« on: 27/05/2017 11:07:14 »
Not all boilers have the ability to sense when all the trvs are shut which can lead to what is known as dry cycling where the boiler just keeeps heating water in its own loop usually around a bathroom radiator with no trv or a bypass loop. However, the room thermostat is rarely in a position to stop this happening.
My 35 year old system was installed with no TRV on the bathroom to prevent the pump trying to push water into a dead end, but auto bypass valves were introduced to prevent the need for this. Nowadays boilers like the one I'm getting have them integrated.
I currently have a boiler for space heating and a separate multipoint for hot water, so I'm replacing them with a combi so that I won't need to build an airing cupboard for a hot tank. With having a multipoint, I'm already used to waiting for the tap to run hot when I turn it on, but a lot of people who don't understand how combis work complain about it. In order to address this, the combi I'm having has been deliberately designed to 'dry cycle' so that the water jacket remains hot in order to reduce the time it takes for the tap to run hot when you turn it on.
With heating though, if it takes x watts to warm the house, dependent on the difference between the inside and outside temperatures at any given time, and the boiler power is y, then the duty cycle of the boiler stat will be x/y, and the frequency that it cycles at will be determined by the hysteresis on the stat. If you already have TRVs to set the room temperatures, then there isn't really anything to be gained by fitting a roomstat in between. For example, if a roomstat was operating at a duty cycle of 50%, then the duty cycle of the boiler stat (whilst the roomstat is on) would just rise to 2x/y in order to maintain the same mean power overall, and hence the same space temperature. The end result though would be that the air temperature in the room will cycle up and down by an amount equal to the hysteresis on the roomstat, whereas without it the temperature variation due to the boiler stat hysteresis tends to get smoothed out by the thermal inertia of the water and radiators.