Vitodens Question

Mike T., Swampeast MO

Situation: Vitodens operating a gravity conversion with TRVs. No low loss header. Small (bathrooms) "wild" radiant floors tapped into the BIG supply mains. Copper tube in Thermofin under ¾" plywood, cement board and ceramic tile. 1 mbh or so design loss for the radiant areas.

Problems:

1) The volume/mass of the heating mains acts like a "buffer". Great for the system as a whole, but BAD for the radiant floors when I'm operating the boiler for maximum efficiency. While the supply temperature produced <I>in the boiler</I> is sufficient for the radiant floors, it's SIGNIFICANTLY reduced just a few feet away from the boiler.

2) Am only using the Vitodens built-in circulator. As the reset curve is optimized, system flow requirement increases and head loss decreases (effectively zero head loss in the piping and the TRVs are a bit more open so they're not putting up much resistance to flow). Head loss through the radiant loops is low, but [it seems] not quite low enough as flow through them (as evidenced by delta-t) is barely a trickle...

Possible solution: 1" x 3/8" tee IMMEDIATELY after the Vitodens supply connection. TINY circulator installed in that 3/8" pipe. 3/8" pipe runs to the little radiant floor supply manifold. Radiant floor return manifold still connected deep in the system to the return main.

Have I completely flipped my gourd or do you think this has a chance of working?

Any other ideas?

Uni R

You've flipped, but long ago...

What about a Monoflo just before the return on the boiler and skip the circ? Would you have enough velocity in the return of your mains to suck water through the under floor system?

Mike T., Swampeast MO

That's certainly an idea! Velocity at the near-boiler return (3-6 gpm or so through a 1" pipe) is actually reasonable... BUT, don't monoflows operate because of delta-p? Am 99.9% certain that delta-p in the radiant loops is already too high. Even last year when I operated the Vitodens at a MUCH higher curve, I had to open the ball valves for the radiant loops fully and still didn't get quite the performance as with the old boiler, B&G 100 circulator and the valves about 2/3 closed.

Being "flipped" has an occasional advantage as you aren't necessarily limited to "normal" ideas...

S Ebels

You're OK Mike

It's the "normal" people that bug me.

I'm assumimg you have 1" piping from the Vito feeding into the old mains. Correct? If that's the case, you could install a tee feeding the radiant loops, followed by a throttling valve of some kind to balance the "head" seen by the radiant circuit. The return for the radiant could be left as is I think.

Glen

same idea -

it is my preference to throttle on the return though. Works for me - and as you are using the on board circ (programmed to what capacity?) may add some longevity to the circ used.

Mike T., Swampeast MO

Steve & Glen

Guess I'm "normally flipped" Used to consider myself a "compassionate conservative" until the term got perverted...

1" supply/return lines from/to the Vitodens flow into the old 1¼" near boiler piping which in turn flow into the two pairs of 3" mains.

The Vito circulator is set for wide-open operation (full variability) at Viessmann recommendation. I still have no idea how the speed is actually determined, but suspect it's more than just outside temperature as it [seems] to be operating at a higher speed with the lower reset curve.

Am not sure if throttling the supply to the rest of the system would work. Increase the head at the throttling valve and flow will reduce. Reduce the flow and the TRVs will open wider to restore the same flow level. Wider open TRVs at same flow means lower head through them so head hasn't changed predictably. Throttle a lot and flow ability through the system as a whole will drop so either the circulator will have to run faster or there won't be sufficient flow through anything save the little radiant floors. Suppose I could replace one of the full port ball valves in the near-boiler with a good metering valve to test.

Would you expect any strange flow problem if I did use the second tiny circulator? One I'm looking at is variable speed DC designed for solar. With enough effort, I could possibly set up a circuit to modulate its flow to keep return temps from the radiant in the desired range.

Thought of using instituting a two-temp system using a Viessmann mixing valve, but the cost is high and the load tiny. Also thought of using a HX off the domestic hot water system but then I'd need thermostats in the baths and two additional circulators.

Any comments and suggestions greatly appreciated. I hate to sacrifice system efficiency for the sake of three tiny radiant floors. Won't always be operating the Vitodens at such a low curve, but can tell from the graphs that the temp available in the mains won't be sufficient with "normal" indoor conditions and an efficiency optimized curve.