I cannot offer any suggestion; my UFH is around 15 years and never been flushed. When the gas guy was around earlier in the year to service the boiler, I did ask him and he said not necessary. Although I installed the UFH myself, he was the one to commission the system.

The UFH are ali lined pipes, all fittings are either brass or stainless and there is some copper pipe. As he told me, if there is no steel in the system, there will not be sludge.

But what do I know? However, I do notice that the actuator sight glass are slightly greenish, so assume there is some kinda 'growth' in the system

Posted By: RexI do notice that the actuator sight glass are slightly greenish, so assume there is some kinda 'growth' in the system

Could be, or is it possibly just corrosion of the copper pipe lengths?

What sort of water is put in UFH systems (I have no idea)? Does it get topped up or is it sealed?

Posted By: fostertom

Posted By: Simon Stilllow flow/low temperature

Those seem to be contradictory. If low flow, then delta t along each circuit must be high, otherwise tiny heat output.

Unless the pipe density is high enough to get the required heat output with low flow/low temp.

Posted By: Peter_in_HungaryUnless the pipe density is high enough to get the required heat output with low flow/low temp.

That doesn't go very far AIUI, to boost output. Once the pipes are at 100c/cs, the screed surface (or spreader plates) is very evenly, fully heated, with insignificant cooler stripes between the pipes - highly diminishing returns, to space the pipes closer.

Looking at it the other way, output is overwhelmingly locked to mean water temp, can only be slightly reduced by wider spacing, or majorly reduced by throttling its flow rate. So if mean water temp is 'low' then output is low - or even lower!

Posted By: Dominic Cooney

Posted By: Simon Stilltop floor

what sort of sytem is it? pipes with spreader plates? I did wonder if a pipe with a tightish bend at the end has finally kinked somewhere? I could imagine that happening with ours in the upstairs rooms, despite my best efforts when installing to try and prevent the chance of it. Just putting it out there as a possibility.

That's an interesting thought, but would only affect one of the loops - all of them have stopped showing any flow on that floor. (I had though initially it was just flowmeters stuck, but it's pretty clear now that water isn't circulating in those loops)

Posted By: GreenPaddyCould it be that there is air in the system? Upper floor ufh are more prone to that for obvious reasons. It does happen from time to time, even if there are auto air bleed valves fitted (which sometimes fail and leak or choke up with scale and fail to venthttps:///newforum/extensions/Vanillacons/smilies/standard/sad.gif" alt="" title="" >)

Likely need a booster pump by a plumber to give it a flush through, to push out air (just had this with a client last week coincidentally). That way you'll shift sludge and/or air. If you just request it to be done, rather than a load of man hours fault finding, it won't be so expensive. I'd put an antiflocculant in it (inhibitor not antifreeze obvs).

We did get air in the system for the first few years - it would get noisy after a while and we'd let the air out and add some water back. However - it eventually emerged that the reason for this was a very slow leak at one of the manifold connections (hidden behind a shelf next to the boiler). We found that, sorted the leaking joint, and have never had air or loss of pressure since.

I do wonder about air in conventional radiator systems - I suspect it's the result of small leaks that just arent noticed because they occur from the expansion/contraction process when the heating is used, but you've got enough heat to evaporate the leak. Whereas on UFH at sub 30C any expansion is going to be so minimal that's not going to happen.

As an aside we discovered a year ago that we'd had a very slight leak on the hot supply pipe to the kitchen sink (at the final connection). But the combination of the heat from the water and a low humidity warm house meant we only ever noticed it - as a flooded kitchen floor - when we were away for an extended period with the heating off. The rest of the time it had alway evaporated long before it reached the unit kickboards...
It happened two years running when we went away in winter. The first time we had a roofer out and 'improved' what we thought were the most at risk flat roof junctions where the leak might have emerged from....

