Viessmann Vitodens Circ Pump Runs Constantly

HeatingHelp

This discussion was created from comments split from: Viessmann Vitodens 200-W WB2B circ pump runs constantly.

jbjb

Just seen this thread dates back a few years, but it might help me with a recent problem which is very similar...

I'm in the UK and have three 49kW Vitodens 200-W in cascade with LLH and Vitotronic 300-K. I'm not using weather comp on the 300-K because it's being sent a 0-10V flow setpoint signal from separate controls which do the weather comp and drive the system pump - I just want a dumb cascade, which produces the flow temperature asked of it. Viessmann have fitted a fixed resistor in place of the outdoor sensor to provide a constant 12ºC outdoor temperature to the 300-K

The 300-K works pretty well and delivers the requested flow temp from the cascade. The issue is that the boiler circulating pumps are running 24/7. The pumps have two cables to them - AC supply and 0-10V speed control. In this application, the system can lie dormant for days and I don't want the pumps sitting there running, wasting energy. I'm very happy for them to do pump exercise when they want to and frost protection, but otherwise I want them off unless the boiler is active!

Let's start with the easy bit. I got excited that coding 2:51 in the boilers would sort this out. But it's not available on the boilers, despite being shown in the installation manual. I suspect that being in a cascade has removed this option somehow - in the coding pages, 2:51 is missing and skipped over.

So, that pretty much only leaves something in the 300-K, but I can find no mention of circulating pumps. The only vaguely-promising option is the ECO threshold in Cascade 2:41-44 but I haven't tried this. It appears to block individual boilers if the outside temp is above the threshold temp set. So, I'd have to set them all to 11ºC (or less) in my case. It's not clear if this would stop the pumps or whether, with ECO threshold set, the boilers would then still be available to the cascade.

The 300-K seems to take the highest of the 0-10V input or its own calculated weather comp flow setpoint. So, at one point, the 300-K was showing a flow setpoint of 29ºC with no external demand. Selecting the lowest curve and slope has got this down to 20ºC but I have a horrible feeling the boilers would fire to maintain this, which might explain why the pumps are running...

If I clear all the heating time programs in the 300-K, will that stop it? If I do this, will the 0-10V external demand input still work?

Sorry, lots of questions. A lovely system, in many ways, but I just need to stop the wretched pumps!

jbjb

OK - some small progress today.

Poking around in the Vitotronic 300-K, realised that it has its own heating and DHW schedules, which I'm not using. Was going to wipe these out. Then realised I could change the system mode from "Heating and DHW" to "Standby". When I do this, the flow setpoint drops to 0ºC. Checked that the 0-10V input can still fire up the plant (which it can), so this looks like a step forward, in that the 300-K itself should no longer be trying to drive the plant.

But... the pumps didn't stop - at least, not while I waited a few minutes to see. So, I'm still looking for that elusive setting to stop the boiler pumps running when there is absolutely nothing for them to do.

Erin Holohan Haskell

@jbjb, I've moved your posts here so they don't get lost at the bottom of an old thread. Thanks.

gennady

These schedules on 300k should be set to 24HRS operation on heat and DHW. What exact model number of 300k do you have? You need to do some disassembly to get to it. It’s on lower right corner, under the cover. better post photo. Also, what controls are installed on the boilers? Are there vitotronic 100 or 200? Older systems had 100 when supplied for cascade installation.
Also, what is the layout number? You find it at address 00.

jbjb

@gennady - thanks for coming back. Boilers have the older constant temp Vitotronic 100s in them as they're only really looking after the boiler and interfacing it to the cascade - all the clever stuff is done in the 300K, by the look of it. Layout is 01 from memory.

OK - I think I have cracked the problems, by a combination of actions.

1) Boilers are supplied with Wilo Strata variable speed pumps with separate 0-10VDC speed control cable. All the pumps had been set to constant head setting 6, which the manual seems to indicate. With the burner running, I checked the 0-10V output from the boiler and it was sending 100%. This got me thinking - why would you fit an expensive variable speed pump and then leave it set at constant speed? So, I changed the pumps to the External Input setting. With the burner running, they appear to run flat out. Several minutes after the burner stops firing, the pumps ramp down and then stop - hooray! Silence, at last. The boiler then seems to fire the pump for a few seconds every hour or so, maybe to exercise it or update the boiler flow temp?

My installer was concerned about taking them off 24/7 constant speed running in case it upset the hydraulics of the cascade. If it's running them all at 100% when the boiler fires, I'm not sure it makes a lot of difference. I don't want pumps running on 2 boilers that aren't firing - that just wastes heat up the flue and into the plant room besides the energy consumption and annoyance of three pumps running 24/7. So, this seems the better option - I'll monitor and see what happens.



2) The other issue I had was finding the LLH warm when it shouldn't have been, even with the 300K schedule set to Standby and showing 0ºC flow setpoint. The external BMS sends a 10% demand signal when it's at rest and this is right on the threshold of what the 300K needs to fire the cascade up. This was causing the cascade to start, including firing the lead boiler briefly, presumably to check it was OK? This was enough to keep the LLH at 40ºC! So I needed to drop the demand voltage signal a smidge to stop this.

To cut a long story short, I've fitted a forward-biased Schottky diode in the 0-10V demand circuit. This reduces the sending voltage by a constant 50mV, which is just enough to shut the 300K down without causing a big temperature error (approx 0.5ºC) over the operating range.

So, all issues cracked and the plant is now doing exactly what I had been hoping for.