Variable pressure

[ian33a]

My first post, so please make allowances!

 

We moved into a property back in May and are now finding that the cost of heating it has been more than we anticipated, even with the price hikes.

 

I started checking out the 20 year old UFH system and uncovered half a dozen duff actuators (which I've replaced) and six circuits where the actuator pins are solid (again replaced). All circuits and zones are now functional. What I have found though is that the manifold pressure isn't constant, on even close.

 

I'm learning about UFH as I go, terminology and methods easily throw me but, I'm gaining knowledge, but it's taking time.

 

The system to me is a rather untidy mess. Nothing was labelled, no design documents. I've documented as I go. As far as I can tell, the whole system is fed via a Grant Eco Vortex oil boiler. It does a great job at providing heat and there appears to be plenty of water pressure - it being a closed pressurised system as far as I can tell (the thermal store being on the first floor and shower roses at equivalent top of tank height). In terms of UFH topology, it seems like a common inflow and outflow to two Far manifolds, one driving the top floor, one the bottom floor. Each manifold has a Grundfos pump and there is one additional pump (not sure what that does tbh). Each circuit is controlled via mechanical blending valves.

 

 

The top manifold drives the ground floor, the bottom, the top floor. Each has a separate wiring centre with wired 240v thermostats.

 

Having replaced the actuator pins (some on each manifold), I've done my very best to bleed everything using the inflow and outflow ports on each manifold. After one false start, I'm now pretty confident that the bleeding has done a reasonable job. All room circuits now get warm and the thermostats demand heat as and when necessary with the associated actuators doing their job and, the rooms get warm!

 

All of that said (and pressures and temperatures were working as expected before I meddled) I now find the following: 

- Zero heat demand for several hours and the manifold pressure gauges drop to almost zero - the top manifold being worse than the bottom one. The inflow manifold temperatures drop down to 20 or 30c (not unexpected with no heat demand) and the outflow temperatures are a little below that (again expected). 

- If a demand is made in a circuit, the associated pump energises, the associated actuator begins to open, the inflow manifold temperature starts to increase and the pressure gauge starts to rise. 

- At some point demand will cease, the pump will turn off and pressure on the gauge will slowly fall, quite slowly.

- Over many cycles the manifold pressure will get to between 1 and 2 bar and cycle up and down as heat is demanded.

- If no demand is made for a lengthy period (I don't know how long - certainly hours) the gauge pressure will gradually fall away and finally go back to the state described in the first bullet.

 

I think I'd expected the pressure to remain relatively constant all the time - but maybe it shouldn't? 

 

I haven't touched any of the flow adjusters and have no plans to go near them. I have played, a bit, with the inflow, outflow and mixer settings - the manifold temperatures being about 55 when I started (but less now). I don't know if I've upset something there - we got a bit desperate over the weekend when we had zero flow under any circumstances, thankfully addressed the next day with more copious system bleeding.

 

Our property gets it's cold supply from a bore hole that we own. The mains is pressurised using a big blue space rocket like thing in the cupboard above the manifolds. I found this pressure vessel by the thermal store

 

 

I don't know if this pressurises the hot water for personal use of for the central heating. I struggle to see where the pipes go - and again, no documentation.

 

So, is the variability in pressure on the gauges normal? If not, any suggestions as to what I have done wrong or simply haven't done at all?

 

I have an engineer coming out later in January (my wife got fed up with me meddling) but would like to understand ahead of time what may be wrong (and possibly improve things) ahead of his visit. It's the engineer in me ... just that I'm not a heating engineer!

 

Many thanks in advance

 

Ian

 

 

 

[ProDave]

Regardless of the source of your water you would expect the heating circuit to be a closed system with an expansion vessel.  It is quite likely the expansion vessel has failed.  Can you see a largish white or red tank anywhere with a single pipe connecting into the bottom?

[ian33a]

Hi Dave,

 

Thanks for your reply.

 

I've looked far and wide but haven't found a red or white one - just the black one, with a factory set pressure rating of 4 bar (but I haven't checked it functionally). I assumed that the CH pressure setting would be around 1.5 bar anyway.

 

Ian

 

 

 

[Radian]

1 hour ago, ian33a said:

Our property gets it's cold supply from a bore hole that we own. The mains is pressurised using a big blue space rocket like thing in the cupboard above the manifolds.

 

Never encountered this situation before and I'm curious to know how it works - from the sound of it your primary heating circuit isn't gravity fed so you presumably have a pump to supply the house from the bore hole and then a pressurised cylinder acting on the heating circuit?

