DIY Doctor

Danfoss TP900SI Room Start On and Off

Postby briricha » Thu Oct 03, 2019 1:14 pm

I have a danfoss room stat and a TP900SI programmable controller in a new house I have just moved into. Weird thing is every now and then the controller shows the central heating coming on for literally 30 seconds and then off again. This happens randomly. Having rung Danfoss they advise that this is due to something call chrono proportioning (I think). Now I understand the basics of if a room falls below temp that heating should kick in to bring it back to temp and then turn off so not sure why it is coming on and off for such a short length of time. Any help appreciated.
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Postby ericmark » Fri Oct 04, 2019 9:07 pm

Thank you for the post, I had not heard of Chrono Proportional before your post, it refers to an advanced control algorithm in the thermostat, however there is also an advanced control algorithm inside the boiler, to work they must work together.

Many thermostats have anti hysteresis software built in, and the simple method is to start cycling the boiler off/on using a mark/space ratio before it hits the target temperature to stop it over shooting, this was common with pre-modulating boilers.

However when the condensation boiler came out, the return water needs to be cool enough for it to work, the original method was the thermostatic radiator valve (TRV) and the by-pass valve, as the TRV's close the by-pass opens and hot water returns which causes the boiler to modulate (flame height is reduced) and the advanced control algorithm means the water temperature actually reduces so the boiler gains the latent heat.

However if you turn the boiler off/on with a wall thermostat it upsets the boilers advanced control algorithm, so it does not gain the latent heat.

The normal way was to use the wall thermostat to only switch off the boiler when the weather has improved, so it is placed in a cool room on ground floor with no alternative heating or outside door.

However this has evolved and now the wall thermostat, or maybe better name is a hub, is connected to the ebus, either special manufacturers thermostat, or OpenTherm, so the boiler is modulated directly by the thermostat.

Some manufacturers use a single wall thermostat not interconnected with the TRV heads, so again location is rather important, it also means third party thermostats don't work very well, it seems the Chrono (Means to do with time) Proportional (another name for mark/space ratio) is an attempt to make a thermostat that can be used without connecting to ebus, that will control boiler in a similar way.

However a 30 second burst is unlikely to get water to a radiator so rather useless, a series of 30 second bursts may work, but even that is unlikely, the basic thing is, when a boiler turns off, any heat in the boiler is lost through the flue, so normally you get the boiler to modulate to it's minimum setting and lowest temperature before it turns off.

The Chrono Proportional makes me think of snake oil, but I may be wrong.
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