OT msg ID 15 - max power and min modulation level

[emmeesse68]

Hi all,

I came across an OT message between my Ferroli BlueHelix Pro 32C boiler and my Honeywell R8810 Opentherm Evohome interface.

The message is ID is 15 and I read in the protocol document it should report the maximum boiler capacity in kW and the minimum modulation level in %.

My boiler responds 35 25 - - that would imply a maximum power of 35 kW - that's not what I read in my boiler specs, which say maximum power is 32kW.

Also, 25% of minimum modulation seems HUGE...Boiler specs say mimimum power is 7,2kW - that's a lot... but it's less than 25% either of 35 or 32kW.

Maybe mt boiler controller isn't well programmed but I'd like to hear from the community... Is your boiler minimum power that big?

- M

[D_Hailsham]

I don't know why ID 15 should report the max power as 35kW. However it's the heating output which is relevant, not the input. For the 32C this is 28.9kW. The minimum is 7.2kW, which is 24.9% of the max. I'm not going to argue over 0.1%.

Yes, the minimum output can be high - on some combi boilers as high as 12kW. This is always a problem with combi boilers, which tend to be sized purely on their hot water performance. Modern, very well insulated houses, normally have very low heating requirements, well below the minimum modulation level of a combi boiler. Fortunately boiler manufacturers seem to be waking up to this.

ID 14 and 15 are used by boiler sequencers for controlling multiple boilers.

[emmeesse68]

Thanks for your reply... by the way, why do you make a difference between input and output power? In my mind there's little significance for such a thing as "input power" for a boiler... It's more of a thing that "outputs" heating power, accepting "gas" as input... Am I missing something?

[D_Hailsham]

Each cubic metre of gas contains a certain amount of energy - approx 38 MegaJoules. Not all this energy becomes useful energy to heat the water. Some goes out of the flue, some heats up the boiler casing. So you may have 38MJ going into the boiler, but only 36MJ is gets into the water (made up numbers). Older cast iron boilers were very bad as only 50% of the energy in the gas was used to heat the water. Modern condensing boilers can be 90% efficient when not condensing and about 97% efficient when condensing.

You pay the gas supplier for the energy going into the boiler, not for the energy going into the water. So the more efficient the boiler, the less money you are wasting on 'lost' energy.

[noether]

Hello Hailsham,

it seems that you are pretty familiar with some terminology here. In particular I am talking about "modulation level", what does exactly mean? Going into the protocol document does not help at all, here is the "best definition" I have found there (related with the post ID14 and 15).

"The boiler capacity level setting is to be used for boiler sequencer applications. The control setpoint should
be set to maximum, and then the capacity level setting throttled back to the required value. The default value
in the slave device should be 100% (ie no throttling back of the capacity). The master can read the maximum
boiler capacity and minimum modulation levels from the slave if it supports these"

to my understanding, if I send whatever "% modulation level" to the boiler, what does the boiler do?

Thanks in advance

[D_Hailsham]

The "Maximum Capacity" is the maximum output of the boiler. The "minimum modulation" is the output of the boiler when running at minimum output, given as a % . So, if you have a 100kW boiler, which can modulate down to 10kW, the Maximum Capacity is 100kW and the Minimum Modulation is 10%.

Parameters 14 and 15 are only used in multi-boiler installations where you need to know the characteristics of each boiler. An Opentherm controller for a multi-boiler installation is more complicated than those used in domestic installations.

You cannot send a modulation level (ID15) to the boiler: it is a "Read" parameter. The controller asks the boiler for this information, then uses it to decide which of the available boilers to use.

The Protocol Document has to be read very carefully.

Presumably Ferolli boilers outputs ID15 in case they are used in a multi-boiler system.