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The Glitch & The Fix

Preventing Short Cycling

November 1, 2007
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The Glitch

The Glitch

Overview: An installer is advised to use a buffer tank to prevent short cycling of a low mass boiler serving several independently controlled space heating zones. The system also needs to provide domestic hot water. The design he comes up with is shown below.

Exercise: Find at least five errors in this layout.

The Fix

The Fix

One rule of working with buffer tanks is to not operate the buffer tank as part of the domestic water heating mode. The indirect tank provides plenty of thermal mass to stabilized the boiler during this mode. Storing heat in buffer tank for domestic water heating only adds to standby heat loss. The fix is to operate the indirect tank as a separate circuit independent of the space heating loads. Notice that hot water from the boiler is drawn through the air separator in either space heating or DHW mode.

The series primary/secondary piping shown in the Glitch lowers the supply water temperature to the downstream secondary circuits. If the DHW tank was allowed to operate simultaneously with space heating, the heat exchanger in the indirect tank would get a relatively low supply temperature due to temperature drop across the space heating secondary circuits. Better to pipe these secondary circuits as parallel circuit to give them the same supply temperature.

The incorrect piping also allows an undetermined percentage of the boiler flow to pass through the buffer tank rather than pass through the primary loop. This could exasperate the problem of temperature drop along the primary loop due to low flow.

Finally, if the boiler is off and heat is being supplied from the buffer tank, there is no way to prevent hot water flow through the boiler heat exchanger. This situation will lead to excessive heat loss from the boiler jacket.

In the Fix drawing, the buffer tank is simply piped in as an alternative heat source. When its secondary circulator is operating, its thermal mass “participates” in the space heat operation. If there is any space heating mode where the buffer tank is not needed, it can be taken offline by turning off its circulator.


1. Buffer tank charging mode: Circulators P2 and P3 are operating and boiler is firing. If zone circulators are off then flow make "U-turn" between closely spaced tees A and B. If one or more zones are on entering flow "splits" at tee A (some to load, remainder to buffer tank through tee B.

2. Supplying zones from boiler only: P2 is on, P3 is off. Hot water from boiler is injected at tee A, and drawn through zones. Return water is pulled out at B and returned to boiler.

3. Supplying zones from buffer tank only: If buffer tank is hot enough, and there is a call for space heating. P2 is off, P3 is on. Hot water enter distribution system at tee A and returns to buffer tank from tee B.

4. Prioritized domestic water heating: Circualtor P1 is on, P2 is off. P3 might be operating if space heating zones are active.  


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Recent Articles by John Siegenthaler, P.E.

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buffer tank

Aaron Weaver
August 17, 2010
This is a great way to add on an inexpensive electic water heating tank that will be used as a buffer tank. Wire in the electric water heater and make sure the stats are set lower than your outdoor reset in winter and now you can switch the water heater/buffer tank on as a secondary heating source for peace of mind when you aren't home.

buffer tank w/system 2000

jim messenger
July 16, 2011
What controls would you use to activate and deactivate P-2 & P-3.Supply & return coming from boiler passes thru a flatplate heat exchanger inorder to isolate radiant heating.What do you thing about constant circulation w/temp controled by in & outside sensors w/use of 3-4 way valve or injector pump

Pellet Boiler/Oil boiler/Buffer

P Shearer
June 30, 2012
hi can anyone give me the best planned way to connect these items,using the oil boiler only as a back up in the event that the pellet boiler fails. It's a 800ltr buffer tank with domestic hot water coil the system will be sealed 1.5bar.



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