This guide is intended to assist in the estimation of the water volume of pressurised commercial heating systems. It is a 'rule of thumb' based on years of experience drawn from equipment manufacturers and system designers. In all cases it assumes that the boiler(s) has been correctly sized to have the heat output needed to satisfy the system. Unless information is available to the contrary, assume that the 'Average Heat Output' shown in the table is equal to two thirds of the total rating of the boiler(s). 1kW of average boiler heat output equals approximately 12 litres system volume. Convert the figure to system volume using the table below.
| Average Heat Output |
System Volume |
| kW |
BTU/hr |
Litres |
| 60 |
204,720 |
720 |
| 70 |
238,840 |
840 |
| 80 |
272,960 |
960 |
| 90 |
307,080 |
1080 |
| 100 |
341,200 |
1200 |
| 150 |
511,800 |
1800 |
| 200 |
682,400 |
2400 |
| 250 |
853,000 |
3000 |
| 300 |
1,023,600 |
3600 |
| 350 |
1,194,200 |
4200 |
| 400 |
1,364,800 |
4800 |
| 450 |
1,535,400 |
5400 |
| 500 |
1,706,000 |
6000 |
| 550 |
1,876,600 |
6600 |
| kW |
BTU/hr |
Litres |
| 600 |
2,047,200 |
7200 |
| 650 |
2,217,200 |
7800 |
| 700 |
2,388,400 |
8400 |
| 750 |
2,559,000 |
9000 |
| 800 |
2,729,600 |
9600 |
| 850 |
2,900,200 |
10200 |
| 900 |
3,070,800 |
10800 |
| 950 |
3,241,400 |
11400 |
| 1000 |
3,412,000 |
12000 |
| 1050 |
3,582,600 |
12600 |
| 1100 |
3,753,200 |
13200 |
| 1150 |
3,923,800 |
13800 |
| 1200 |
4,094,000 |
14400 |
| 1250 |
4,265,000 |
15000 |
| kW |
BTU/hr |
Litres |
| 1300 |
4,435,600 |
15600 |
| 1350 |
4,606,200 |
16200 |
| 1400 |
4,776,800 |
16800 |
| 1450 |
4,947,400 |
17400 |
| 1500 |
5,118,000 |
18000 |
| 1550 |
5,288,600 |
18600 |
| 1600 |
5,459,200 |
19200 |
| 1650 |
5,629,800 |
19800 |
| 1700 |
5,800,400 |
20400 |
| 1750 |
5,971,000 |
21000 |
| 1800 |
6,141,600 |
21600 |
| 1850 |
6,312,200 |
22200 |
| 1900 |
6,482,800 |
22800 |
| 1950 |
6,653,400 |
23400 |
| 2000 |
6,824,000 |
24000 |
| 2500 |
8,530,000 |
30000 |
| 3000 |
10,236,000 |
36000 |
- If the system is open vented, the volume should be increased by 10% (multiply by 1.1).
- Sentinel X100 to X400 inclusive are nominally dosed at 1% of system volume.
- All other products should be dosed in accordance with the manufacturers' instructions.
- The guide does not apply to domestic systems.