Two different temperatures fo UFH and Megaflow cylinder

[evgeny]

Hi,

We are renovating a flat. We will have Underflow Heating in the whole flat and Indirect megaflow cylinder for DHW.
We want to use Worchester 30CDI system boiler to indirectly heat Megafloow and heat UFH.
For UFH we need to heat water max upto 60 degrees; to heat megaflow cylinder we need water temperature as high as possible (probably 80-90 degrees or similiar)
Normally there is only one temperature set for Central heating on a boiler. But 30 CDI boiler has diverter valve and separate temperature control used when different water temperature is needed: normally for different temperature for DHW and central heating, when 30 CDI is installed as a combi (without Megaflow cylinder). When system with unvented cylinder is heated this diverter valve and independent temperature control is disabled.
My question: is it possible to use this diverter valve and independent temperature control to supply Megaflow cylinder with water 80-90 degrees and UFH with 60. It might be a bit more efficient than supply UFH with 80 degees water and then mix it again with cold water to bring water temperature back to 60 for UFH pipes.

 

[WHPES]

Yes, standard paractice. You don't need to do anything different with the main part of the circuit though - the UFH unit is supplied with 80 degrees typically but the kit includes a thermo mixing valve to lower temp before it goes round the UFH circuit. Too detailed to discuss here but I suggest you do a bit of reading up - plenty or resources on the net.

 

[evgeny]

Yes true, you are referring to standard prctice supply 80 degee water to UFH manifold, mix with cold water in manifold mixing valve.

What we would like to do is to supply water from boiler with temperature 60 degrees to manifold, so that UFH manifold do not need to mix it with cold water at all.

The difficult part is to supply 80 degees water to megaflow cylinder at the same time. The question is can we achieve it by using this diverter valve in 30CDI?

 

[WHPES]

Don't see how you can do it other than by having a circulator for the UFH and mixing UFH return water with hot water to get 60 degrees in that circuit alone. If boiler is only putting out 60 degrees Megaflow is going to be slower to heat up and you are going to get hysteresis problems and the boiler is not going to run efficiently as it will always be firing.

 

[evgeny]

True again for the standard heating boiler,as there is only one CH temperature set possible.

It is different for 30CDI: there two versions of it 30CDI Sytem amd 30CDI Combination. Both are essentially the same probably slightly differently prewired.

30CDI is able to supply water with two different temperatures. When 30 CDI is used as a combi: 60 degrees can be supplied to DHW and 80-90 to CH.

My question is when 30CDI is used as a system boiler: second temperature control in 30CDI is not used. It is disabled.

The idea is: use first control to supply 60 degees to UFH (if 30 CDI would have been used as combi this 60 degees would go to DHW) and second control to supply 80-90 to megaflow cylinder.

 

[WHPES]

My misunderstanding. Not familiar with that boiler (I only do oil fired work). I'd contact Worcester Bosch Tech support as they are pretty helpful with stuff like that.

 

[evgeny]

I have tried. The were not very helpful. It seems to be a non-standard setup, and they were unable to advise.
This is the reason I am writing to this forum. Maybe someone will advise if it is at all possible reasonable

 

[WHPES]

If the outputs can be different, it's probably more down to the controls so maybe have a word with a good spark.

Be careful with potential hysteresis issues with any thermostats!

Probably easier to supply everything with the 80-90 degrees output. I expect it would be more efficient as you will get better heat transfer.

 

[Cds]

Looking at the installation instructions there is no indication of separate temperature control of the two circuits when the diverting valve is used. My best guess would be that it will not work.

There are a few boilers available that provide two circuits that can be controlled at different temperatures. Have a look at Viessmann, Keston and Eco Hometec (there are probably others) as I think they all do boilers with either separate circuits or circuits that can be run at different temperatures depending on the demand.

 

[evgeny]

Could you give any specific reference? Cannot find anything in the internet.
And neither Worcester nor Vaillant have any boiler with two independently controls curcuits ?

 

[Plucky]

I can see what you are getting at but this can not be done on this boiler.

If you can get a boiler to do it then it will be good for your gas bills! Cheaper to heat water to 60 degrees than to heat it to 80 degrees only to blend it back down again!

 

[unguided1]

why on earth do you want the temperature in the megaflo to be 80-90 degrees as i recall the pressure temperature relief valve is preset to fail at 90-95 degrees. I assume the reason you want the underfloor temp to be 60 degrees is because it is under a wooden floor, if its under screed you should be able to run it down as low as 35 degrees

 

[doitmyself]

You have asked the same question in another post on this forum. Which one do you want us to answer, as I don't feel like repeating myself?