[JohnMo]

If your UFH is being supplied via a thermal store, this will only have a header tank either built in or an insulated one above it. So your UFH is an open vented system, not a pressurised system.

 

That is pretty much how ours is, and the UFH gauges register 0.2 bar pressure. 

 

Your gauges are showing how much pressure the pump is having to develop to pump water around.  As the heating demand is switched off the drop is just pressure depleting to show the system head from thermal store header tank.  10m of head is 1 bar pressure on the gauge.

 

If your UFH is taking it's feed from the thermal store, your pumps may need to be turned down. Keep reducing its speed until you see a change on the flow gauges, then take up to next speed.

[ProDave]

25 minutes ago, ian33a said:

Hi Dave,

 

Thanks for your reply.

 

I've looked far and wide but haven't found a red or white one - just the black one, with a factory set pressure rating of 4 bar (but I haven't checked it functionally). I assumed that the CH pressure setting would be around 1.5 bar anyway.

 

Ian

 

 

 

Can you post some pictures of your hot water tank and pipework and other gubbins around it?

[Nickfromwales]

I think the black vessel is the expansion for the DHW ( domestic hot water ) coil inside the thermal store. If all taps are closed and you heat that from cold, the water held within the DHW coil needs to expand somewhere, and that is where it goes. Very difficult to comment without seeing where every pipe goes to / from in detail of course.

 

4 hours ago, JohnMo said:

If your UFH is being supplied via a thermal store, this will only have a header tank either built in or an insulated one above it. So your UFH is an open vented system, not a pressurised system.

 

That is pretty much how ours is, and the UFH gauges register 0.2 bar pressure. 

 

Your gauges are showing how much pressure the pump is having to develop to pump water around.  As the heating demand is switched off the drop is just pressure depleting to show the system head from thermal store header tank.  10m of head is 1 bar pressure on the gauge.

I think the same. The static head ( gravity pressure ) is displayed on the gauges and then that value rises when the pumps force water into the manifold rails. As there are differing values of resistance, with x number of actuators open / closed, those values will be variable but will reflect the pump creating pressure between itself and the available runoff into the loops of UFH. Rolling back down to zero / near zero says this is indeed likely to be a gravity based heating system vs sealed and pressurised. I assume you have not had to top up via a filling loop and have a gauge and ( typically ) red heating expansion vessel ( as Dave asks )?

 

A sealed and pressurised system kit would look like this ( LINK ) so if this is not present you are on gravity, also if you've been venting a lot you would have run out of stored pressure by now and would have had to 'top-up' via the flexible silver hose ( as seen in the image ) so this further reinforces the gravity assumption here.

 

The big blue rocket is a cold mains accumulator ( a 1/2" thick rubber balloon inside a pressurised can which the borehole pump 'inflates' each time it runs. This prevents erratic unmanageable water delivery to the taps etc and smooths things out. It also prevents the pump kicking in frequently for small take-offs of water, thus extending the longevity of the borehole pump.

 

Air in the system is a killer of UFH as the flow rates around the loops are sub 5l/p/m, so they are just not flowing fast enough to 'blow' the air out by the force of circulation. I would recommend getting these all purged properly, and having the automatic air vents replaced without hesitation. The items circled in yellow here;

 

 

These automatic air vents simply unscrew from a 1/2" BSP thread and can be bought from most local plumbers merchants, albeit they will likely come in brass then and not chrome. The ones you have may well have a black rubber o-ring for hand tight installation so you may want to order the same again, or the brass ones will work with a few turns of PTFE tape on them. Here's a link to suitable replacements if you want to order online ( LINK ) vs the "buy local and PTFE them" option  

This fix will allow the air to be expelled without you constantly monitoring / babysitting them, and work automatically to release the air whenever it presents itself. Without these functioning properly it is likely you are missing air which then gets sent back around the loops, infinitely.

 

The purging I refer to above is carried out thus; ( this is based on the manifold being on the 1st floor and the ground floor loops being 'below' it? If this is on the ground floor and there are loops above it, you will need to tackle this differently ). 

 

Turn off the red and blue manifold isolation valves after the pumps to isolate the manifolds and loops of pipe.

Remove the old ( knackered ) air vents, catching a couple of litres of water with some old towels.

With the air vents all removed, using the empty port for access, you use a 1/2" connector to link a hose running to a drain to the lower manifold rail.

You connect another hose from the outside tap to the other manifold rail ( or if cold mains water is available locally within the plumbing tap into that ) likewise.

