(Oh no, not again!) Balancing CH system

[Velvet]

I've googled, read the faq's, but I still have some questions.

GCH in house, with a Combi boiler. 7 rads in total, semi-detached house, all bar one have TRV's. Room stat present in non-rad area. Boiler located on ground floor, has 'thick' (22mm?) pipes from boiler which then drop via 'T' pieces to the smaller 'normal' (15?mm) pipes that run under kitchen floor to the downstairs rads, thick pipes continue upstairs (at some point dropping to smaller size to the upstairs rads).

Not 100% sure of order of rads downstairs, despite there being only two. Upstairs it's bathroom (directly above boiler location) then through bedrooms, and then drops down to feed downstairs hall rad as the last one on the 'upstairs' leg.

Boiler appears very capable of heating entire house and then some. I've no cold radiators anywhere, though I suspect the kitchen one could do with being a double rather than a single - this is the coldest room of the house (not *that* much cooking goes on as I mostly live on my own, plus it's north facing, and has unheated west-facing hall to side door too).

I'm finding that the heating has to be on for 3-4 hours in order to get the whole room up to temp. The TRV's do their thing, cycling on and off. Lots of cycling on and off in some cases. I suspect this might be partly down to the radiator being fiercely on due to the boiler being so energetic (even when set to minimum), so the TRV turns off prematurely, rad cools, room stops heating up, TRV comes on again... only to go off again a short while later due to radiator being burnyhot again.

So I thought I'd balance the system (it's clearly not, since all lockshields are fully open everywhere) to see if this would improve things.

I've borrowed one of those point-and-click temperature sensing things, which is great. But...

What I've found is that with everything wide open, there's about a 6C drop between flow and return at the boiler. Pump is integral to boiler btw. On individual rads, there's a negligable drop on the return from the rads. They'll happily sit there at around 63C both sides of the rad, with slight fluctuation as the boiler cycles on and off. No short cycling. Boiler apparently has an integral bypass (a bit of pipe, unclear if there's any automatic valve on it at all). Manual also states that a flow valve should be fitted between flow and return in order to limit flow to acheive design temp drop across system. I figure that'll be my bathroom rad then, since it's non TRV.

Struggled to get a 11C drop across the radiators unless they're only just cracked open on the lockshields. Boilers rated for 11-17C drop across the entire system, depending on rads served etc.

What I'm struggling with is the drop measurement though. If the boiler's firing, it's quite a long cycle, and it brings the flows up to about 62C at each rad. I can then get an 11C drop. When the boiler's not firing and just circulating, the drop reduces quite a lot, though eventually (now that I've throttled back even the bathroom rad by a looong way) the boiler's not fired for long enough that I'm seeing a flow of about 54C and a return of about 48C. Boiler *eventually* kicks in and fires again. It's not overheating due to lack of flow (all the TRVs are still wide at this point despite the lockshields beingn throttled back a long way to almost closed for almost all rads) and can't hear kettling either.

So, when measuring this drop, at what point in the cycle do you do it? with the boiler firing, so you get maximum input, and wait for maximum temp of return from the radiator?

Is it worth me throttling back *every single radiator* in order to get anywhere near 10C drop across them, and something between 10-17C drop across the boiler? Isn't that going to put more stress on the pump? And will it actually improve the TRV characteristics?

I've balanced a system before in my old place, but that was a) all on one level, b) no TRV's, c) very clearly defined system with boiler at one end, radiators in a line away from that, thus making it easy to balance with the furthest being fully open and still seeing a 11C drop across it, with progressively closer rads to the boiler being more and more throttled back.

This place? it's all bizarre and odd and a bit confusing.

[Velvet]

Oh, and I've just run around shutting all the TRVs to establish if the flow rate through teh bathroom rad is sufficient should they all shut. It's not, because it short-cycles and kettles. It's not *that* likely during winter that it'd be on from the room stat yet the trvs would all be shut, but it *does* happen regularly in spring/autumn, due to the location of the room stat, and it's useful to warm the bathroom up to help with condensation issues in there :)

So I'm going to have to rethink all this - it *clearly* needs a higher flow rate through the bathroom rad, regardless of what the others are throttled back to.

Am I doomed to failure with my thought that overly eager rads are causing mayhem with the trvs, and open all the rads up a bit more (and settle for a 4C drop across them, or something?)

