Vitodens - primary/secondary or direct?

Mike T., Swampeast MO

With a straight gravity conversion system the Vitodens won't be able to find any delta-p and you will have to use primary-secondary. It would damage the circulator without.

With TRVs you'll need to verify that the combined Cv rating of the TRVs at typical maintenance flow provides sufficient resistance to the circulator. A differential pressure of 1/2 psi provides sufficient flow for some TRVs but this might not be enough restriction for the circulator to operate without undue wear. In any event a gravity conversion would seem to test the limits of the boilers' sensitivity to delta p.

Holly Gates

Primary/secondary on the vidodens?

Hello there, I am looking to replace a conversion boiler in a gravity system with a Vitodens 200. Right now, I just want to drop in the new boiler (with new near boiler piping), but eventually I want to repipe everything to get rid of all that huge pipe cluttering up the basement and install TRVs on the radiators.

I've spoken with someone that sells these units, as well as the company rep in my area, both of whom recommend doing primary/secondary using the Viessmann low loss header. This seems reasonable to me, especially with the current piping and no TRVs, but I wanted to get some more opinions. My feeling is that if it is just as good to put the boiler in the main loop the inclination might be to do that and avoid the expense and extra power consumption of another circulator and the header. However I think there is certainly a good argument to doing primary/secondary, especially with the current gravity setup.

After repiping so the radiators are in a reverse return parallel configuration with TRVs though, it seems like the best thing would be a variable speed circulator set up to maintain a constant delta P across the radiators feeding the main loop. From what I can tell, this would might be achieved by putting the Vitodens in the main loop but would be lost by powering the main loop with a run of the mill circulator.

Anyways, any third opinions for me? Thanks,
-Holly

heatboy

Use the Low Loss Header, Holly.

The Low Loss header will serve/act a debris seperator. You could have some nasties in the system and by using the header, most of the stuff will settle to the bottom of it. The HX on the Vitodens, while bigger than all of the other condensing boilers HXs, has pretty narrow passage ways.

This is a picture of one we are making in the shop for an application exactly like yours. The mixing valve is for some radaint we installed, but the entire remaining house consists of cast iron radiators. Electrical consumption, especially on the 6-24 and 8-32, is tiny since the integral pump varies it's output.

hb

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Glen

The vitodens/low loss header/TRV combinations is a perfect setup for constant circ as well on the secondary side. You'll probably get many opinions on this one!