- Fully modulating
The heat output from a fully modulating combination boiler or a gas water heater can be varied from full off to full on, giving you fine control between the minimum and maximum temperatures it produces.
This is performed by the boiler's gas control valve, varying the amount of gas it allows through, in direct relation to the water outlet flow.
Therefore a mixer shower fed from a fully-modulating combination boiler or a gas water heater will allow total control over the blend temperature the shower delivers from full cold to full hot.
- Step modulating
The gas valve within a step modulating combination boiler or a gas water heater, can only be incrementally adjusted (i.e. off, low, medium or high).
Should a mixer shower be fed via a step-modulating combination boiler or a gas water heater, you would find that once the required blend temperature through the shower is obtained, the boiler would continue heating the water until the next level is reached (i.e. low to medium, or medium to high).
It would then shut off the gas supply until the heated water had moved through the boiler, causing a cool shot of water to pass through the shower valve before the boiler re-ignites. With these temperature fluctuations, the shower experience would not be considered comfortable.