DHW control of Baxi boiler with storage tank

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DHW control of Baxi boiler with storage tank

Postby spee981 » Mon May 15, 2017 6:23 am

Hi all!

I'm new here and have just overcome the complications of a full renovation of the heating system of my house. My choice was a Baxi Duo-Tec Compact+ boiler due to it's size and support for OT. I'd like to have some control over it's behavior but at least to monitor what it does and how, that's why I also purchased an OT gateway right after the heating system was deployed. I have not bought a Baxi thermostat yet, though, since I could not get it's user's manual to be sure about what it supports in terms of programming (and it's relatively expensive due to the lack of information I could get about it's capabilities). It's BAXI 7104336 or it's wireless counterpart BAXI 7105432. I think these support weekly CH and DHW programs, but others reported that there are too few daily CH setpoints to be convenient for them. Anyway, prior to investing in a room thermostat I wanted to see whether I could control on my own using the wireless room sensors all over the house and create DHW based on our measured needs.

So I started to experiment with the GW these days, attached the gateway to the boiler and had immediate success: they started the conversation and after some time I was able to start/stop central heating and set the control setpoint. After some investigations I was also able to control the DHW setpoint of the boiler by issuing SW serial commands from otmonitor, and discovered the HW=1 command as well (comfort mode on the interface), which enabled DHW. Added alternative commands 3 and 26 for slave configurations and DHW temperature respectively, so that the monitor get more info about the slave, and afterwards I saw the DHW properties and the measured DHW temperature reported to the monitor periodically. And I got stuck: even if I enable DHW and the measured DHW temperature goes below the setpoint (ie. below 37 degrees currently), nothing happens. When the gateway is not attached, the boiler on it's own started to heat the DHW tank, but with the gateway nothing happens. I think I miss something at this point: I can't figure out how to instruct the boiler to switch the valve (do I need to instruct it at all?) and start heating the DHW. For CH it was easy, I only issue the CS=x command and it sets the CH flag automatically, then the boiler starts to heat, nothing else is needed. I can switch it off with CS=0 or CH=0, both are OK. But nothing happens for DHW, regardless of what is set by SW command, what's the measured DHW temp and whether I issue HW=1 or not.

Here's a snippet from the logs, maybe it helps someone to figure it out even faster:

Code: Select all
18:12:50.379235 R80380000 Read-Data DHW setpoint: 0.00
18:12:50.500077 BC0382500 Read-Ack DHW setpoint: 37.00
18:12:51.439296 R00390000 Read-Data Max CH water setpoint: 0.00
18:12:51.556307 BC0395555 Read-Ack Max CH water setpoint: 85.33
18:12:52.499241 R801A0000 Read-Data DHW temperature: 0.00
18:12:52.614386 B401A2433 Read-Ack DHW temperature: 36.20
18:12:53.556307 R00120000 Read-Data CH water pressure: 0.00
18:12:53.674230 B40120133 Read-Ack CH water pressure: 1.20
18:12:54.292459 Command: AA=3
18:12:54.310325 AA: 3
18:12:54.617057 R80000200 Read-Data Status: 00000010 00000000
                              - CH enable: disabled (0)
                              - DHW enable: enabled (1)
                              - Cooling enable: disabled (0)
                              - OTC active: not active (0)
                              - CH2 enable: disabled (0)
                              - Summer/winter mode: winter (0)
                              - DHW blocking: unblocked (0)
18:12:54.733293 BC0000000 Read-Ack Status: 00000000 00000000
                              -  Fault indication: no fault (0)
                              - CH mode: not active (0)
                              - DHW mode: not active (0)
                              - Flame status: flame off (0)
                              - Cooling status: not active (0)
                              - CH2 mode: not active (0)
                              - Diagnostic indication: no diagnostics (0)
                              - Electricity production: not active (0)
18:12:55.675299 R80190000 Read-Data Boiler water temperature: 0.00
18:12:55.792588 B40192299 Read-Ack Boiler water temperature: 34.60
18:12:56.734301 R10014100 Write-Data Control setpoint: 65.00
18:12:56.874376 BD0014100 Write-Ack Control setpoint: 65.00
18:12:57.793291 R00030000 Read-Data Slave configuration: 00000000 0
18:12:57.912067 B40031F1B Read-Ack Slave configuration: 00011111 27
                              - DHW: present (1)
                              - Control type: on/off (1)
                              - Cooling: supported (1)
                              - DHW: storage tank (1)
                              - Master pump control: not allowed (1)
                              - CH2: not present (0)
                              - Remote water filling: available/unknown (0)
                              - Heat/cool mode control: heat (0)
18:12:59.116930 R00110000 Read-Data Relative modulation level: 0.00
18:12:59.120128 BC0110000 Read-Ack Relative modulation level: 0.00
18:12:59.937003 R001B0000 Read-Data Outside temperature: 0.00
18:13:00.371953 BC01B1400 Read-Ack Outside temperature: 20.00
18:13:00.969371 R801C0000 Read-Data Return water temperature: 0.00
18:13:01.086373 B401C1B00 Read-Ack Return water temperature: 27.00
18:13:02.028290 R900E6400 Write-Data Maximum relative modulation level: 100.00
18:13:02.145350 B500E6400 Write-Ack Maximum relative modulation level: 100.00
18:13:03.087286 R801A0000 Read-Data DHW temperature: 0.00
18:13:03.223381 B401A2433 Read-Ack DHW temperature: 36.20

