summer which in turn can dramatically lower your electric bill. A less expensive option may be to purchase either a duct mounted whole home PCO type of air purifier or a standalone one. You will need one that either has an ionizer or PCO (Photocatalytic Oxidation) technology. Keep in mind that ionization will produce ozone so for some people can cause issues, however it is very good at getting rid of cigarette smoke. I hope this helps.

James Bowman, National Technical Manager, HVACR

13 Mar

Is this installation legal?

User botlit asks if this photo of an installation is illegal or not? What do you think?

Answer: As long as the vent on the regulator meets the distance requirements to doors, windows, fresh air vents, sources of ignition, etc. it should be legal. Copper can be used as long as it’s flared – or sweated with material that has a melting point above 1,000°F.
– jst1265

26 Jan

HRV issues/Condensation on ducts.

Stenas asks: I moved into this house in February and it was fairly cold last winter. We’ve had a bit of a cold snap and its been 14 to 30ish for the last week. I went downstairs and noticed a puddle of water on the floor and checked the HRV until above it. There is water dripping down the ducts.

I cleared out the closet and the water is dripping from under the duct insulation (grey wrap tapped at the ends, tape seal was/is loose).

I know this is an issue but is there anything I can do in the interim while I wait for a HVAC contractor to come out?

As an aside: the unit is a Venmar Constructo 1.5ES it hasn’t really worked since I bought the house. If you turn it on its stuck in high mode and you cannot use the singular on unit button to change it to low or panel controlled.

Is the unit not being on adding to the problem?


A: The condensation is caused by the surface dewpoint of the duct being below the dewpoint of the surrounding air. Insulate the duct with a vapour barrier. You could do it yourself.  – Deux1

12 Dec