Buffer Tank Sizing

Buffer Tank Sizing

We are showing how to size a buffer tank when you have an on/off or modulating boiler. When sizing the buffer tank, it should be sized for the smallest zone and a pre-determined run time. The runtime is your choice, usually between 5 and 10 minutes.

Buffer tanks reduce short cycling of the boiler. Most boilers are normally 100% or more oversized for the heating system. If the boiler is properly sized short cycling will automatically be reduced. When a high efficiency boiler is used the higher the turndown the less chance of a need for a buffer tank.

Let's look at sizing a buffer tank for an on/off boiler

V = t(Qhs - Qloadmin)/(500 * delta-T)

Where:

V =Minimum buffer tank volume (gallons)

t = Minimum heat source on time in minutes

Qhs = Rated heat output of heat source (Btuh)

Qloadmin= Minimum concurrent heating load when heat source is in demand Btuh

Delta-T = Change in average tank temperature during minimum heat source on time (f)

Let's assume the following:

t = 10-minute recovery time

Qhs = Boiler output 68,000

Qloadmin = Zone output 5000 Bth/h

Delta-T = 20f

V = 10(68,000 - 7000) / (500 * 20f)

V = 63 gallons

 

Sizing a buffer tank for 10:1 high efficiency boiler

V = t(Qhs - Qloadmin)/(500 * delta-T)

Where:

V = Minimum buffer tank volume (gallons)

t = Minimum heat source on time in minutes

Qhsmin = Minimum stable heat output of heat source Btuh

Qloadmin= Minimum concurrent heating load when heat source is in demand Btuh

Delta-T = Change in average tank temperature during minimum heat source on time (f) Note: The smallest zone must be less than the minimum output of the boiler or no buffer tank is required.

Let's assume the following:

t = 10-minute recovery time

Qhs = Boiler output 68,000

Qloadmin = Zone output 5000 BTh/h

Delta-T = 20f

V = 10(6800 - 2000) / (500 * 20f)

V = 4.8 gallons

Although this will run longer cycles I am not totally happy with a high efficiency boiler running a minimum of 10 minutes. I would choose 15 - 20 minutes run times.

The difference if all were the same except a 20-minute run time. would be as follows.

V = 20(6800 - 2000) / (500 * 20f)

V = 9.6 gallons

You can see to get a better run time would not be a drastic tank size change. You can see a delta-T change will change the tank size and run time.

What if our boiler of choice is only a 5:1 turndown boiler.

Let's apply this.

V = 10(13,600 - 2000) / (500 * 20f)

V = 11.6 gallons

But, a 20-minute runtime would change the tank size to:

V = 20(6800 - 2000) / (500 * 20f)

V = 23.2 gallons

It's your choice if you want to use a buffer tank what run times do you want and what size tank do you want?

Disclaimer: The information found on this website is for informational purposes only. All preventive maintenance, service, installations should be reviewed on a per job situation. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc or its personnel accepts no responsibility for improper information, application, damage to property or bodily injury from applied information found on this website.