Vaillant Ecotec Pro 28 - Modulation problem
Hi Folks,
Sorry in advance for the long post.
I've just installed an older version of the extension kit (EK01-TC-UK-03) wireless thermostat and a couple of SRTs. I have an older Vaillant Ecotec Pro 28 (pre 2010) with the older circuit board that doesn't have the blocks for Ebus, RT and Burner off. It has a long block with numbers 3 4 5, 7 8 9 and the + - for Ebus. I have connected two wires from the + - on the extension kit to the + - on the boiler and I have bridged terminals 3 and 4 on the boiler to set to always on. I have a the timer switch installed on the boiler faceplate and have set this to on. I have set up the thermostat according to installer instructions for Vaillant Ebus, D07 etc.
The setup works but doesn't seem to modulate. I am able to change the flow temperature by turning the dial on the boiler and it heats up to that temperature every time it fires. It seems that the boiler is working as if in relay mode? (on/off only). If I set the flow temperature using the control on the boiler then when the Tado system calls for heat the boiler always heats up to the temperature that I set, it does not modulate up and down. Also the boiler cycles on and off after short periods with pump runs, which I believe it's how Tado controls heating in relay mode.
I contacted support and this was their response.
"We have checked your system and it seems that tado° does modulate your boiler, but because the temperature difference for your set temperatures are very small that is why it isn't noticable.
Modulation is noticable when the temperature difference is larger."
This was my reply:
"Thanks very much for your reply and for looking into my situation.
A couple of things.
What exactly do you mean by "difference for your set temperatures are very small". I have two set temperatures for the single zone I have set up so far, daytime 19 degrees and night time 16 degrees. The temperature hasn't fallen below 16 degrees at night so far.
Could you give me an example of how the boiler would modulate if the temperature difference were larger? What would a large enough temperature difference be for the modulation to be noticeable? Normally with a boiler that modulates it will start off with a higher flow temperature to get up to the set point then it will modulate down to a lower temperature to maintain the set temp.
For example I have a friend with a similar Tado system and when the boiler has achieved the set temperature the Tado control reduces the flow temp considerably to maintain that set temp (say from 60 degrees to 45 degrees flow temp). In my case the flow temp while the boiler is firing never changes at all even when the set temp has been reached. It just switches off."
There are a few reasons I don't think the boiler is modulating. I can alter the maximum flow temp for the boiler using the control knob on the boiler and that is then the temp that the boiler heats to, if I increase or decrease this temp using the boiler controls, then the boiler heats the water to that new temp. If the Tado controls were modulating the flow temp then I wouldn't be able to set the temperature manually and the flow temperature would depend on the Tado controls, not what I set it to. All the Tado controls are doing is switching the boiler on, it then heats up to the temperature I have set, then they switch it off and the pump runs for a while, then they repeat this over and over, on and off. Isn't this is how the boiler would be controlled in relay mode?"
Tado response:
"In your case the temperature differences were 2 degrees in the last couple of days. The system was modulating, but very little. That is because there was not much need to modulate.
based on the heat request, the heating system modulates the heat it supplies to reach and maintain a specific temperature.
This you can notice that when the set temperature is reached. the boiler doesn't fully turn on all the time to maintain that temperature."
This doesn't seem right to me as the boiler was heating every time to the maximum flow temp that I set and then cycling on and off to maintain the temp rather than reducing the flow temp.
With the Tado system working through Ebus I shouldn't be able to influence the flow temp with the boiler controls, right?
Any thoughts would be much appreciated.
Comments
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@dmcmason I don't know if this is relevant or helpful, but in October I installed Tado with my Vaillant Ecotec 831 Plus combi boiler. Initially I connected it in relay mode, as it was with my old timer/thermostat. But after only a couple of days I rewired the system for e-Bus. This worked perfectly in that Tado was asking for flow temperatures as low as 33C all the way up to the set maximum of 50/60/75 or whatever I tried for experimentation.
