How do combis control HW temp?

[Set Square]

Just been staying with son & wife in their new (to them) flat which has a Vokera Linea 24 (or somesuch) combi boiler.

It seems to produce hot water to the kitchen tap at one flow rate only. Open the tap too much, and it's luke warm. That's understandable - since it's limited by the heating capacity of the boiler - although I would have thought that 24kW could manage a higher flow than it does, even in December with a low input temperature.

What seems less understandable though, it that if you turn the flow *down* below what appears to be the optimum, it *still* runs cold. It's as though it says to itself "I don't need to work very hard to keep up with this" and then promptly turns the burner right off!

I've never liked the idea of combis - but I didn't know they were as bad as

*this*! Is this typical, or is this a duff one, or are we not driving it right? As far as I know, son has no instructions for it. I've look on the Vokera website, but that's being "re-built"!

I would have expected a half-way decent combi to control the flow rate itself in order to maintain the set output temperature, and to have moderated - or at least cycled - the burner to maintain the temperature when the flow rate is externally lowered by a partially open tap. Is this too much to hope for?

[BigWallop]

The longer the water stays in the tank, the hotter it gets. To fast a flow through the tank and the water will not get a chance to heat as much.

[Stephen Dawson]

Its probably naff, as most Vokeras are.

[Set Square]

Do they all have tanks? Anyway, I'm not sure that would explain why it suddenly goes *cold* when you partially *close* a tap through which decent hot water is running.

[Lee]

Normally the cold water flow rate into a combi is throttled to give a reasonable compromise between flow rate and temperature rise. There not being much point in high flow rate with a low temperature ;)

Maybe the secondary heat exchanger is scaled and the boiler electronics thinks the primary water is hot enough because the cold water is not sinking enough heat away?

Interesting, don't think I've seen one with an active flow control - sounds like a good idea though, but there's no doubt a practical reason why they don't seem to have this feature.

Think this is going to depend on the *minimum* designed flow rate. Ours claims 0.5l/min minimum and quite happily moderates on DHW ;) Don't most do this now though?

Lee

[Stefek Zaba]

Only sometimes: some, but not all, combis modulate and regulate this way. In our previous place we had a Worcester of some sort (fading neurons suggest "9.24" as part of the model number), and that was reasonably good at keeping up a useful and roughly-constant temperature across a variety of flowrates, though turned down too low meant the burner would go off completely.

Whether the Vokera at your lad's is supposed to do this I don't know. The other thing to bear in mind is that it has a setpoint, which some helf-n-sathety-minded installer might've tweaked down low, and that all the setpoint can monitor is the temp at HW exit - if the run to the pipes is long and uninsulated for much of its length, that'll eat up quite a lot of the heat before the HW ever reaches your (lad's) taps.

HTH - Stefek

[Lee]

Sorry that should be minimum flow rate of 1.5l/min ;)

Lee

[Set Square]

That probably explains it! When I'm washing greasy dishes, I like to give them an initial swill under a trickle of very hot water before putting them in the bowl of soapy water. With a stored hot water system, this is easy. With a combi, it's nigh impossible!

[Set Square]

I'm not sure - it's a few years old. There's a chart inside the lid which states max and min heat outputs - but this might be a one-off gas pressure setting rather than a real-time modulation.

There is a knob to set HW temperature (not graduated in degrees). That was turned fairly well - but not fully - up

and that

Not a problem in this particular case. The boiler is right next to the kitchen sink!

[Andy Hall]

You can work out the theoretical maximum flow rate.

Say that the cold water temperature is 8 degrees at the moment (probably about right) and you want 50 degree hot water.

Using the formula

Heat applied = mass x specific heat x temperature rise

gives

24000 = mass x 4.186 x 42

giving 136.5 grams/second

Since a litre of water is approx. 1kg, this equates to 8.2 litres/minute - not very exciting.

Possibly there's a thermostat/temperature sensor fault as well.....

It seeems so, doesn't it.......

[raden]

In message , Set Square writes

Well, the flow rate shouldn't be dynamically affected by the boiler

Not normal operation

There is a modulating gas valve which adjusts the burner rate to keep the output temperature constant.

It sounds to me like it's either a faulty pcb or temperature sensor (more likely, from your description)

The temp sensor is a temp dependent resistor, which if it's not the right value (for whatever reason), will give an incorrect temperature reading to the pcb which will then send an incorrect voltage to the modulating gas valve. Working flat out on hot water, you should expect to see something in the region of 24v dc on the modulating gas valve

[raden]

In message , BigWallop writes

[BigWallop]

Yes. We know !

[raden]

In message , BigWallop writes

So where does the tank enter in the plot ?

I raise you 2 !!'s

[Ed Sirett]

They aren't really as bad as this one for the most part. This sounds like it is time to descale the 2ndry heat exchanger. Almost all the calls I get about this model seems to be this problem and dunking the 'sardine can' in HCl (S/fix patio cleaner) seems to fix them.

[BigWallop]

:-) It's the bit inside the boiler where the water gets heated up. :-) !!! I raise you another ! :-)

[raden]

In message , BigWallop writes

There is no tank, just a heat exchanger - basically a coil of pipe

!!!!!!!!!!!!!!!!!

ho diddly ho

[Stefek Zaba]

"Nigh" nuffink - it *is* impossible. Even with modulating burners, a combi can't reliably deliver a trickle of very hot water for long - you turn the flow down below the minimum wot the combi's happy with, out the burners go. You get one pipework's volume of hot water, then tepid going to cold. Then you have to turn the tap on full to get it to light up again.(Cue, of course, for IMM to come along to describe some fantasy system involving two heat banks, a storage tank, and of course an electronic descaler ;-)

Stefek

[Set Square]

Yes, he's been remarkably quiet in this thread - I wonder why?!

[raden]

In message , Set Square writes

Didn't he say he was going away ...

the same day that someone was going round Enfield stabbing people IIRC

Just a coincidence, I'm sure