The one line of arithmetic behind a buffer tank
When a boiler fires at a minimum output higher than the load in front of it, the surplus has to go somewhere, and the only place available is the water in the system. Heat going into storage is the boiler output minus the load. The heat a gallon of water can absorb over a temperature swing is its weight times its specific heat times the swing, so the burn time you get is the stored energy divided by the surplus.
Turn that around and the volume needed is the surplus times the run time divided by the fluid constant times the swing, with run time in minutes and the constant being the familiar 499.8 for water. With the defaults on this page — a 40,000 BTU/h minimum, a 10,000 BTU/h load, a 10 minute target and a 20 degree swing — the surplus is 30,000, and the volume is 30,000 times 10 divided by 499.8 times 20, which is 30 gallons. Twenty gallons are already in the pipework and the boiler, so the buffer to add is 10.
The same numbers give the cycle rate without a buffer. Twenty gallons swinging 20 degrees hold about 200,000 BTU-minutes of capacity against a 30,000 BTU/h surplus, which is a 6.7 minute burn, followed by a 20 minute coast while the 10,000 BTU/h load pulls the temperature back down. That is a 26.7 minute cycle, or 2.3 cycles an hour. Add the buffer and the burn stretches to 10 minutes, the coast to 30, and the rate falls to 1.5.
Why the worst case is not the design case
Look at the table this page prints of volume against load. The smaller the load, the bigger the surplus, and the volume rises in a straight line as the load falls. At a load of ten percent of the boiler minimum, the required volume is nearly what it would be at no load at all. There is no tank size that covers every condition, because the limit as the load goes to zero is a tank sized for the entire minimum output.
So the load you enter is a decision about which conditions you care about, and it deserves thought rather than a default. Reasonable candidates are the load of the smallest single zone that can call on its own, the whole-house load at a mild outdoor temperature you expect often, and the load at the temperature where the reset curve puts the system in shoulder season. The heating degree day calculator is one way to see how many hours a season actually spends near those mild conditions, which is the real question hiding underneath.
Ways to attack the same problem without a tank
| Lever | What it changes | The catch |
|---|---|---|
| More water volume | Longer burn at the same surplus | Space, weight, cost, and more mass to warm up |
| Wider temperature swing | Volume falls in proportion | The emitters see a wider supply temperature range |
| Fewer, larger zones | Raises the smallest load that can call | Less independent control room by room |
| Lower boiler minimum output | Cuts the surplus directly | A property of the equipment, decided at purchase |
| Combining zone calls | Delays a call until several add up | Control strategy, and rooms wait longer |
The swing is the lever people reach for last and it is the strongest one in the arithmetic, because volume and swing are inversely proportional: double the swing and halve the tank. What it costs is that the emitters see the whole swing too, so the room temperature wanders more, and if the boiler is meant to condense, the top of the swing may push return temperatures above where it does.
Where the model is thin
It assumes a fully mixed tank, a steady load through the burn and a boiler that holds its minimum output constant. Real tanks stratify, which is often deliberate and useful and means the usable volume is not the label volume — the field for usable share exists for that. Modulating boilers do not sit at one output; they slide. And the load moves during the burn because the room is warming.
The piping arrangement matters as much as the gallons. A tank in series with the flow, a tank used as a hydraulic separator with four connections, and a tank as a dead-header all behave differently, and the difference is not a volume correction. That is a design question for whoever is responsible for the system. For what the pipework itself already holds, the pipe volume calculator; for the flow the circuit needs, the hydronic flow calculator; and for the zoning arrangement that decides how small the smallest call can be, the zone flow split calculator.
Questions people ask
How do I know if my boiler is short cycling?
Count the burner starts over a fixed period at a steady mild outdoor temperature, and compare the burn length to what the equipment documentation says about this boiler. The arithmetic above predicts it from first principles: divide the water volume times the constant times the temperature swing by the surplus output for the burn, and by the load for the off period. If the predicted cycle is a few minutes and the observed one is a few minutes, you have found the mechanism rather than a fault. What counts as too many cycles is not something this page will tell you, because it depends on the equipment and belongs to its manufacturer.
What size buffer tank do I need?
Surplus output times run time divided by 499.8 times the temperature swing, with run time in minutes and volume in gallons, then subtract the water already in the system. The surplus is the boiler minimum output minus the load. The answer swings enormously with the load you choose, which is the honest part of the problem: on the defaults here, half the boiler minimum needs 20 gallons of total system water and a tenth of it needs 36, for the same equipment and the same run time. Decide which load matters before reading the volume.
Does a bigger temperature swing let me use a smaller tank?
Yes, and exactly in proportion — double the swing and the volume halves. It is the cheapest lever in the equation and it is not free. The swing is what the emitters see, so a wide swing means supply temperature wanders across a wide band and room temperature follows it with a lag. On a condensing boiler a wide swing can also lift return temperatures out of the condensing range for part of each burn, which is a direct efficiency cost. How wide a swing is workable depends on the emitters, the control strategy and the equipment.
Is the water already in my system enough?
Sometimes. An old cast iron boiler with cast iron radiators and steel mains holds a remarkable amount of water, and that mass is why those systems rarely short cycled even with crude controls. A modern low-mass boiler with home-run PEX and panel radiators can hold under ten gallons in the whole house. Add the boiler water content from its data sheet to the emitter volumes from theirs, work the piping out with the pipe volume calculator, and put the total in the field above. The result is often the argument for or against a tank on its own.
Can I use my indirect water heater as a buffer?
That is a piping and control question rather than an arithmetic one, and it goes to whoever is responsible for the system. What the arithmetic can say is that the gallons only count if they are actually in the heating loop during the heating burn and actually swing through the temperature range you entered. A cylinder held at a fixed temperature for domestic hot water is not swinging, so its volume is not doing buffer work. Anything that connects a heating circuit to domestic water raises cross-connection questions, and a system with glycol in it must never be able to reach potable water.