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The argument for water heater sequencers

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#1 Philip Clausen

Philip Clausen

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Posted 17 April 2012 - 08:20 PM

… about this project
The argument for water heater sequencers

For our multiple water heater system, short cycling has been reduced or eliminated by implementing an automatic control strategy that maximizes overall efficiency and proper load distribution. Proportional, integral and derivative (PID) logic staging makes sure each water heater is operated ONLY as needed. Thus, the supply is matched to the demand – (i.e. capacity = heat lost from the building + the hot water usage). Priority has been placed on those heaters (3 of 9 total units) that have a turndown ratio of 5:1 to further reduce burner starts and purge air requirements.

Features and Benefits include but are not limited to:
Evenly distributes wear and tear - i.e. all units can be arranged to run an equal amount of cycles and / or time

Reduced fuel consumption through short cycle elimination

By incorporating time of day based logic, the installed system anticipates demand rather that reacting to it

The installed system “fails ON” so that there is no risk of control failure causing a shortage of hot water

A setback temperature is employed to compensate for the loss of control that our mixing valve exhibits during low flow conditions

A ‘hobbs’ meter has been designed and incorporated in the software that operates this control system – the purpose is to record the cumulative run time (with 1 second resolution!) and the cumulative cycles for each of 9 units. The total operational cost can be easily calculated based on current fuel cost and this total can be subtracted from the billed cost for this meter … what remains is the cost of cooking
We can’t control what we don’t measure!

Domestic hot water systems typically have a design capacity that ensures an adequate supply of hot water on the peak day of the year i.e. minimum cold water temperature from the supply source and maximum demand for hot water. As domestic hot water requirements change, (low occupancy, higher incoming water temperatures, time of day etc.) water heaters cycle on and off. When a water heater meets the desired load too quickly or is off for only a short period before turning on again, this is known as short cycling. In other words, the unit will short cycle when the demand is less than the maximum design conditions. Our system is sized adequately to meet the maximum design conditions – typically less than 2 hours in the morning - (our 9 water heaters operate continuously without cycling during peak demand times), the majority of the time our system needs to meet only a small partial load due to low demand. Short cycling during these none peak times is a symptom of ‘over capacity’.

Short cycling results in excessive heat loss up the exhaust stack during the pre-purge and post-purge steps of the water heater cycle as the purged air is heated and forced through the heater to remove any remaining combustible gas mixture. Purge temperatures can be as high as 100+°F.

A short cycling water heater is typically 10% to 15% less efficient than a normal operating water heater at low firing rates – only 3 of our 9 water heaters have the ability to vary their firing rates.

Short cycling can also drastically reduce the life of water heater components, such as gas train parts and combustion air motors, from years to months. For example, if a water heater is cycling on and off every five minutes, these parts are being subjected to 100,000+ cycles per year. Operating under these conditions, a water heater’s life could be reduced by a significant amount, especially since flame temperatures can be as high as 2,500°F.

Any and all feedback is welcomed!

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