Startegy to set maximum relative modulation

This Forum is about the Opentherm gateway (OTGW) from Schelte

Moderator: hvxl

Post Reply
fox
Starting Member
Starting Member
Posts: 10
Joined: Sun Sep 25, 2022 1:44 pm

Startegy to set maximum relative modulation

Post by fox »

I would like to share my ideas about dynamically adjusting the maximum modulation setting.

First, on my boiler, I've noticed the boiler never listens to the max relative modulation setting when its 3way valve is in DHW position. It's only listening to me when heating up the heating installation with CH pump on.
I can understand.
Modulation is much higher with DHW:
- DHW flows at high rate (CH rate is not that high)
- DHW is never ending new cold water needing heat up (CH water is a closed loop and the water coming back to the boiler will finally be warm)
I'm happy not to impact DHW with modulation limiting because:
- DHW ends up on our skin, our body is sensitive to even 1°C fluctuation during a shower, so we need an extremely stable boiler algorithm to drive the modulation level. Limiting it would probably lead to cold shower disaster!

So, max modulation level only controls the max modulation of central heating. It's perfect because I was afraid to impact the showers as well!

My idea is following:
- I know that my boiler has a maximum rating of 25kW (at 100% modulation)
- I know that 1% modulation actually means 10% of its maximum rating (2.5kW)
- There is a given quantity of water in the CH installation. I haven't measured it yet, but I will do it one day, maybe by draining the whole installation, taking note of the cold water meter reading before and after putting water back in the system.
- We know that the fluid is water. To heat up 1kg of water by 1°C, we need 4.184kJ of energy.
- We know the current boiler flow temperature
- We know the setpoint
- I have 8 radiators with 8 ZigBee TRVs. Before heating is turned on, my system knows exactly how many radiators have opened valves, hence how much water will tried to get heated by the boiler (we could simplify and say each radiator has the same size, and neglect the length mismatch between each pipe run loop). My system also knows if each TRV will demand a lot of water flow or little.

Suppose half of my TRVs are opened and suppose the whole installation volume is 80L. We'll have 40L to heat up.
Suppose flow temperature is at 18°C and we would like it to be 40°C. DeltaT = 22°C

To increase the temperature of 40kg of water by 22°C, we need a total energy of 3679kJ.

Now, we can decide what is an acceptable duration for this process to take. 10min for example.
We need 6131W of power for 10min so that 3679kJ will be added to the system.
6131W is 24% of the boiler maximum rating.
So it means 16% of maximum relative modulation.


Of course this computation is not very precise: it neglects 2 big things:
- the flame start-up modulation which initially never listens to the max modulation setting (flame always startup at 38% for 10s before slowly obeying and going down to the target we have set)
- a heating installation is designed to DISSIPATE a lot of energy :D My computations consider that heating from 20°C to 21°C is the same as heating from 59°C to 60°C. It's obviously not the case, the more the target is high, the more we dissipate energy, so we need more than the results of my computation.


Then there is also the empirical way without any computation. I tried to "measure" time taken in different situations but there are so many measures to do... What when heating water to 30°C with 3/8 radiators open, what when heating water to 50°C with 7/8 radiators open...

How do you think about such idea to drive the max modulation?

It's nice that we cannot control the modulation directly and that we can only be more strict that the boiler itself. It avoids to transform the house into ashes... 🔥 :D

When I control it manually, I try to decrease the modulation more than the boiler when reaching the target, to try to never extinguish the flame (less boiler cycling, less wear on components, lower exhaust temperature and metal dilatation, long time in condensing range).
Post Reply

Return to “Opentherm Gateway Forum”