Buffer tank sizing guide

Our buffer tank sizing guide will show you the formula that is used to properly calculate the best sized buffer tank for your home.

buffer tank

Buffer tanks are simply thermal stores.  They hold a certain amount of water, which increases the volume of your heating distribution system.  We have written another article on whether you need a buffer tank and I recommend you check that out.


Buffer tank sizing is important:

Correct buffer tank sizing is fairly technical but also important.   This is because it prevents rapid cycling of your boiler when the loading is below your minimum boiler output.

Rapid cycling is a big problem.  It results in a big increase in CO2 and other emissions like dust (making your home more unhealthy), and reduces the efficiency of your boiler costing you more money.

I will try and explain in the simplest terms I can.  But most heat pump (and biomass boiler) manufacturers will give you the correct buffer tank size for your home.


Buffer tank sizing formula:

So here it is.  This formula will give you the most appropriate buffer tank sizing for your home (in gallons).  As this is the Green Home, we have focused on connecting a buffer tank with a heat pump.

Bt = t (Oh – qload) / (500dT)

I told you it was technical.

Lets look at this in more detail and break the formula into 4 parts.


  1. Bt = the buffer tank size you need (in gallons)
  2. t = time operating (boiler run time – is usually 10 minutes for a heat pump)
  3. Oh = Boiler DOE output (the amount of heat your heater puts out) in BTU/hr
  4. qload = the deman placed on your system from the smallest zone of heating (i.e. the smallest room like your bathroom) in BTU/hr
  5. dT = the delta T of your boiler (the change in temperature. I.e. the final temperature – the initial temperature) in deg. F.


So using this formula you can make an educated guess at your buffer tank sizing requirements.

Example:  t = 10 minutes, Oh = 120,000, qload = 15,000, dT = 30f


Bt = 10*(120,000-15,000)/(500*30)

Bt =10*(105,000/15,000)

Bt = 70 gallon buffer tank

I hope this quick guide has given you some insight into how buffer tanks are sized.  Most people will not have access to the figures needed in the formula so it is easier to check with your manufacturer.


Image courtesy: MyTub

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