Hi RikkiDees,
Firstly let me state that I am NOT a plumber.
We also have a Heat Only Boiler - for reference only an Ideal Logic Heat H15, paired with Drayton Wiser Thermostats.
When a Heat Only system was installed (prior to 2023) then the designed flow and return temps would typically be 80'C Flow, 60'C Return giving a 20'C differential.
This is BAD as condensing boilers only start condensing when the return temp is 54'C or lower. The lower the Return temperature the more efficient your boiler will be to a maximum of around 94%.
Approximately:
80/60'C - 80% efficient
74/54'C - 85% efficient
64/44'C - 94% efficient
So turning up your Flow Temperature will increase your gas use and decrease your efficiency (meaning you use more gas).
1) Is your boiler running efficiently?
When was it last serviced? There are seals and electrodes that can deterioate that adversely affect your boiler performance.
2) Is your system dirty/blocked
A dirty system (sludge) will impede flow and reduce the heating output of your radiators.
3) Is your system balanced?
Our house is only a few years old but the system was not balanced correctly. This means adjusting the flow through the radiators so that the liquid doesn't skip radiators.
4) I guess it is possible that the Flow Sensor is faulty (but I would expect you to have other issues, eg Hot Water would be too cold or too hot)
5) Can you fit OpenTherm Controls (Nest in OT Mode/ EPH and a few others).
Whenever possible you should fit OpenTherm controls (or the manufacturers equivalent standard).
Discover the benefits of Opentherm technology for your home heating system. Learn more at BoilerCentral.
www.boilercentral.com
*) I looked through the manual for the Baxi 400 Heat Only Boiler and unfortunately it doesn't appear your boiler has OT Controls and the Baxi Usense Smart room thermostat only works with Baxi Combi Boilers.
**) You can also purchase "Auto Balancing TRV's" and "Auto Balancing Valves"
Danfoss radiator thermostats maintain constant desired room temperatures, individually room by room. The RAS-B2 is a 2-in-1 temperature control and automatic balancing valve, combining a thermostatic radiator valve and pressure controller for two-pipe heating systems.
www.danfoss.com
I hope that helps in some way