Combi Boiler and Thermostat Behaviour

Spies

I have a Honeywell CM727 thermostat, the actual thermostat portion is in my Living Room.

I notice that instead of constantly calling for heat until the target is reached, it cycles up to 6 times per hour, sure this on and off isn't the most efficient way to heat a house as a lot of heat will be trapped within the pipes under the floor when they boiler is 'off' between those periods?

Would it not be better to have a constant call for heat until target temperature is reached and just let the boiler modulate/cycle with its own built in logic?

The reason I ask, is because I'm planning to add a smart relay which I can use to call for heat using my Home Assistant instance and it doesn't appear to do the modulation bit that my thermostat currently does.

[Deleted User]

Honeywell sets 6 cycles per hour for all its smart thermostats connected to gas heating systems. I had Evohome in a previous home. The Honeywell rationale for it is that it prevents boiler cycling (which in itself is inefficient) and it reduces over and under swings of the set temperature. There are, however, opposing views:

https://www.heatgeek.com/what-is-boiler-modulation/

doodling

Hi,

The thermostat is attempting to accurately control the room temperature to the setting you requested.  If it just ran the heating system continuously until the set temperature is reached then you would get overshoot.  As the heat in the water in the pipes is generally lost within the house then that isn't a problem - in your specific case I would be insulating any pipes (like those under the floor you mention) which aren't in the house thermal envelope.

You can usually adjust the control parameters to specify a minimum run time per cycle and number of cycles per hour.  That might help if you have an unusual situation (e.g. a very powerful boiler servicing a very low demand where the cycles end up so sort that the water heated by the boiler never makes it to the radiators until several cycles later) but generally the default parameters are fine.

In an ideal world, the thermostat would directly control the boiler modulation (and if you bought the thermostat sold by your boiler manufacturer it might) but central heating boiler and control system suppliers are making that as hard as possible by having a whole host of incompatible control arrangements (even when they notionally comply with a standard), largely in an attempt to force people to buy as much of the control system from the boiler supplier as possible.

How are you planning to call for heat with your smart relay?  Will the relay enable / disable the output from the thermostat or will it be an additional call for heat?  In the latter case, what will control the house temperature?

Astria

If it's anything like Hive, they try to do "smarts" - turn the heat off before the target is reached, hoping that residual heat will actually get to the target temperature. It stops the thermostat from overshooting the target temperature. If it's set for 18c, you could actually end up with 19c. I noticed that it didn't actually work so well - it turned it off before then came on again 10 minutes later because it didn't reach the target, then it was trying to predict the temperature falling and firing up the boiler before the temperature dropped to try and keep the 18c perfect. However it's not perfect and can never be perfect because it's not a multi zoned heating system.

I disabled it all last winter - much prefer the boiler only coming on once per hour at maximum with my own system. The boiler never short cycles whilst doing this because it's correctly sized and can modulate down (which is another thing they are trying to prevent)

doodling

Hi,

[Deleted User] said:

Honeywell sets 6 cycles per hour for all its smart thermostats connected to gas heating systems. I had Evohome in a previous home. The Honeywell rationale for it is that it prevents boiler cycling (which in itself is inefficient) and it reduces over and under swings of the set temperature. There are, however, opposing views:

https://www.heatgeek.com/what-is-boiler-modulation/

Direct control of boiler modulation by the thermostat is almost certainly best, but as per my previous comment, boiler manufacturers make that difficult unless you buy the whole control system from them.  Depending on your boiler design and heating load, the system will have to revert to on-off control at low loads anyway so that functionality has to be included in any heating control design.

Spies

How are you planning to call for heat with your smart relay?  Will the relay enable / disable the output from the thermostat or will it be an additional call for heat?  In the latter case, what will control the house temperature?

It's going to be wired in parallel so the traditional thermostat can still operate. It just bridges the live and live return which signals the call for heat.

BUFF

Astria said:

If it's anything like Hive, they try to do "smarts" - turn the heat off before the target is reached, hoping that residual heat will actually get to the target temperature. It stops the thermostat from overshooting the target temperature. If it's set for 18c, you could actually end up with 19c. I noticed that it didn't actually work so well - it turned it off before then came on again 10 minutes later because it didn't reach the target, then it was trying to predict the temperature falling and firing up the boiler before the temperature dropped to try and keep the 18c perfect. However it's not perfect and can never be perfect because it's not a multi zoned heating system.

I disabled it all last winter - much prefer the boiler only coming on once per hour at maximum with my own system. The boiler never short cycles whilst doing this because it's correctly sized and can modulate down (which is another thing they are trying to prevent)

Hive is "smart" in terms of being connected but in terms of boiler efficiency it is poor - it is a Class 1 thermostat (the lowest that you can get).

doodling

Hi,

Spies said:

How are you planning to call for heat with your smart relay?  Will the relay enable / disable the output from the thermostat or will it be an additional call for heat?  In the latter case, what will control the house temperature?

It's going to be wired in parallel so the traditional thermostat can still operate. It just bridges the live and live return which signals the call for heat.

What is regulating the troom temperature when your smart relay is on?

What disables the thermostat output when the smart relay is on?

There is no problem with doing what you propose but whether it results in a useful heating control system is a different question.  Without a suitable control loop driven by a temperature sensor, whether that is a simple on/off approach or a time proportional approach like the Honeywell, you are going to end up with something that overheats the house and wastes money.

Spies

The temperature sensor is a BLE sensor so that'd my reference point for actuating the relay. 

The live return from the current thermostat will go to the Switch on the relay which I can detach from the output of it (as to block heat calls when the house is empty for example), I can use logic to determine whether to allow the switch event to activate the output or not.

doodling

Hi,

If you're using the relay as an off switch or to control the temperature below the thermostat setting then that should work fine.

The only wrinkle is how the thermostat will respond to not being in control of the boiler.  I would expect it to ramp up its demand as it tries to get the room up to temperature without success - and this would result in a massive overshoot when the thermostat regains control which it might take a while to recover from.