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Discuss Viessmann boiler short cycling in the Central Heating Forum area at PlumbersForums.net

D

doitmyself

I'm not aware of any motorized valves.
There must be one, or two, somewhere. They control the flow of water through the CH and HW circuits so they only get heated when required. If you dont have them, both circuits would be heated whenever the boiler was alight.

How were hot water times/temperatures controlled before the Usense was installed? There must have been valves to stop CH and HW heating up when not required?

With an Opentherm system it's normal to have a diverter valve which allows either the HW or the CH to be heated, not both at the same time. The reason for this is that HW required the flow temperature to be constant at about 70C, while CH will vary the temperature as required. A diverter valve may have been installed in the boiler (its a Viessmann option); check with the installer.

As I said, the Usense is designed for use with some Baxi combi boilers. Like all combis, they have a diverter valve built in and automatically disconnect Opentherm when a tap is turned on, so the boiler runs at max output. The HW feature on the Usense is there for preheating only.
 
OP
V

Valdas

There must be one, or two, somewhere. They control the flow of water through the CH and HW circuits so they only get heated when required. If you dont have them, both circuits would be heated whenever the boiler was alight.

How were hot water times/temperatures controlled before the Usense was installed? There must have been valves to stop CH and HW heating up when not required?

With an Opentherm system it's normal to have a diverter valve which allows either the HW or the CH to be heated, not both at the same time. The reason for this is that HW required the flow temperature to be constant at about 70C, while CH will vary the temperature as required. A diverter valve may have been installed in the boiler (its a Viessmann option); check with the installer.

As I said, the Usense is designed for use with some Baxi combi boilers. Like all combis, they have a diverter valve built in and automatically disconnect Opentherm when a tap is turned on, so the boiler runs at max output. The HW feature on the Usense is there for preheating only.
I think boiler does all the switching as it has separate pipes for CH and HW. It behaves very similar to combi boiler. When the temperature inside the tank drops then boiler stops CH flow and switches to HW circuit. There is an indicator on boiler's display telling which circuit is currently running.

Viessmann boiler has an option to set HW temperature to maintain in tank so before thermostat was installed I simply set 60C HW temperature using boiler's control panel.

Here is how it looks.
91764956_246189669763240_1379886616117510144_n.jpg
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It seems to me that you found the problem stating in post #11 that the SP has to be > 30C to get the boiler away which indicates that the minimum weather compensated SP temperature should be > 30C, if the SP is say 32C then the boiler shouldn't fire until 25C (SP-7) and must not reach 39C (SP+7) to avoid cut out.
It might be true. I unplugged weather compensation sensor yesterday and haven't noticed short cycling since then. Weather is getting warmer and boiler runs not often so need more time to be sure short cycling is gone.
 
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J

John.g

What is the HW SP now?.
If you look at page 78 of the attached you will see the selectable weather compensation curves, yours seems to be set up on a very low curve but doesn't really make a lot of sense as it runs out of settings, if this is your curve setting table would suggest setting somewhere between "15" and "20".
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Should have asked, "what is the CH SP now" above.
 
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D

doitmyself

yours seems to be set up on a very low curve but doesn't really make a lot of sense as it runs out of settings, if this is your curve setting table would suggest setting somewhere between "15" and "20".
I thought the curve was very flat, but then remembered that the OP said he had underfloor heating (on both floors??)
 
OP
V

Valdas

What is the HW SP now?.
If you look at page 78 of the attached you will see the selectable weather compensation curves, yours seems to be set up on a very low curve but doesn't really make a lot of sense as it runs out of settings, if this is your curve setting table would suggest setting somewhere between "15" and "20".
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Should have asked, "what is the CH SP now" above.
CH SP is controlled by thermostat. Monitoring tool shows CH SP is 7C when there is no need for heating. When thermostat decides it's time to start heating, it sets CH SP to 50C (which MAX CH water setpoint) and the boiler starts up at once without short cycling as temperature spike during ignition doesn't exceeds 50C. After that thermostat lowers CH SP and when it's time to stop heating it lowers CH SP more and the boiler cuts off.
The heating curve I've provided is from thermostat which overrides boiler's heating curve. Those heating curves from the manual are quite steep and fit more for radiator based heating.
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I thought the curve was very flat, but then remembered that the OP said he had underfloor heating (on both floors??)
Yes it's all underfloor heating, no radiators.
 
K

Klump99

I had a similar problem, for me it was the flow rate sensor, and this was with 100w as well.
 
J

John.g

It makes sense to me to kick the SP up initially to get the boiler to stabilize and then reduce it, you said you had removed the weather compensation sensor so on what basis is the boiler now controlling its SP? the curve you gave looks like the boiler is still looking at a outside temperature from somewhere.
Can I assume it doesn't/wasn't doing this (initially increasing the SP to 50C) when running on the outside temperature compensation and have you the curve for this?.

Edit: I'm mixed up I think, the curve you gave is for the outside temp compensation? so can you provide the boiler curve or its basis for changing the SP?.
 
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D

doitmyself

I'm mixed up I think, the curve you gave is for the outside temp compensation? so can you provide the boiler curve or its basis for changing the SP?
Yes, the curve supplied by the OP is for the weather compensator.

Theree is no such thing as a boiler curve. The boiler is told by the thermostat, via Opentherm, what the required SP is. How this is calculated is known only to the stat manufacturers. I would assume that it will depend on the difference between actual and required room temperatures and take into account the rate of temperature drop.

To quote the Opentherm Protocol:

The slave [boiler] does not need to know how the master [thermostat} has calculated the control setpoint, e.g.
whether it used room control or OTC [outdoor temperature control], it only needs to control to the value. Likewise, the master does not need to know how the slave is controlling the supply.
 
J

John.g

Thanks, that clears it up for me, I had thought of the room stat as just a on/off switch without communication. It is interesting how it starts up at 50C., its a pity that the OTC doesn't seem to do likewise but I suppose that is only a sensor.

I have seen a relations Vokera Vision 20S with a cruder version of this called SARA, if the room stat contacts stay closed for longer than 20 minutes then the boiler SP temperature is increased as long as the "manual" boiler SP is set between certain limits.
 
OP
V

Valdas

There was not a single short cycle after disconnecting outside temperature sensor for more than 2 weeks. Without OTC thermostat sets 50C SP when it's time to start heating and boiler overcomes the spike without switching off.
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I had a similar problem, for me it was the flow rate sensor, and this was with 100w as well.
Was the boiler short cycling because of rapidly rising temperature during the ignition phase? What has changed after replacing flow rate sensor?
 
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D

doitmyself

There was not a single short cycle after disconnecting outside temperature sensor for more than 2 weeks.
I've been working on the 24 hr log you sent me. I have converted it to an Excel file to make analysis easier.

Would it be possible to send me another 24 hr log, without the outdoor sensor, connected, so I can see exactly what happens?

Heres a chart showing how the boiler runs over the 24 hours. The oudoor temperature uses the vertical scale on the left, All other lines use the left hand scale where 100 is either 100C or 100%. The thing I do not understand is why the boiler is switching up and down between modulations which are 20% apart when on heating, but more refined when on hot water. It's as if TPI (from the usense?) was controlling the boiler temperature. The actual room temperature stays very constant at 22C so I haven't bothered to show it.

Usense with Outdoor Sensor.png
 

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