[stevehull]

There is no set amount of short cycling. It all depends on the outside temperature and the difference between that and the conditioned space (delta T).

For example, consider a 24,000 BTU single stage heat "machine" (resistance, gas, geothermal, propane, whatever). Assume the neutral point for heating is 65 degrees F in a well insulated 2400 sqft home.

At a moderate heating load (outside temp of 40 F), the duty on time might be 10 minutes on 20 minutes off. But at zero F (large heating load) this might be 30 minutes on and 1 minute off. Assume that zero is the design low temperature.

Yes, this heating unit would "short cycle" when it was 55 degrees F out, but for a single stage, non-proportional unit, this unit is perfectly sized as at the maximum load it is on virtually 100% of the time.

However, if it is zero out and the unit is not on at 100%, then waste heat is generated and/or you have paid for a unit too large for your application. If it is purely resistance heat, there is virtually no standby loss, but if there is any vent, then there is standby loss with decreased efficiency.

Many well intentioned HVAC installers put in too large a heating system as they think that it will "heat up the house faster". It may do that by 1-2 minutes, but at a large waste of efficiency.

Newer gas furnaces (and some electric resistance heaters) use modulating gas values (variable ac amps for resistance heat) to keep the furnace on for as long as possible. This minimizes the heat blast and then cool down so despised by many on forced air heat.

For AC, the same is true. Too large a unit does not run for long times thetreby removing latent heat (condensation). The thermodynamiocs of AC merans you remove water andf that phase change allows the temperature to decrease. But if too large an AC unit it too will short cycle and cool dowen too quickly without removal of latent moisture. Mold is the common result (where I get called in).

The ideally sized heater/AC will run 100% at the design loads for your climate.

The new variable speed (10-120%) Copeland compressors in some new geothermal heat pumps allow tremendous SEER and COP (50+/6+) by maximizing run times.

So, check the duty cycle on times on your coldest night (about now in January) and I can discuss later how to modify natural gas/propane units to increase the duty cycle on time and thereby increase your heater efficiency.