what makes hot water flow with a combi?

[dog-man]

In a combi boiler situation. (Genesis 80)

Is it the pump or the water pressure that makes the hot water flow when a tap is turned on?

[David Hearn]

I believe there's a flow sensor in the boiler. When the tap is opened, water flows, boiler detects this and starts heating.

I'd be surprised if there was a pump on the hot water side of the combi, as effectively it would be pumping mains water, which is a no-no.

D

[John McLean]

"David Hearn" wrote in message news:e4dn3f$llb$ [e-mail]...

There is a water pressure switch inside my combi. I have had 3 or 4 attempts by british gas over the past few years to give reliable bath and sink washing up water. These attempts were made at the time of repairs due to other problems: - no CH etc. The latest attempt was made specifically 3 weeks ago, when they replaced the pressure switch when we had no hot water at all. It is always ok when they try out the system after repairs. I still have the same intermittent problem, I have found, that if I use the room thermostat to call for CH when there is no bath water, the boiler fires and since it is a common heat exchanger, we then have bath water. We are moving from this house shortly, otherwise I would have the system replaced but for what unit and what system?. The boiler is 5 years old, although the main pcb and other items, including the heat exchanger have been replaced, as a result of trial and error fault finding. This was done under repairs to an annual servicing/maintenance contract with british gas. There have been occasions, when we have had no heating of any form for a week at a time, the latest in january of this year. After discussions with the various servicemen attending for faults, despite their laptops, records are not kept of specific problems, with specific makes or models of boilers. Each incident requiring repair, requires another suck and see. If I had to replace this combi unit, I would consider reverting back to the original storage cylinder system, at least I would have had an immersion heater for bath water. I had to improvise for bath water at the last incident in the depths of winter; but this is not a topic for the diy fraternity. (We had enough portable gas and electrical heaters). What is the experience of others to gas systems?

[Ed Sirett]

The water comes out of the hot taps due to the pressure of water in the mains.

There are so many types anything I describe will only be approximate. I have no knowledge of a Genesis 80 (which manufacturer is that?) Anyway in general:

1) The boiler has to detect when the DHW is flowing. A variety of mechanisms are used. 1) A paddle inside a fitting swings moving a switch - used on some Ferroilis at least 2) A magnet spins in the water flow which is detected by a Hall effect probe - Vaillants since 2000). 3) Differential pressure acting on a diaphragm driving a switch. This is very common. The source of the differential pressure can be: i) Hydrualic resistance of the secondary heat exchanger (some Vokera and Vailllant Sines +others). ii) Venturi (very common). iii) Pitot-static pressure (into v. across stream pressure). 2) In all models except some Ferrolis the water is heated indirectly by the gas so the pump has to start to circulate the primary water. This is true even on boilers where the DHW heat exchanger lies inside the primary heat exchanger ( SD or C&M for sure & one of the Potty Lynx models).

[Doctor Drivel]

Firstly get a top make of combi, like Viessmann - they fail less often and their own service people are better than British Gas.

You can have an in-line instant electric heater in the combi draw-off to give showers and hot water a one tap if the combi is down. There are floor mounted combi versions with electric backup, for CH and DHW.

Knowing what is available is half the battle.

[Andy Hall]

On Sun, 18 Jun 2006 15:34:00 +0100, Doctor Drivel wrote (in article ):

Nowadays that would not be difficult

Avoiding dealing with British Gas is the other