Thanks in advance for any help!
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Re: DHW control of Baxi boiler with storage tank

Postby spee981 » Wed May 17, 2017 8:43 pm

I've just found this topic:
domoticaforum.eu/viewtopic.php?f=70& ... mp;t=10893
It's about Data ID 99, which controls (?) the operating mode to be HC1/HC2 or DHW. Since Baxi 7104336 is the recommended thermostat for my boiler, and it supports storage tank DHW programming, and sends MSG 99 as well, I think this message can be the key.

I'm already trying to extend the current gateway code with support for sending this message, but I have not found any specs which mention this data ID at all (and the link in the post is not alive anymore). Does anybody know anything about ID 99 ? I think it's safe to assume it is a write data message, and it contains flag8 data in high byte. Any idea or hints?
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Re: DHW control of Baxi boiler with storage tank

Postby hvxl » Thu May 18, 2017 11:10 am

The way the DHW setpoint works is quite different from the control setpoint. For DHW you basically set the desired hot water temperature you want to get from your taps. With the DHW enable bit you indicate when you want the boiler to keep a preheated amount of water. These two things are only very loosely related.

When there is a demand for hot water, the boiler will try to provide that at the specified temperature. If it has some amount of preheated water, it will be able to faster provide the water at the correct temperature. However, there is no guarantee that the preheated water is kept at the configured temperature. It would actually be very energy-inefficient if the boiler did that. So the temperature of the preheated water is probably going to fluctuate within a fairly large range.

To check if things are working properly set the DHW enable bit for a longer period of time and check that the boiler periodically heats its hot water reserve.
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Re: DHW control of Baxi boiler with storage tank

Postby spee981 » Thu May 18, 2017 1:15 pm

OK I'll try what you suggest, and wait even more. I gave up waiting here, btw:
18:26:29.017516 B401922B2 Read-Ack Boiler water temperature: 34.70
As soon as I can I'll redo the testing.

May I still not succeed, I plan to send 90630000 and 10630001 using the interface firmware, and check how the boiler reacts to them. I already know that for read message 99 it replies unknown ID.
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Re: DHW control of Baxi boiler with storage tank

Postby spee981 » Sun May 28, 2017 11:36 am

Msg 99 did not help, it's a read message not a write, but the boiler does not respond to it at in either case.

While using the interface firmware I failed to maintain continuous communication and/or did not send the proper series of messages, and the boiler registered a permanent error code 83 (communication error with the controller). Afterwards I was still able to control it with the gateway, but it refused to heat (and flashed the error code) while having nothing attached to it's OT terminals. Fortunately it still created DHW, but it would have been tedious to remove the gateway all the time when I want DHW, then reattach it to be able to heat...

So I bought the BAXI controller at last, and although it's display is horrific, it looks well at least. I mean, instead of meaningful words and sentences it uses 3-letter abbreviations only (which must have been fine 20 years before, but nowadays it's not impossible to to I18N in virtually any uC available on the market). E83 went away, and the BAXI controller communicates through the GW using OT messages.

And the solution for the boiler not creating DHW for me previously (for this case at least) : even though the boiler reports CH2 as "not present", the BAXI controller sets the CH2 control bit. If and only if CH2 and DHW enable are both set it does create DHW, for the DHW bit alone it only activates the function and shows it on it's display, like I described previously. When CH2 is set, and the DHW temperature falls below the DHW setpoint, the boiler fires, and starts heating DHW.
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