The problem I found was not with Tado, but with the boiler. Maximum CH output is 24kWh, but minimum is a whopping 9kWh. The boiler can and does modulate down - in fact I rated it down to 9kW only - but that's far too powerful when Tado has closed off most of the valves and there is only one, perhaps two radiators chucking out heat.
Water in the system might be at, say 27C and when Tado called for a paltry 33C the boiler flow reached that in a few seconds, turned off the gas, overran the pump for five minutes and then entered anti-cycling mode for the next several minutes. Basically the boiler was cycling ineffectively until eventually Tado ran out of patience and asked for a higher temperature.
After a few weeks of this I gave up with e-Bus and returned to relay operation. Now I get proper burn times and some actual heat reaching the radiators instead of wasting energy pumping tepid water nowhere.
So the question is, can your boiler modulate sufficiently low or are you hitting the same or similar problem? Of course, if Tado is always asking for full heat every time then maybe e-Bus is not working as it should.
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@eezytiger Thanks for your reply.
That's really helpful. My boiler also only rated down to 9kW so that could be an issue. But it seems to me from observing the flow temps on the boiler that it isn't modulating noticeably, it tends to heat right up to the maximum that I have set and then shut down and run the pump before anti cycling and keep repeating this. I haven't seen it run at lower temps than the max. You mention your boiler was running up to a set maximum on Ebus. Do you mean that you were able to set a maximum flow temp and that the boiler ran up to that level while using Ebus? That's what seems to be happening with my boiler, but I thought you weren't supposed to be able to manually set the maximum with Ebus? I thought the Tado system decided what the maximum was depending on the situation?
I had a reply from another person from Tado today...
"Your boiler flow temperature is spiking quite alot, the instances you see when the temperature is at the max are almost always only a few minutes long.
This usually means there's some kind of circulation issue, either not enough water pressure, or air bubbles trapped in the pipes, pump or radiators, or the return flow temperature is too high.
The last one you can easily fix yourself by removing a TRV from the radiator closest to your boiler.
For the other ones, check the water levels and try to bleed the radiators to get rid of any air trapped in the system."
I'm not sure what he means by the flow temp is spiking a lot but I checked the points he raised and system pressure is fine and no air in rads. I checked the return temp and boiler says it's about 50 to 52 degrees at a flow temp of 60. Not sure if this is classed as "too high".
My system is not well balanced so I might try balancing it to see if that helps reduce the return temp, but I still think it's not modulating as I never see flow temps lower than the max I have set.
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@dmcmason when using e-Bus I was able to set maximum flow temperature using the CH dial on the boiler. Tado appeared to honour this maximum, but was free to request a lower flow temperature and frequently did. I monitored the e-Bus demand temperature using boiler code d.9, and general operation from some other code values.
There was usually a little bit of temperature overshoot, maybe 2-3 degrees, but flow temperature interests me less than return temperature, which I wanted to keep low enough to maintain effective condensing operation. I.e. below 55C.
Whilst the boiler and system was heating up I would only see a temperature drop of around 10C between flow and return at the boiler. Once the boiler shut off the gas these numbers would narrow to next to no difference as the pump continued for a further five minutes without new heat being added. I think this kind of drop is fine when you are using lower temperatures than 70-80C and several rads are shut off. The system simply can't lose the heat fast enough.
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@eezytiger Thanks for that.
I had already been using some of the d codes, 40 and 41 mainly but hadn't spotted d9 which is really useful. After monitoring this I can see that the boiler is in fact modulating and seems to be modulating more now that the weather is a bit milder, so wondering if the controller is taking the outside temp into account. It is still only requesting flow temps about 6 or 7 degrees below maximum, but at least I can see that modulation is happening.
The boiler is still short cycling but after hearing your experience I'm guessing my situation is similar. Except you said that that you weren't getting decent flow temps and the flow temps I'm getting seem to be higher. I have asked Tado about the short cycling so will see what they say. I think if I balance my radiators so more heat is getting to the last couple that might reduce the return temp. At the moment I've got about a 10 degree difference as well.
I've got a guy coming to service my boiler soon who specialises in Vaillant and installs smart heating systems so hopefully he might have some thoughts.
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