Scalding water from combi

[Martin Angove]

Combi boiler supplying a couple of taps and a lorryload of radiators on (I think) two circuits (err... that is the radiators are on two circuits).

Child turns tap on and is scalded by water that seems to me to be well above 60C (lots of steam etc.)

Fiddling with the "tap temperature" dial is interesting; the first water you get is scalding, but after a while it settles down to a lower temperature, related to the setting of the dial.

The boiler has a temperature indicator on the front under the flap; a row of neons or LEDs or something indicating up to 80C. After running a tap for a bit, they go down to 40C or 50C, but climb right up to 80C when the taps are closed.

Now I think this might related to the hot weather; the radiators in zone

1 *all* have thermostatic valves which are obviously, in this weather, not keen to pass any water. There is no obvious sign of a heat-dump loop. Zone 2 has a room thermostat which is again, obviously, off (indicating 25C this evening). The boiler (CH) is controlled by a timeswitch which no-one has seen fit to turn off, so is calling for heating, and the function switch on the boiler is also still selected for both heating and hot water.

Is it possible that because there is a call for CH but nowhere for the heat to go that it's being dumped into the DHW? The boiler has a "preheat" function. If so, should we be looking to take the TRV off one of the radiators, or installing a room thermostat for zone 1? Zone 1, BTW has one "cold" area (entrance lobby), a corridor and several smaller rooms which heat up much better.

Or should we be looking at another fault?

I only thought to pose this question here a few minutes ago and so I've no further details of the boiler (make, size etc.) but if it helps I can get them very easily. The boiler is likely to have quite an output as it is running 9 radiators in zone 1 and a further 9 (IIRC) large ones in zone 2.

Any thoughts appreciated.

Hwyl!

M.

[Adrian Boliston]

On my baxi combi the "preheat" dial governs the initial tap temperature as this is using hot water stored in the combi's tank. I find setting the "preheat" to the six o clock position means the water is not so scalding as in the 9 o clock setting.

[Martin Angove]

Well, yes. The dial has a range of about 12 o'clock to 9 o'clock. It was normally set to 6 as yours, but as I said even so the first water was scalding, after running the tap for a bit it settled down to a more normal temperature. Setting it to 3 still gets scalding water as a first step, but after a while it becomes tepid.

Hwyl!

M.

[Pete C]

Hi,

One way round this could be a blending valve, but I'd think about adding a longish parallel loop to the HW supply from the boiler, and a valve in the original branch. Then close the valve a little so half the water goes through the valve and half through the loop.

If the loop volume is sized to be at least the volume of the 'slug' of scalding water from the boiler then the temperature of the water should average out much better.

Bit tricky to explain but would be cheap to try out.

cheers, Pete.

[Martin Angove]

Except for two things: first this system is only about 6 months old and hence still under a service contract of some description, secondly it wasn't until yesterday (Sunday) that anyone noticed this problem. I'm sure that if it had been a problem from the start someone would have noticed by now.

By the way, I didn't get my OP back on the list, the first I knew it had got through was Adrian's reply. Has anyone else missed the OP because it contains a lot more detail than the above paragraph and I could quite easily repeat it?

Hwyl!

M.

[Pete C]

Why is the CH on in summer BTW? Have you tried switching it off to see if it helps? :)

This sort of problem has been discussed before though, see post 42,44 for an explanation:

Came through fine on individual.net

cheers, Pete.

[Dave Plowman (News)]

Why switch it off? I've got a programmable thermostat to do that for me. ;-)

[John]

Without knowing what boiler you have and whether there is a heatstore in it its difficult to know. I would look first of all for there being a weep-by of a valve between boiler and domestic heat exchanger, it may be that you have a bit of scale or dirt causing a valve or diverter to fail to close off properly. There should be a bypass circuit for the heating before the zoning valves, preferably via a route which can absorb some heat such as a radiator in an airing cupboard or a towel rail but it should be possible to determine if the TRVs all being closed is affecting things by removing the head of one such as the bathroom one. If the worst comes to the worst a cheap and cheerful thermostatic blending valve could be fitted to the tap circuit. These can be obtained for around thirty quid upwards.

[Martin Angove]

Because this is a church and there isn't anyone officially designated as "in charge of setting heating clocks" etc.

Brilliant, thanks, Nov '04 was during my 6 months (ish) "off list" and so I missed that discussion. I must get into the habit of Googling rather than relying on my own memory :-)

Rather odd that demon seem to have lost it then, especially as it was posted through them!

Hwyl!

M.

[Pete C]

I see, just thought maybe there would be some connection between CH and DHW even if the thermostat was off.

What make and model is it? Maybe someone else here has come across the same thing.

If in a church it would probably be best to fit a 'NHS TMV3' spec thermostatic valve, as there are probably older people around whose reactions could be a bit slow and be very vulnerable to scalding:

cheers, Pete.

[Ed Sirett]

Make and Model (I'd hazard a guess is a Pott Lynx ?)

Usually when there is a heating demand and nowhere for the primary water to go - the O/H stat sends the boiler into lockout.

If it's a lynx then I think it might just have an internal bypass (or the diverter valve is not returning 100% to the CH position after HW mode.

If the CH control was up then then primary water in the boiler could be well over 75C. If it's a Lynx then it has a tube-in-tube heat exchanger. The gas heats the primary water which directly heats the DHW in the inner tubes. hence the symptoms.

18 average radiators could well only need 24kW. Say an average of 1200x600 single panels with fins.

[Martin Angove]

They certainly have; I was pointed in the direction of a thread from last Novemeber :-) As for make and model, I don't have those to hand at the moment.