Velvet

[Velvet]

Velvet wrote: (and snipped)

Right, another idea. The bathroom rad's definitely the first to heat up, and the only one without a TRV. What about... shutting off all other rads, and closing the bathroom one in steps, till just before the boiler starts threatening to kettle. That'll leave me with the minimum flow rate (there is nothing in the manual for the boiler about this at all!) - and I can then balance the rest of them (albeit they will be further away from the boiler than the bathroom rad and thus theoretically should have the LSV's opened wider than the bathroom, which appears to mean pretty much wide enough to fit an articulated lorry through)?

Oh, and I've read BOTH balancing faq's. I think a good thing would be to simply reduce the pump speed, but I can't find anything in the manual about that being possible to do on this combi (it's a non-condensing 5-ish years old Saunier Duval SD30e, which is apparently a Vaillant in disguise?) - it just says restrict the flow using a valve across flow and return.

At least setting it all back how it was when I started it easy, I suppose :) LSV's to fully open - no faffing about writing down number of turns needed to close them before I started fiddling :)

Velvet

[Phil Addison]

The physical ordering is of no consequence. Just feel the flow and returns as the system warms up. If all valves are fully open, the last to warm up is 'furthest', and its LSV should be left fully open.

I'm puzzled by that. TRVs are not on-off devices so they should not cycle. They are linear and gradually reduce the flow as the room heats up, until the flow is just enough to maintain the required room temperature. I suppose what you describe could happen if for reason the heat from the rad is directly affecting the TRV, say if there is a draught blowing hot air on it. It is never ideal having them so close to the rads as they necessarily are fixed, as they are supposed to sense the room temperature not whether the rad is hot or cold.

Good plan.

Its called an Infra Red (IR) thermometer. Did you also put some black PVC tape on the pipes to take readings from? IR thermometers give strange readings off some surfaces.

I don't understand what this 'flow valve between flow and return' might be. It sounds as if they mean bypass, but you say that is integral. It is common for the bathroom rad with no TRV to be used as a bypass. On the other hand you say there is a room stat; the rad in the room with the room stat should have no TRV.

Forget about 11C drops - as I say in my FAQ there is NO requirement for that - it is a common misconception. The aim is to achieve EQUAL drops across all rads, whilst one rad (or more) has its LSV FULLY OPEN. You need to ensure the TRVs don't close at all during balancing. Open them to max settings, or preferably loosen or remove the TRV head to ensure this.

62C is rather low for flow temperature if the boiler is set to maximum. Is that the highest it ever reaches?

Did you wind up the room stat? That could be cutting out the boiler.

See FAQ. You want the boiler firing, and wait long enough for all flow & return temperatures to settle to a steady value.

The 'furhest' is the one that is having the hardest job to get hot. It's LSV should be fully open.

I think you need to read the FAQ again!

uk.d-i-y FAQ is at Google uk.d-i-y archive is at NOSPAM from address to email me

[Phil Addison]

Yes, that would be the way to set the LSV on a rad being used as the by-pass, but I thought you said your combi has built in bypass?

Just balance the rest as per the FAQ. You might finish up with a different temperature drop across the bypass rad than all the others, but so long as you are warm enough its nothing to worry about.

I recommend this one

then I would, wouldn't I? ;)

Yes, if the temp drop you finish up with is less than the 11C typical you can reduce the pump speed.

As I said in the reply to your other post, I don't follow that. Maybe someone else with that manual can clarify it.

Phil The uk.d-i-y FAQ is at

Google uk.d-i-y archive is at NOSPAM from address to email me

[Phil Addison]

The room stat should be set so that it cuts the boiler out before all the room stats close, then this problem won't arise. It is not a perfect solution because it means the boiler shuts down when one room at least is still asking for heat, but short of putting in a flow detector switch it's the best you can do.

Phil The uk.d-i-y FAQ is at

Google uk.d-i-y archive is at NOSPAM from address to email me

[Martin]

Correct me if I'm wrong, but I thought you must not fit a TRV to the radiator in the room/area where your CH thermostat is located for obvious reasons.

Martin

[adder1969]

It means using the pump valves or similar to restrict the flow rather like you would use the valves on a radiator. I'd be surprised if the pump wasn't adjustable but if you set it low then it means it takes longer for the house to heat up.

I've balanced mine by feel and it seems ok. No idea what the temp drops are.

For the "bypass" you need enough water volume so that it doesn't overheat and boil. If the boiler gets very hot then you need more water than it it doesn't. If you don't have enough water in the regular pipework then a radiator will add some volume. If the bypass loop is too open you'll get boiler cycling. if it's too closed you might get kettling. Auto bypass valves supposedly give you the best of both worlds - all the heat to the rads when they need it and then flow through the bypass when they don't.

