Combi boiler - Hot water 'heat dump' too hot

[Dave Plowman (News)]

Don't be silly, Phil, that would require some original thought.

A lose lose situation for IMM.

[Phil Addison]

Hi Ed, Thanks for getting us back on topic. Sorry if my post was confusing, but I am passing on a problem I got from a reader of the FAQ site, who does not seem able to access the ng.

However, she says the combi was recently installed, so scaling (3) should not be the cause, and her description appears to rule out 1 & 2.

The problem seems to be that from when turned on the water initially comes out very hot, then goes cold, then warms up to the correct stat temperature. I wonder if it could be because she has the tap turned on only a small amount initially, so it overheats and shuts down, then she turns the tap on more in order to make it re-light. Could this last action could produce cold until the over-heat resets, and then the greater flow enables it to modulate properly and warm up to the right temperature?

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

NOSPAM from address to email me

[Phil Addison]

That was meant to be addressed to the whole thread. I obviously didn't expect any sense from IMM. I also changed the subject line to keep the junk separate, at least for newsreaders that thread by subject.

Phil

[richard]

IMM , every post a gem. long may you reign.

The retort puts me in mind of Tom Baker narrating "Little Britain" fab.

Rich

[IMM]

WE need people on here who read properly. Will both of you two please go way to alt.cant.read.

[IMM]

Anyone who comes out with that should just shut up about heating and water systems. What a dork! Some mothers.....

[IMM]

Most do. You find that a cylinder removed from the airing cupboard and a small rad installed is greatly appreciated. The space it releases is phenomenal. Using Elson tanks can make a difference. The square shapes take up far less space. Gledhill now make the XStream unvented cylinder in a square shape, with all connections on one side.

[Dave Plowman (News)]

Oh. I was hoping you'd say it would explode, as in your earlier post...

[Dave Plowman (News)]

[Ed Sirett]

Generally yes. In fact this last week I installed a small heating system completely from scratch. I removed the existing multipoint and installed the boiler on the first day. Closed the flow and return isolators, filled it, bled it, and the user had hot water again by the end of the day. Not all combis will tolerate this, the model in question was a Vaillant

828, but most would I think.

[Ed Sirett]

Sorry I have now read the OP properly. The combi is working correctly and the 'feature' does occur on many models.

I'm familiar with several models although some models might not exhibit the problem though most do I would guess. A model name would help here. In fact the only ones which I reckon are ones with a sperate internal bypass pipework.

On many models the secondary heat exchanger also serves as a part of the bypass ciurcuit. This is easily implemented by arranging for the diverter valve to bleed primary water through the secondary heat exchnager when in CH mode. So in winter the 2ndry HE gets to the temperature of the primary flow. This means that the contents of the 2ndry HE are hot, this slug of very hot water will comes out first in DHW mode.

To mitigate the problem try turning the radiator tempertaure down. Of course if the rads are not over sized then the house won't get heated. 8-(.

All the boilers which have double primary heat exchnagers (eg Ferolli) or integral tube-in-tube type like (Pott. Lynx, Saunier Duval) will also have this problem and likely a worse version.

HTH

[IMM]

particularly

Most combi's will act as a stand alone multi-point as long as the CH is is charged with water. What this know-it-all-fool wants to know is if a combi will operate with the CH side dry. They will not.

[IMM]

It just will not work.

[Dave Plowman (News)]

Thanks for a comprehensive answer, Ed.

Mind reading again, idiot?

[IMM]

Answer to your question is: you are an idiot.

[Holly]

post...

My HW cylinders seem to be in danger of explosion now.....

Holly

[Pete C]

Hi,

Does it bleed water regardless of the flow in the primary and can it be blocked somehow? If there is a differential pressure valve in there or it can be blocked it may be possible to use a lower pressure one elsewhere.

cheers, Pete.

[Ed Sirett]

I'm fairly sure that modern Vaillants and Vokeras work as I describe. The diverter-valve does not entirely block the path through the primary side of the secondary heat exchanger. There is nothing smart about this it is simply a deliberate hole in the valve. The effect is to slightly reduce the output of the boiler in CH mode, since the output in CH mode is usually far in exces of requirements this is not a problem.

The only way to prevent the primary water doing this wouod be to modify the diverte valve. The effect of doing this would be disasterous.

1) The boiler would have no bypass. 2) On Vaillants there would be no sensing of primary water temperture leading to a catastrophe.

[The Natural Philosopher]

"You left out a 'there is', a second 'at', and there is a superfluous 'to' and three commas are missing."

3/10.

EXCERPT FROM IMM'S SCHOOL PSYCHITATSIC CAREER REPORT

---------------------------------------------------- Must try harder.

This child is held back by conviction of his own correctness.

I strongly doubt he would be capable of being even a plumber.

Incapable of coming to terms with reality, he is gullible and defensive. Mild sociopathic tendencies.

John works best in a situation in which he is given clear simple instructions, and there is no ambiguity, and some else is doing the thinking for him.

Left to hs own devices he grabs for any piece of printed matter that has one or two longer words on than he can understand, and uses it as an effective Bible. His deep supsicion that other people are laughing at him and trying to manipulate his (very limited) intellect does not extend to the printed word.

He would hae made an ideal Camp guard at Buchenwald, or Roman Catholic Priest, but today is likely to never find satisfaction for his socipathic tendencies, and will probably end up stalking, becoming an Internet geek, or join the political party that tells the biggest lies, in the fewest words.

[IMM]

How about a "z"s here and there.

No need. Brilliant as I am.

Too true, not a plumber. 10/10

Are you saying both are the same? My oh my!!! This one needs professional attention.

I will never join the Tory party.