Two Zone Hotel Recirc Systems

Summary: Tall hotels require two or more pressure zones for domestic hot water.  Here are some approaches to the hot water recirc systems.

Hotels over 15 floors generally have two or more pressure zones for the domestic hot water.  These pressure zones are controlled with Pressure Reducing Valves (PRVs).  Assuming there is one hot water boiler and storage tank system, the PRVs separate the hot water storage tank from the hot water piping zone.  If a conventional recirc system is installed, the recirc pump must pump through the PRVs.  If the pressure drop through the PRV is 60 psi, then the recirc pump must be selected with a pump head to include the 60 psi plus the pressure loss through the system which is generally about 5 to 10 psi.  The required pump must then have a total head of about 70 psi.  The result is a pump which uses significant horse power to generate the required flow. 

An alternate approach is to avoid a recirc loop that includes the PRV.  Rather, provide a recirc pump for each pressure zone and return the recirc water back to the header down stream of the PRV.  Of course, this creates a recirc loop that does not pull new hot water from the storage tank.  So how do we keep the loop from gradually going cold during the night?  The answer is to provide a separate source of heat in the loop. 

One 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 heater.

Another approach is to utilize a heat exchanger to transfer heat from the low pressure hot water loop to the high pressure hot water loop.  The heat exchanger acts as a pressure isolator and allows the heat of one loop to move to the other loop without pumping across the pressure drop of the PRV.  The drawing below shows how this is done. 



Note that the upper level is the low pressure zone.  This makes sense because the natural head loss due to elevation eliminates the need for a PRV.  The lower floors are served by a zone downstream of a PRV.  The heat exchanger primary takes it’s heat from the main upper zone riser which is always at 120 degrees due the recirc action of its recirc pump.  The secondary of the heat exchanger is then the source of hot water for the hot water zone served by the PRV.  The water temperature of the secondary of the heat exchanger will be slightly less than 120 degrees due to the approach temperature of the heat exchanger, but it will be satisfactory for purposes of keeping the loop warm.  Of course, a practical adjustment of the water temperatures would be to have the low pressure loop initial temperature set at 125 degrees.  Any initial temperature up to 127 degrees is generally considered safe since the water temperature drops before reaching the guestrooms anyway.

Here is an enlarged diagram for piping the heat exchanger.  Note the cross flow to assure efficient heat transfer. 

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