I think, if this is a known and unavoidable "feature" of this boiler, that we may well be going down this route. It is more likely to be a youngster who would fall foul of this problem; the oldies either wouldn't use the loos in church or would be so quick washing hands that they'd only get the pipework water. We have an inordinate number of under 5s on a Sunday morning, and more than a few in the evening too.

Hwyl!

M.

[Martin Angove]

[...]

I'll dig this up in the morning.

The flame telltale goes out, but only after the temperature telltales have reached 80. No "fault" telltales light (apparently a fault is indicated by a flashing temperature telltale).

The CH control is quite probably in that region; the radiators get good and hot.

The 9 in the extension probably average this, but the ones in the church are doubles and may be slightly larger. Put it this way, before the extension we had four big (10kW?) gas convectors in there which just about coped in the depths of winter. Building the extension up against the long North wall of the church (it's a tin hut) helped no end, but the remaining two convectors struggled. Now that the new radiators are in, it gets up to temperature very quickly.

Thanks for the interest.

Hwyl!

M.

[Martin Angove]

It's a Baxi Combi Instant 105e which, according to the manual I've just downloaded from the Baxi website, has a small amount of stored primary water which preheats (non stored) mains water... that is, if I understand it correctly.

In that case there appears to be nothing which can be done about it unless preheat is turned off. So long as the "preheat" function is selected, whether or not CH is demanded, the primary water is heated to its normal set point and it is this set point which determines the initial temperature of the DHW.

I'm a bit confused about the preheat dial though; the manual implies that you can control the DHW temperature using the dial, but doing so turns preheat on. If preheat is turned off (dial fully anticlockwise) then there is no manual control of DHW temperature, and it comes out at a fixed 50C. Or does it mean that *this* dial sets the preheat temperature? In which case we do still have a problem as even with the dial at the 3 o'clock position the initial slug of water was scalding hot...

Hmmm...

Hwyl!

M.

[Tony Bryer]

I am our church heating person but by long-standing tradition my name does not appear on the list of office holders. I tell people that it's because when Henry II turned up at church and it was cold he had Humphrey de Furnace beheaded, and after that no one would take the job unless promised anonymity: if you preach a dull sermon, no one will say anything; if you make a mess of the flowers, no one will say anything; if you're the organist and play out of tune no one will say anything. But if it's a bit cold *everyone* will make a point of saying so!

At the risk of further thread drift, an obituary for a church boiler from our September 2001 newsletter

"There are those whose contribution to our church life is very obvious and others, who we value no less highly, who stay out of the limelight, quietly getting on with their allotted tasks. Potterton - no one ever found out his first name - was one of the latter. For 25 years he devoted himself to bringing a sense of warmth to our fellowship. Show him people arriving cold and wet and he'd put heart and soul into sending them home with a warm feeling. It has to be said that he weighed 31 stone, had a hearty appetite, and insisted on hiding under the gallery stairs on the grounds that he'd been put there when he first came in, but the few of us who knew him well valued what he did.

But sadly he was beginning to show his age: several times last winter he fell asleep, leaving others to apologise for him, and when one evening he unleashed a torrent of bad smells we knew his days with us were numbered. So a few weeks ago Norman Kearon and Tony Bryer had the unenviable task of escorting him off the premises. He didn't want to move but with the aid of a ramp he was eventually did. His place has been taken by slim

6-stone twins* from Kent with, we are assured, appetites to match. They look forward to giving you a warm welcome in the years to come no matter what you weigh!" *Keston Celsius boilers

[Owain]

Who's "in charge of defending compensation claims in court" ;-)

I don't see you having much choice about fitting a thermostatic valve, regardless of boiler type, on public liability grounds alone. You also now have Disability Discrimination Act to comply with (people with impaired sensitivity or movement who cannot sense or adjust the water temperature themselves).

Owain

[Ed Sirett]

I'm not familar with this model. The preheat _may_ be like the Vaillant. If you turn it off then turn the DHW temp knob to required setting then it stay off. If you turn it to maximum and then turn the knob down to the required setting the preheat is on.

As others have said a £30 mixing valve is the best way to go along with proper use of the heating controls.

I can't see anything in the manual about pre-heating. It might just be like most other combis that when the heating is on high the initial supply of DHW is scalding. Even quite expensive models have this 'feature'.

Ironically in winter this feature may show up less, as the boiler might be a bit under powered and can't reach 80C.

[Martin Angove]

On that front we have a single-lever mixer tap in the disabled loo which supposedly complies.

Hwyl!

M.

[Martin Angove]

The first mention of preheating is in the "description", para 1.1.2 and section 1.2. The preheat "switch" and DHW temperature dial are one and the same. There is no way to set the temperature without having preheat on.

Quite probably.

It's all a little confusing to a non gas engineer. According to the installation manual the preheat water is "stored" in the expansion vessel. The diagrams show that in CH mode this vessel is effectively out of circuit, but if so, how can it be acting as an expansion vessel? In DHW mode the primary water flows through the expansion vessel to the secondary heat exchanger.

It's the temperature of the water in the expansion vessel which determines the initial temperature of the DHW, so if it's out of circuit the question is how is it reaching what is presumably the full temperature of the CH flow?

Hmmm... on the subject of CH and expansion vessels, how much water would

18 or so radiators likely contain? Any rough figures for water in pipework (e.g. l/m of 28/22/15mm pipe)?

Hwyl!

M.

[Owain]

That alone doesn't resolve the quandaries that (a) a maximum temperature limit is required; and (b) people with such attributes might not use the amenities targeted at the disabled.

Does the tap have to be labelled bilingually as well?

Sosej![1]

Owain

[1] I think it's my favourite Welsh word.