To avoid having a bypass radiator you need enough water volume in the pipes to avoid kettling etc but it often depends on the boiler type.

[Velvet]

(snipped)

My boiler has a bypass pipe in it, from one side of the heat exch unit to the other, from what I can make out. It's about the same thickness as a bit of microbore (yuck/spit/hiss) so the flow rate through it is going to be pretty poor, and the volume of water also practically nonexistant (we're talking maybe a foot, foot and a half length at absolute best).

I've never adjusted a CH pump in my life so not really sure what I should be looking at on it to twiddle. If someone could give me a few clues as to what to look for (bearing in mind it is part of the boiler, I know where the pump is on it - rather than being external to the boiler type). The manual is a bit crap generally for the boiler, so it's not completely infeasible that the pump *is* actually adjustable, and they've just not actually said that anywhere (though the bit about using a flow valve across flow/return does rather make one wonder).

I definitely don't have the water volume in the boilers own bypass in order to prevent kettling, as I discovered later that night when I shut all the trvs down fully. Even with the bathroom rad half-open on the LSV there was insufficient flow to prevent kettling, and even when the CH boiler stat was at minimum too.

[Velvet]

You're right, they apparently fight. I do actually use the kitchen rad to heat the side hall/downstairs WC at the moment, and the stat is in the WC (conviently close to the boiler in the room next to the WC). No rad in the WC, the side hall is heated via the kitchen, and the kitchen is the biggest room that's also north facing, so always the last to come up to temp despite being the 2 or 3rd closest rad to the boiler (greatest heat losses from that room so always starts from a colder temperature).

Despite effectively being one area, by the time the TRV brings the room up to temp, the lag in that heat reaching the side hall/WC and thus the stat is just right in order to then shut the boiler off. Whilst the kitchen is coming up to temp, the other rads have been cycling on and off quite a bit (usually prematurely, due to the fierce heat from the rads, which is still only a theory of mine). All in all it works really well, but only because of the specific environmental conditions that are present with the kitchen/hall/wc/stat.

It's not practical to have the stat in the room without the TRV on the rad - that's the bathroom, so wild fluctuations in temp due to window being open to ventilate to prevent condensation, as well as heat from having a bath, for example.

[Velvet]

Phil Addison wrote, and Velvet snipped lots and replied at various points:

I know which is the furthest - the thing is that even with all the LSV's fully open, within just a couple of minutes there is very little difference in flow/return over even the furthest radiator?

Hadn't put black pvc tape on the pipes to measure though. They're painted white. Was wiggling detector around and making sure it was very close (but not touching) in order to get reading off the pipe rather than pipe & skirting behind. I'll black tape them tonight and see if it improves things.

They do gradually close down, but they seem to close down before the room is up to temperature. The radiator goes cold (well, usually cools as far as slightly above room temp) and the TRV opens up again, radiator gets toasty again, TRV starts closing down again. The house has DG so it shouldn't be all that drafty, and I have the doors closed on all rooms (bar the livingroom-kitchen door, just due to being in/out often).

It's as if the room is being heated in a staircase effect. Blast of heat, TRV closes, heat circulates around rest of room making it a bit warmer (we're not talking huge rooms here either, standard 3 bed 1930's semi) - area around rad cools down from what it was, TRV opens up again, blast of heat released, TRV shuts down again whilst heat makes it's way into room (this isn't happening over the space of just a few minutes, but neither is it taking half an hour to shut the TRVs off).

I think they mean a bypass. Though they seem to be saying 'adjust it so the heat loss across the boiler is between 10-17C, and these are the available pump head's you'll have 'spare' at any point on the graph'. The bypass in the boiler is incredibly small and can't see it actually functioning to avoid kettling. Think microbore sized pipe from one end of heat exch to the other. That's the 'bypass'. There's no adjusting valve on it at all.

Yep, I did make sure all TRVs were fully open. Couldn't figure out how to get the heads off, so I settled for windows open on a cold night (not too breezy) and all cranked to max. With all LSV's open, the most I can get is a 7C drop across the entire system (though I'll re-measure with black tape!). Over any radiator, it's barely measurable. Less than 1C in most cases. I think this means the pump's too fast, but can't find anything in the manual about adjusting that as I said elsewhere.

Nope, that was the boiler set on middle-of-the-road. Should it be set to maximum when balancing it all?

