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Thread: Boiler Temperature

  1. #1
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    Default Boiler Temperature

    I installed Evohome last Autumn mainly because I was going on holiday and wanted to be able to switch CH on 24hrs before returning home. For this reason all my old conventional thermostatic rad valves are still in place and I'm quite happy with this at the moment. I also installed a wireless cylinder thermostat and BDR91 relay to control the hot water.

    It seems to me that there must be an optimum efficiency temperature for the water to reach in the boiler - too hot and it will be cycling on/off or too cold and it will be continually trying to reach temperature. I imagine this is more important for an oil fired boiler such as the one I have.

    Also does Evohome have any facility to measure the boiler water temperature? e.g. similar to the way it measures HW. Without this then setting the boiler temperature is totally reliant on the boiler thermostat settings which I don't think are very accurate.

  2. #2
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    Quote Originally Posted by Bignomis View Post
    I installed Evohome last Autumn mainly because I was going on holiday and wanted to be able to switch CH on 24hrs before returning home. For this reason all my old conventional thermostatic rad valves are still in place and I'm quite happy with this at the moment. I also installed a wireless cylinder thermostat and BDR91 relay to control the hot water.

    It seems to me that there must be an optimum efficiency temperature for the water to reach in the boiler - too hot and it will be cycling on/off or too cold and it will be continually trying to reach temperature. I imagine this is more important for an oil fired boiler such as the one I have.
    In a full Evohome system with HR92's there are two things it can do to control the heat output at each radiator - it can adjust the water flow through a radiator by adjusting the valve, (which is per radiator) or it can adjust the total heat output from the boiler via the boiler relay or OpenTherm - which affects all radiators, or a combination of both, which is what it does in practice.

    Most efficient would be to use the lowest flow temperature which can meet the demands of the room which has the highest heat demand and reduce the flow of the individual radiators based on that so those which need less heat restrict their flow. This is what the system tries to do.

    However on your system you don't have any HR92's so the only thing it can do is ask for more or less heat from the boiler. It will do this to meet the target temperature of the single "zone" you have, which is the controller.

    If you have a BDR91 all it can do is turn on for longer or shorter periods of time in each TPI cycle. If the room is too cold it will turn on for longer if its too hot it will turn on for less time in the next cycle. So efficiency doesn't really factor into it because it because it can only optimise in one dimension - more heat from the boiler or less heat from the boiler to meet the target of the one room it knows about...

    This is less efficient than if all the rooms were controlled by the Evohome because with a single thermostat system you generally have to set that zone which is measured to a higher than desirable temperature to ensure the boiler continues to run enough to heat the rooms with the manual TRV's adequately. And in fact that room has to be running even if you don't want it to be at all.

    In our house the room which originally had the thermostat was the hallway and because it only has one tiny "outside wall" (the front door) it is better insulated than all the other rooms, so it would easily get up to temperature and shut off the boiler causing other rooms to be starved for heat and drop below the desired temperature, so we would have to manually turn the hallway up even further, despite not wanting to spend any significant time in the hallway.

    A full HR92 system allows all zones to call for heat as needed and allows any room to be individually heated - for example we can heat just the living room without having to heat the hallway if we wanted to.
    Also does Evohome have any facility to measure the boiler water temperature? e.g. similar to the way it measures HW. Without this then setting the boiler temperature is totally reliant on the boiler thermostat settings which I don't think are very accurate.
    It's called OpenTherm.

    With OpenTherm control the Evohome asks for a specific flow temperature based on the heat demands of the zones and the boiler does everything it can to maintain the requested flow temperature and will modulate accordingly. (And yes the OpenTherm protocol does allow the boiler to report the measured flow temperature back to the Evohome, although I'm not sure if the Evohome actually uses this information)

    With a BDR91 the Evohome doesn't know what the flow temperature is, but it doesn't really need to. Nor does it really matter if the flow temperature setting of the boiler is "accurate". Using TPI it will modulate the total heat output from the boiler by changing the proportion of time that the relay is on. This will affect the average heat output of the boiler.

    The only time the flow temperature setting on the boiler has any real effect is in a maximum demand situation - in other words when the house is heating up. In this condition the boiler will be running at the set flow temperature most of the time, so the temperature you set will affect how quickly the house warms up, and also whether it can indeed fully warm up in the middle of winter.

    However once the zones are getting near their desired set points the boiler relay will start cycling on and off proportionally to find the right "balance" - in this situation the exact flow temperature setting doesn't matter as long as it is "high enough", as the average flow temperature is going to be below the set maximum flow temperature.

    About the only control you have with a BD91 is the cycle time and minimum on time settings. These default to 6 cycles an hour and 1 minute minimum on time for gas boilers. I've read that oil fired boilers should use 3 cycles per hour however I don't have any direct experience of oil fired boilers so that is just what I've read. Perhaps someone else reading has an oil fired boiler and can comment.
    Last edited by DBMandrake; 25th October 2020 at 11:39 AM.

  3. #3
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    Thank you DBMandrake for replying to my post and explaining so clearly. I have heard of OpenTherm but didn't understand what it did so thank you. I can also see that my old boiler will not be compatible with OpenTherm but obviously something to research in advance if it needs replacing.

    I guess if I wanted to know the boiler water temperature accurately I would have to do this using a separate device.

    When I installed the Evohome I asked the chap who does the annual servicing for advice on the number of cycles but this completely fazed him so I left at six cycles and one minute minimum. Any views on this would be welcome.

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