Hacking combi to run dhw and heating at the same time

[sonicboom]

I am running a low temp heating system (3kw heating + weather comp) with a viessmann 100-w boiler than can churn out 26kw in heating and 30kw dhw. Dhw doesn't go above 40c @ 4lpm which is about 7kw. It is frustrating that considering how low the energy usage is that the combi can only run dhw or heating and not both together.

 

I am temped to remove the seal rings from the diverter value. Has anyone been ballsy enough to do something so drastic ? What possible issues could I come up against ?

 

The alternative would be a cylinder but that has its own issues. It's annoying that viessman and co have introduced so much tech into newer boilers without thinking about the fact that lower consumption makes it possible to run both things at the same time.

Edited Saturday at 19:00 by sonicboom

[sharpener]

Don't see why it wouldn't work from the heat transfer point of view. But the various sensors and software may view it as an error condition and it will be difficult to hack that.

As a less elegant solution you could fit an external heat exchanger to do the HW and plumb it up as a second heating zone, then the boiler would not be able to tell the difference. Except you wouldn't want the WC to function when there is a demand for HW. I have had to get round that with an external relay as Vokera did not have a solution for this.

[JohnMo]

It sounds like your flow switch isn't working on the DHW side. The flow switch should push the burner output to the max. Or do you have the settings just wrong in the controller and you have set DHW to 40 instead of 60ish.

 

Heating and DHW should be different parts in the boiler logic. DHW setting be completely different to heating.

 

Easy to set a combi to run a cylinder. And a way better solution compared to combi. Heat pump cylinder, will give 55 Deg water from 60 Deg flow temp in 25 to 30 mins (210L). You need a resister to move boiler from thinking it's running weather comp to not, it then flicks over to achieve flow temp set in the boiler controller.

[marshian]

Is the output of the boiler capped for HW for either temp or boiler power?

 

I can understand the output being capped for CH because 26kW boiler feeding a 3kWh heat loss is massively oversized but it makes no sense to me to have only 7kW and 40 deg water

 

Have you got the ViCare App so you can see what the boiler is doing?

 

In my app I can set HW temp different to CH (100-W heat only with 115L cylinder and HW demand box) I'm pretty sure the app would enable you to increase the HW set point to say 50 and the boiler would modulate according to HW flow.

 

I really don't see the point of trying to hack the boiler to do both at the same time - with a heat loss of 3kWh you aren't going to see a house temp drop even if you are running a really big and deep bath!!!