You then remove one actuator and open the outside tap.

 

Cold mains water will then blast through the one loop, and out to the drain, taking any air / crud with it. Once satisfied this is running freely without spluts of air, you refit the actuator and move to the next actuator, and repeat the purge cycle for each loop, one at a time. Keep going until all loops are purged on both manifolds, then remove the hoses / connectors and fit the new air vents in their place.

Open the blue vales slowly to allow air in the manifolds ( not loops ) to rise to the air vent at the higher manifold rail and then reopen the red valve.

 

Remember that the thumb cap on top of the new vents stays OPEN, and do NOT shut them after installing them. These are designed to be left open for life, so venting air in your absence.

 

Maybe get a plumber out and offer to assist with this process if you are not confident with this. Start there, and I think you'll be back running smoothly in no time, or we will soon know the next fault!

 

Enjoy.

[ian33a]

Thank you all, ever so much, for your replies. I was especially pleased when JohnMo offered his thoughts last night but decided to mull things over (in my sleep) before replying. Then, over night, the confirmation from Nick was especially pleasing too. I did take some pictures of the thermal store, or at least, tried to. It's really difficult to take anything that is meaning full as the units appears to have been assembled and a cupboard built around it and the the manifold and a doorway through the middle and in to the bathroom - hard to visualise, I appreciate. All I can show is an open cupboard with a load of pipes ... but here goes:

 

 

The black expansion vessel is about a third of the way down the picture on the left hand side of the store. The coil of wire isn't part of the thermal store - it's part of a temperature monitoring system for something else. Not shown here is the expansion tank. This is stored on a platform above the ceiling of the walkway through and sits above the tank thus confirming what has been suggested : this is a vented system. There is no red pressure vessel, anywhere, as far as I can tell.

 

Everything that you have described is what seems to be happening. In a quiescent (rest) state the amount of static pressure falls away to near zero on both gauges. There can only be a metre or so of static head between the top of the highest manifold and the top of the thermal store so this is consistent with there being no pressure. It does, indeed, build, with heat demand and is somewhat variable depending upon how many zones demand. Looking back (before my meddling) I only ever looked at these pressure gauges a few times and they always had pressure. I guess I just never looked at them with interest at a time when the system was at rest. You can't imagine the relief that your confirmation of what we have here is OK. I was really concerned that I had caused a massive issue which was unsafe for family visiting over Christmas!

 

With regards to bleeding, and the automatic air vents, certainly, I'll have these looked at (there's no way my wife will allow me to do this right now!). The affected manifold is on floor one and the circuits are below on the ground floor. When I bled the top manifold, I did the following: I connected a mains feed to the inflow (just below the top inflow temperature gauge - just as in the picture) and a garden hose to the outflow manifold. Then I closed the inflow and outflow gate valves. I then made sure each of the circuits was closed and opened just one (at the far end). I opened the gate valves on the hoses (inflow first) and ran water through the circuit for about 10 minutes. I lost the will to live waiting any longer. Some air came out at first but, after that, it was just a gentle flow of water. I then closed off that circuit and moved to the next one, repeating the process. Once complete I closed off each of the gate valves on the hoses.

Then I put the mains pipe on the outflow and the garden hose on the inflow. I opened the manifold gate valves and ran water through the mixer and the pump.

 

If I was to change the auto air vents, it seems like a similar process to this, as, Nick, you have described. I think I will have a go at this once Christmas is over - no sense paying somebody to do it in January when it seems like I have had a fair bit of practice already - plus there's a sense of achievement in managing to do it myself (with assistance from my ever patient wife).

 

Out of interest, if I had to bleed the lower manifold (which drives UFH on the same floor), what process needs to be applied for bleeding (just in case changing the auto air vents threw up issues)?

 

 

 

Edited December 23, 2022 by ian33a

[JohnMo]

If your flow through the loops is steady and the pump is a consistent steady noise (not noisy), then there is unlikely to be air in the system.  You will hear air movement going through pump, if a blocked slug of air you get no water flow through that loop.

[ProDave]

If it is a vented system, then the pressure in the pipes will always be low and should be constant, any expansion due to heat pushes the water back up into the header tank.

 

I would expect a pressure gauge to register very little pressure, so the question now become WHY does it increase when it gets hot?  I would just expect the expansion to raise the level in the header tank slightly and the increase in pressure to be almost nothing.

[dpmiller]

a pump's flow can add or remove a surprising amount of pressure