Problem with underfloor heating

[MikeH]

Thanks so much for the time you've taken over this.

Yes, I found that documentation on the Danfoss site, but have to admit that I didn't really understand it. Now I know how those folk who can't assemble flat-pack furniture from a few cryptic drawings feel. :-)

What I actually have is the layout depicted in the top-right drawing on page 2 showing a combination of a VMV with a RAVK. The primary pumped flow from the boiler goes into port B, and the return to the boiler (along with the UHF secondary return) connects to port A. The valve output on port AB goes into the UFH pump, the output of which feeds the UHF loop.

I understand that it's being used as a mixing valve, but I don't understand the difference between the top left (which is titled 'Mixing Valve RAVK...") and the top right configurations on page 2 of the Danfoss data sheet. In the top left the primary flow goes into port A, but in the application using a VMV and RAVK in the top-right of the page shows the primary flow going into port B. I think I must be missing something fundamental here... Is there some significance in the two different symbols for the load used in the two diagrams? Either way, I have the layout as shown at the *top-right* of the page (and this also matches the layout documentation provided by the supplier of the UHF kit).

I do hope I haven't misled you in my earlier posts by omitting to mention the RAVK element, but my assumption was that the VMV bit is the physical valve whereas the RAVK is the mechanism for actuating it - in this case via the use of a remote temperature sense probe on the output from the AB port.

The original pump is on the first floor in the airing cupboard along with the hot water tank. In the airing cupboard there are tee connections into the main primary flow and return pipes to/from the boiler (which is in the loft immediately above the airing cupboard). New pipe runs take the additional primary flow and return down to the ground floor where the UHF kit is installed (additional pump, VMV mixing valve, manifolds, zone valve, etc). The UFH is therefore effectively fed with it's own primary flow and return from the boiler - is this what you mean when you say...

"The diagram only makes sense if the flow and return pipes are connected to the same pipe, giving negligible pressure difference between them. . The secondary pump will draw water from the passing primary as it needs it, but the primary needs a flow route to keep it going past"

has omitted to provide the minimum 3 litre/min bypass that is required by the boiler.

Does this make any more sense?

Apologies again if I've misled you in my earlier descriptions of the system.

Mike

[The Natural Philosopher]

Something has occurred to me...you HAVE got the pumps the right way round, so that the two pumps don't work against each other when the valve is open?

[MikeH]

To answer one of my own questions, I've just realised that my confusion over the drawings on the Danfoss data sheet arose because I hadn't noticed the difference between 'normally open' and 'normally closed'. On reflection I guess it's perfectly reasonable to have the flow and return connections 'reversed' at the valve, depending on the application, because it will allow the actuating mechanism to work 'in the other direction' and still achieve the same outcome from the valve

- if you see what I mean.

If this doesn't make any sense, just ignore my earlier statement about not understanding why one diagram should show the flow connecting to port A while another shows it on port B - despite how it might appear from my ramblings in the paragraph above, I think I now understand that bit. :-)

Mike

[MikeH]

Yeah - they're the right way round with respect to one another. That was one of the things I checked at the start... but thanks for the suggestion. The more theories the better as far as I'm concerned. :-)

The ONLY time it misbehaves at all is when there's no other load on the system. At all other times the UFH works fine and the temperature control is perfectly stable at 45 degrees C. The problems only start when neither the C/H or H/W stats are calling for heat and the UFH is turned on. When that happens the primary circuit is closed (apart from the UFH bit) by the combination of the original 3-port diverter valve and the additional 2-port zone valve that shuts off the flow to the H/W cylinder coil.

My gut feeling is still that the VMV is effectively closing the primary circuit and that the solution is the provision of some sort of bypass to meet the 3 litre/min boiler requirement....

But I'm willing to be shot down in flames on this - I know damn-all about the subject really.

Mike

[MikeH]

Is a 'cross-over bridge' a connection between flow and return? If so, could it be achieved using a towel rail (as I want to fit one anyway).

At the moment there's a zone valve on the output of the UFH pump where it feeds the UFH loop. That valve is activated when the UHF calls for heat, and the pumps and boiler are in turn activated when the valve is fully open. Presumably the additional zone valve you propose is to prevent flow through the cross-over bridge when the UFH is not on?

Is the positioning of the cross-over bridge important? If I was to use a towel rail I would need to tee off the primary flow and return within two or three feet of the VMV mixing valve.

Mike

[Aidan]

Yes. You usually need a regulating valve in it, or you get a roaring short-circuit across it and everything else gets starved of flow.

It's usually just a plain pipe. The towel rail would be controlled by a TRV, or the room would be liable to overheat. You could try it, operating towel rail & UFH with an electric thermostat, I can't see why it shouldn't work. If the flow rates were set exactly, the cross-over return would be just above the UFH return temperature when the mixing valve was at 100%, barely warm.

If it was set up as I've described, there will be no flow in the UFH loop unless the UFH pump runs, so that valve would be redundant.

i don't follow the question. The primary flow through the cross-over is generated by the primary pump. Flow through the UFH loop is generated by the UFH/secondary pump.

I think the answer to that lies on the Danfoss data page. The maximum pressure difference across the 15/20 VMV is given as 0.6/0.5 bar. It gets smaller for bigger valves, presumably the port area is larger. A UPS 15-50 will produce 0.6 bar at 0 flow, but you've got two pumps, with very little flow generating a pressure difference across the valve.

The Danfoss diagram entitled "Combination of ABV and VMV" looks plain wrong to me; little/no flow through the boiler with the mixing valve in some positions. Add a cross-over pipe between boiler flow & return connections with a regulating valve and put in a primary pump (unless it's in the boiler) and it will work. It's similar to what you need.

[MikeH]

Thanks Aidan, I think I've pretty-well got it now (at last!).

Your help is much appreciated.

Mike

[freddyuk]

So Mike did it all work out? I am doing the same project with a RAVK valve and as i see it there i a direct feed from the UFH return pipe into the RAV K so it will dra returning cool water to keep temperature stable while it is calling fo heat as it needs much cooler water than the CH and DHW.I think i can d this with only one pump

-- freddyuk