Yep, it definitely wasn't cutting out from the room stat. Can't find any info on what temp diff the boiler would decide to re-fire to heat the water again from the manual. I suppose the expected drop temp has a bearing on that figure, so maybe I can deduce from that.

Ah, I must have missed that in the FAQ. Not sure I'm going to be able to do this then, but I'll crank the boiler up to max and have another go - I think in the past even on max it tends to push out enough heat fast enough that it will soon cycle off again. Maybe it's massively overspecced for the CH demands the rads are placing on it vs the HW. All the rads are singles, apart from the front hall (which heats stairs and landing). Most rads are 3' long single or smaller, the only exception is the hall double is a 2' double, and the livingroom a 6' single.

Didn't think so :)

None of them struggle to get hot, they're all really very good at getting hot (the kitchen one is undersized for the room I think, it gets plenty hot, it's just a cold room, same size (and a bit more) than the livingroom, yet about 1/3 the area of rad to heat it). It's just the way the TRVs seem to be acting before the room heats up that's been annoying. And the fact that I knew every LSV was wide (I'm beginning to think that doesn't actually matter in this instance though!).

lot. Though I'm not sure the black tape or boiler being on max was mentioned in there - I could be wrong though!

Thanks for all the replies, I know it's been done to death here time and time again, so I really do appreciate people still being willing to reply and help me see the wood for all the trees :)

Velvet

[Velvet]

Velvet wrote (and wrote and wrote, then snipped it all).

Aha!

I've experimented, having found a control box on top of the pump marked up with I, II, and III. No prizes for guessing which one it was set on...

It's now set to the other end of the scale. Rads are now warming up more in the manner I would expect (ie, I can finally be sure the one in the hall really IS the furthest from the boiler). All LSVs and TRVs fully open, boiler's set to max, and I'm waiting for it to get up to temp before I shut off all the trv's and work out how far I can close the bathroom (non trv) rad before the boiler complains. That'll give me a baseline for that radiator, and I should (hopefully!) then be able to throttle back all the others (apart from the index, which is the hall) to what's needed for them to be a little more balanced. There's really very marked difference in the flow reaching the hall rad (water has to go up to the bathroom from the boiler, across to the front of the house via all the upstairs rads, and then down the wall, to the hall rad by the front door). I'm thinking I may find the pump speed needs to be set for the middle, not sure if the up - across - down - up - back - down to boiler again means it's still only once the head or if it's twice...

Velvet

[Velvet]

Well, that's interesting. With the pump set to I (lowest) the water slowly circulates around, but seems to heat up all the rads barring the furthest (downstairs in hall, but fed from upstairs circuit) reasonably quickly (burnyhot within 5 minutes). Boiler temp on the flow reaches

80C and sits there quite happily while it's burning, and with all the rads wide, it happily sits there for a good long while burning at this temp, with the pump set to I.

This includes the two rads on the ground floor circuit, so I don't think it's just convection doing the work - I'd expect both downstairs rads to stay cold if that was the case due to the way the plumbing is at the point where upstairs and downstairs diverges (sideways T with

22mm continuing up, 15mm continuing down, 22mm sideways horizontally to boiler).

However, with all the rads shut except the bathroom (and this wide open), the boiler kettles. Same if I have the kitchen rad wide (closest downstairs one to the boiler, and the only other candidate for having the TRV disabled on it). So two wide, pump on min, kettles and short cycles. Set pump to middle and it stops the kettling, though a little short cycling going on, with periods where it's doing it right (and modulating up and down in the longer burn cycle).

I'm not sure where this has got me. If there were no TRV's I'd set the pump on minimum, balance the entire lot on the basis there would probably be enough flow with that pump speed, and if not, knock the pump speed up to half way, and balance for that.

But the TRVs mean that if I balance the rads, when they shut down, it throws the *entire system* out of whack, not enough flow rate through the remaining radiator, boiler kettles, and I'm back to having the pump on full tilt (which leads to very noisy valves, and all the other things too).

If I leave the kitchen open wide enough to stop the kettling (in addition to the bathroom rad being wide open), then I can see in order for it not to turn off on the TRV it's going to be open that wide constantly, the roomstat will have to be set to turn off before the TRV starts to shut, which then leads to the roomstat turning off prematurely if a) the radiator's kicking out heat at a constant rate, b) I'm cooking, c) for some reason it's warmer in there than another part of the house.

Fair enough that another part of the house may have TRV's 'open', but if the flow rate through those other 'on' radiators isn't at least equal to the flow rate lost from the kitchen one having turned itself off (or even down), the boiler's going to start kettling again...