Posted By: fostertomI see. Highly insulated. 29C, if that means flow temp, sounds right. But at that, your delta-t (difference between flow and return temp) must be really small - ideal for an ASHP - as your mean water temp must be a few degrees above your 20C room temp. You can only get that small delta-t if you have a highish flow rate. You have two pumps, so unless really tiny, two at 1/3 max could be quite powerful, between them. Any idea of the actual flow rate?

why do you only get the small delta if the flow rate is high? the house temperature is stable so once it's at equilibrium the return is never coming back below 20C and most of the time the flow is only a few degrees above that anyway.

How would I work out the flow rates - the pump for the upper floors has a max flow rate of 3m3 p/h (and is set to about 1/3 power)
the lower floor pump has a max flow rate of 5.7m3 and is set to power level 1 (of 3)

Posted By: fostertomI'll try and work out the parameters - you've given many of them, and I'll try and work out the rest. Confirm the upper floors' 25/29C and the lower floors' 1C less, are the modulated flow temp?
My GBF Search is missing, so I can't look back and find your original description - anyone provide the link?

What are you actually trying to do? My issue is that the loops on my top floor arent circulating water, the rest of the system seems to be working as it has done for the last 10 years (and when the top floor was working the house maintained a steady, comfortable, temperature at low cost (It looks like this year our gas is going to cost us about £600. 35% of annual consumption is hot water. Those heating costs ignore the electricity to run the pumps of course, which run 18 hours a day through the heating season.)

And the only time I touch the heating controls is if we're going away in winter for 2 weeks or more to put it into holiday mode - permanent setback. The pumps run when the outside temperature is below 13C so i don't even turn the heating off during summer - that effectively happens automatically.

sure - please do - if you want any more house or system details let me know. I've suggested the approach on various forums over the years and there are always people who'll pipe up and tell you it can't possibly work, doesn't account for solar gain etc but it just sits there and does it's stuff.

Looking back, and ignoring our labour for the DIY install, the system cost £1200 for the screeded floors and £3400 for the overlay (plus the heatsource - boiler/pumps etc)at 2014 prices. I'd wondered how much we really needed to heat the upper floors but having part of the system fail its clear theres not enough transfer from downstairs for that to work.

I'd clearly not fit a gas boiler now but it would be an interesting choice between air-air (with the benefit of cooling) and air-water. The one surprise to me is that you hear it working in a silent - there is an identifiable 'hiss' from the water in the concrete floor (but not from under the wood). My experience of a few peoples modern air-air systems is that they're impressively quiet running at low power to maintain a warm room.

Did all the upstairs circuits just stop working at about the same time as each other, while the downstairs continued as normal? Are there separate manifolds for upstairs and downstairs zones?

That would fit with some kind of airlock in the return pipe from upstairs to downstairs, or sludge/scale blockage or control problems at the manifold.

If the system has gulped in a lot of air, it would have to run at low pressure at some point, this can happen if the bladder has popped in the expansion vessel. Does the pressure look normal when the system is and isn't running?

If you drain a little water out of the system, is it black, or still clear?

If the water is clear with a green tint I'd be surprised if there's anything other than a tiny trace of sludge, if any. If there's enough gunk in the system to have blocked pipes/fittings/valves I'd expect thewater to be filthy

Yes - pressure on the front of the boiler had dropped. I've added it back with water but I suspect the expansion bladders have/are failing after 10 years.

I've let some water out of the top manifold and it's a bit green but still transparent. The particles in in didn't feel gritty and I could break them up with my finger and mix into suspension.

Adding some images - the manifold and close up of flow gauges on upper floor
Also a diagram of the system. I've never really thought about it before as its just seemed to work but the manifolds are in series - theres no bypass - as fast as I can see water goes from the pump to the first manifold supply side and the return side (after that water has flowed through floor circuits) flows to the second supply manifold.

That seems odd. Its over 10 years ago when I did the install but I don't remember every buying any 22mm copper pipe so the final connections to the manifolds must have been done by the heating engineer who specced/installed the boiler and worked on design with me (and persuaded me to go with this approch)

and close up

Lastly, I've had a response from Viessmann on the diffence between heating curve temp and the flow temperatures.

The flow temp is actually 0.7*(average temp of last 20 hours) + 0.3*(current temp) fed into the heating curve.