Boiler cycling
It’s currently mild and my boiler, with a switching thermostat, is only on for about 10% of the time say 10 mins every 100 mins. I do have a modulating boiler but it has 28kw max and 7kw min output so even at lowest power it would still need to switch off completely within a 100 min period to achieve a low enough mean output power. Hence it would need to cycle as much as currently. So how will it help me to use the opentherm type modulating controls provided by tado?
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Once the room is up to temperature Tado will request a lower flow temperature to maintain the room temperature, which in turn will reduce the amount of gas used. Without Opentherm (or Ebus or similar) the boiler would just re-light and try an achieve say a 60oC or more flow rate as it wouldn't know why it's being turned on. With Opentherm or ebus you would get a lower flow rate of 35-40oC.
But yes, you are right about minimum turn down rate of many boilers being a hinderance to saving money as they can't turn down enough to allow a room or two to operate without the cycling Olympics.
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There is plenty of scope for the burn time to increase as currently the duty is only 10%. I don’t know if the boiler control can apply the maximum modulation from 28kW to 7kW. It probably doesn’t really matter so long as the return temperature can be reduced to 35degs. I did take a single measurement of 50 degs for the return temperature with the dial set at 3 (from 6). I can see if I can get a reasonable temperature in the house with the dial on 1. I would like to be able to measure the return temperature continuously if someone can suggest a good gauge. The ones on Amazon have very mixed reviews.
When you think about it, in my lifetime we have gone from a family surviving in a house with a single 1 or 2kW heater, to requiring 28kW. And I’ve not actually been conscious of the change.
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I have 24kW boiler which can modulate down to 3.5 kW, connected via OpenTherm to Tado. I have 7 radiator thermostats, 1 room with underfloor heating (water set to 27C) + 4 radiators with non smart thermostat which are always fully open.
Once the rooms with the smart TRV reach the target temperature they request so low heat from the boiler that even with the 3.5 kW minimum output my boiler is cycling.
I do not know exactly how low flow temperature the Tado thermostat requests, but I see that the boiler kicks in when the current flow is ~32C, heats the water to ~42C, start reducing the burner but at ~37C flow temp it turns it off and then just circulates the water in the system until it reaches again 32C, this takes approximately 3-4 minutes.0 -
You could buy a dual channel thermometer. But for longer term there’s Sonoff THR16 with DS18B20. Fit those and log away.
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I use cable ties and slip a bit of pipe insulation over them.
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@Valaki1234 I believe that when my boiler kicks back in as you describe above, it doesn’t come back on at maximum even though I’m using an on/off thermostat. I think the boiler modulation is related to the temperature differential between the current flow temperature and the demanded flow temperature set by the control on the front of the boiler. If I’m using opentherm does it operate as if it is changing the demanded flow temperature?
I do have a problem with the monitoring provided by any of the smart thermostats as they can only monitor and display what the thermostat is calling for. It doesn’t tell you what this means in terms of gas usage or efficiency. Im not sure how far you can go in optimising the system without bringing a smart gas meter into the picture. For instance as I reduce the flow temperature it will take longer for the house to heat up and the thermostat will call for heat for a longer time. If I only look at the time the thermostat is calling for heat I might assume I’m burning more gas. However we are told that the opposite is true.
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For me the solution to eliminate cycling was:
- Removed all SRTs with dumb ones which are fully open - to maximize emitting surface
- Set the minimum flow temp on the boiler to 35C
- Maximum flow temp is set to 40C (both in the boiler and in Tado)
I kept the Tado thermostat with some wireless sensors. With this setup even when the outside temperature is mild (e.g 10C) the boiler does not cycle. E.g: it turns on for 20-30 mins constantly then turns off for 30 mins. Before that it turned on-off every 5 minutes because Tado requested soo low flow temperature.0 -
@Valaki1234 Maximum flow temp is set to 40C? Is this not extremely low? Could you ref me where I can find recommended boiler flow temp for combi boiler for tado please.
... and in Tado - how do you set this in Tado?
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@spinnekop : it depends on your heating system. With UFH it is high. With big radiators + proper insulation: okay. With small radiotors/no insulation: low.
In my case I have big radiators and 5cm insulation which is not that much but the property is a terraced house thus heated from the sides and the surface on the front and back is small. 35-40C flow is okay for all rooms except the coldest one, I needed to increase the max to 45 to reach 22C in that room when outside is ~0C.
You need to use the 'Tado for installers' app to configure the maximum flow temperature Tado requests (plus you need to set the proper configuraiton in your boiler to allow that maximum value therefore you need to set this in two locations).
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