I know the theory behind them, but... are TRVs really such a good idea in practice? I'm *really* beginning to wonder if they're suitable for this boiler and this house!

[Velvet]

I own this place, and I still forget that (after having rented the previous place for 13 years) I could, should I want, fit something like that to *my* CH :)

I may investigate that. And maybe those automatic bypass valve things. If they're any good?

[Phil Addison]

[big snip]

YES, you don't want the boiler cutting out while you're balancing.

Not a temp diff, it will fire on/off according to the flow temp stat on the boiler.

[more snip]

Phil The uk.d-i-y FAQ is at

Google uk.d-i-y archive is at NOSPAM from address to email me

[Phil Addison]

So you do have one that is slower than the others, therefore the system is not balanced. Next step is to leave that ones LSV wide open and close off the others, using a recording sheet from the FAQ to keep track of what's happening, until that slowest/furthest one is heating as quick and as hot as the hottest of the others.

You should still include that bathroom rad in the balancing, before you start to adjust its control valve.

Some combis allow you to have TRVs on all radiators because they have an internal (automatic) bypass. Are you sure yours doesn't?

No. As already said ALL radiators should be balance first. This makes sure they all get a fair share of the water flow. Throttling one back afterwards is ok, as that will then just make slightly more available EQUALLY to all the others.

Also the wider open the bathroom rad finishes up, the less kettling you will have, though I suspect when properly balanced that problem will disappear anyway. There is no point in turning it down to 'just not quite kettling'. Only turn it down if the bathroom gets too hot (but obviously no to the point of kettling).

That's normal, and is the reason for balancing. The other rads are effectively short-circuiting the hall one, so you have to increase the resistance in them so they take less flow and make more available for the hall. Once a rad has enough flow to fully heat it, giving it more flow wont get it any hotter, but will steal flow from the furthest rads which then wont get so hot.

Use the slowest speed that all gets the rads fully hot. Rads are normally spec'd to give full output with 11C drop across them, so there is no point in running the pump faster than what's needed to get in the region of 11C.

Eh??

Phil The uk.d-i-y FAQ is at

Google uk.d-i-y archive is at NOSPAM from address to email me

[Phil Addison]

That's a good start, now do the balancing.

Where did convection get in on the story? The water is pumped all round, no convection needed.

Better check what you mean by kettling!! Its normal for the to make a bit of a whistling/singing noise when its going flat out, with maybe an occasional knock sound, but it should not be making serious banging/clanking sounds. If it is kettling, it might be worth cleaning it out with flushing fluid, and refilling with a good inhibitor. Google for posts on flushing and inhibitor.

Again lets check definitions: Short cycling occurs when the flow rate through the boiler is insufficient to take away the full heat from the flame, so the temperature rise across the boiler is too great ant the flow reaches the max stat temperature and cuts it out while the return water is till not up to temperature. The cool return water cools it and so it fires up again. With the specified (ie max) flow rate the rise across the boiler the rise is normally only 11C so the flow temperature slowly rises as the rads heat up, and is only a steady 11C above the return.

Shutting down rads obviously reduces the flow rate, and can cause short-cycling if the bypass is inadequate.

Nearly! Set the pump on slow, then balance, then speed the pump up is the overall drop is above the 11C figure. You do not need to achieve exactly 11, just equal across each rad.

No, the system is still balanced. Shutting some rads down simply mans more pressure available to supply the remainder.

Yes, could be. See above. However, the house room-stat is supposed to turn off the boiler before all TRVs close. And incidentally, the TRVs do not normally close completely, they shut down linearly to allow just enough water through to maintain the room at the set temperature. I know you say yours are going on and off, but I don't know why. See if balancing sorts that out.

Indeed, choosing a place for the room stat is a problem. The usual solution is to make the hall rad the non-TRV one and fit the stat in the hall - but not too close to the rad. A more recent option is to get a wireless room-stat and try it in different places.

Are you sure? You don't see to grasp that they are linear devices, and if they are banging on/off something is wrong.

Phil The uk.d-i-y FAQ is at

Google uk.d-i-y archive is at NOSPAM from address to email me

[adder1969]

I wonder if the system is clogged up a bit and/or the high temp stat isn't working or there isn't one fitted. You could modify the bypass to avoid havign the microbore and maybe fitting and auto-valve would mean that the super-hot feed water gets fed back into the boiler and it'll then shut down rather than allowing the water to overheat.