13 Sep 2014

aging lennox rooftop unit short cycling in heat

HVAC Solutions DIY How To No Comments

aging lennox rooftop unit short cycling in heat

In this post I am going to share my experience with anyone out there that needs help with an difficult very common problem with the older Lennox roof top units. I will ad age and models but right now dont have that information.

Lennox made a very good roof top unit that was painted white and is installed on top of commercial buildings all over NY and maybe the world. On average they are 5 to 15 tons and they are built almost exactly the same regardless of size. They are the original installations and are all over the Elmford White Plains Rockland county area and if you have a free standing bank or public building anywhere in NY it’s also going to be on your roof.

The symtons

The symtons are in the winter you will have poor heating and if you can listen to the unit from the return plenum you will hear the burner cycle¬† on and off and never really staying on long enough to heat the space prperly when it’s very cold. Although the thermostat still wants heat and the fan continues to run. There are also problems with cooling but with the Lennox unit this particular problem shows it self better in the heat cycle. Although the ac is just as compromised by this condition.

You can expect poor cooling in the summer, freezing up, people adding freon. Water leaking thru plenum etc.

The solution

The solution to your problem involves real maintenance not just changing filters and belts like has been done for the last thirty years. The evaporator coil is clogged with dirt. When there is a call for heat and the burner cycles on, there is a lack of airflow caused by a dirty and partially clogged evaporator coil. Because it’s partially clogged a limit switch near the coil trips when the burner starts to reach temperature due to the very slow moving air. Shutting off the burner causing cold air to be blown into the space. If your in a Mack Cali suite in the Elmsford area then your problem is compounded by an open return common in that area. It was very common years ago to not install return ducts into the space below but to leave the return open to the ceiling and just put perforated tiles in the drop ceiling for some return air back to the unit. This type of installation is helpful to the building owner because it helps warm up the roof in the winter and it helps keep it cool in the summer but it is far from being beneficial to the tenant. The tenant may be paying double if not more for there utility bills then what it needs to be, most of those roofs are not insulated under the corrugated metal.

The good news is you dont need a new unit. Don’t let anyone say its just weak or needs a heat exchanger. We used to pull those out and never really find anything wrong. A little rust spot here and there, isn’t the problem! I worked for a company that changed dozens of those heat exchangers and left them on the roofs. Then when a lift could come in they had me go around and remove them. I would with out even a second thought put all of them back in the unit, many of those tenants had there heat locked out during the winter waiting the replacement. For a little rust hole so small you wouldn’t have been able to stick the tip of a pencil in.

The coil on the Lennox unit is not easy to get to and there is the chance a little water will leak down while cleaning it. But on a nice day start by removing all the panels. Then A hose must be brought to the roof, connect to a small electric power washer, get a bottle of Simple Green and find a caring qualified mechanic that will do a good job and not destroy your office. And after your finished you will have a unit that heats and cools just fine.

PS while the unit is open clean the condenser coils and the rest of the machine!

These machines were built like tanks and if properly maintained will last a very long time.


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