Hot Water Recirc Booster Heaters Simplify Hotel Commissioning

Summary: A small water heater in the recirc line is a simple way to avoid low flow recirc temperature drift when the storage water temperature is greater than 120 degrees.

Hot water mixing valves function effectively when hot water is in heavy use and cold water is mixing with the hot water.  However, when there is no net usage of hot water, such as at night in a hotel, the recirculation line can become progressively warm until it reaches the temperature of the hot water storage tank.  This temperature is often at 160 degrees, and the first user of hot water in the morning gets a slug of extremely hot water.  Of course, the common answer is to configure the mixing valves properly and balance the low flow condition.  The flaw in this concept is that many plumbers are not experienced in performing this work and it falls on the shoulders of the engineer to come to the hotel during the first few weeks of operation and perform the balance.

But there is a fool-proof alternative that requires no guesswork.  The method is to place a small hot water heater in the recirc line to compensate for the temperature loss in circulation.  Since the heater has its own internal thermostat, the only commissioning effort required is to set the water heater thermostat to 120 degrees.

But what BTUH rating is needed for the hot water heater?  The answer is not easy because it depends on the heat loss rate of the hot water piping in the path that is being recirculated. One could compute the heat loss of the piping, but that is an arduous task.  The approach I use is to start with the basic assumption that the recirc flow rate will be designed to achieve a return temperature with maximum temperature drop of 15 degrees.  This matches real experience where we supply 120 to 125 degree water with a return temperature of about 110 degrees.   Since we all have experience in selecting the flow rate in the risers to achieve a delta T of about 10 to 15 degrees,  this approach has a practical starting point as compared to computing the heat loss of a piping system.  So multiply the total circulation flow rate by 15 degrees and by 500.  For example, if the hotel has 20 risers with 1/2 gpm per riser, the total flow is 10 gpm.  This results in 15 degrees x 10 gpm x 500 = 75,000 BTUH.  That is a small water heater if it is gas, or a 20kw heater if it is electric.

Aside: It is interesting to think about the implications of this heat loss as it relates to energy waste.  This heat loss is not only a direct loss of heating energy, but it also requires constant mechanical cooling to remove this wasted heat.   So this is a double argument for better hot water system insulation.

One Response to “Hot Water Recirc Booster Heaters Simplify Hotel Commissioning”

  1. Two Zone Hotel Recirc Systems | Mark Robison, P.E. Says:

    […] source of heat is an electric or gas hot water heater.  Refer to Hot Water Recirc Booster Heaters Simplify Hotel Commissioning for sizing this auxilary […]

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