Pre heating water before it enters combi

[Goldspoon]

After a discussion with a mate and some research of my own I now believe that it is not recommended that one feeds pre-heated water into a standard combi boiler (rather than mains cold). Forgetting any pitfalls associated with pressure or safety for the moment (assume hot can be at approx. mains pressure because it is pumped from a cylinder) and assuming that a mixer valve can be used prior to delivery to ensure the combi gets a certain max. temperature... can somebody tell me why?

I know that some modulated GAS combi boilers will accept hot water but understand that oil combis can never do so (why?).

So the first question is... why should one not feed hot water into an unmodulated combi instead of cold water?

Question 2 is the same but what about warm water?

Is it because an unmodulated combi will add temperature to hot water whether it needs it or not and this might cause it to get to steam temperature (so if 55 degrees goes in and combi usually heats to 46 degrees then it will actually throw out 101 degrees water - but this confuses me as surely the boiler will note the 55 degrees and just cut the flames so no more heat added??).

Hope the above is understandable.

 

[scatmanjohn]

If you put "pre heated" mains water through a combi@ say 55deg then the boiler just would not fire up.

What would be the logic in doing that?

You and your mate sound a bit confused me thinks

 

[tonybhoy]

Alpha have a device that raises the incoming mains water temp by passing the mains round the flue before it enters the boiler.

Gas saver I believe,costs about £600.

 

[Goldspoon]

If you put "pre heated" mains water through a combi@ say 55deg then the boiler just would not fire up.

What would be the logic in doing that?

You and your mate sound a bit confused me thinks

Not at all confused as to the logic.

If you can pre-heat the water and if the gas fires less (or not at all) then you use less gas and save money.

 

[Paul Barker]

most modern combis are fine used this way.

Because the manufacturer might fear you would want to attach another manufacturers product to theirs they will not happily give you assurance. Because they will not make a sale.

The question you should ask them is this. "can i buy another one of your combis and put the output of the first into the second to boost my hot water?"

 

[spudkey]

many combis use plastic componets on the cold inlet this why they tend to say max inlet temp of 25. Preheating inlet makes sense look at the alpha gas saver & the flow smart.

 

[Paul Barker]

many combis use plastic componets on the cold inlet this why they tend to say max inlet temp of 25. Preheating inlet makes sense look at the alpha gas saver & the flow smart.

Not to mention solar smart.

 

[Agile]

Similar plastic components are used on the CH parts of the boiler so that should not present any problem.

Most manufacturers who prohibit preheating seem to be scared of legionella. The recommendation is that mains water should not be stored between 20° and 50° for this reason.

Most manufacturers who allow preheating try to avoid the legionella problem by storing preheated water at 55°+ and using a blending valve immediately before the boiler. That can enable a solar preheat to be used with a combi boiler.

Oil and other non modulating boilers would be forced to turn on/off if the inlet water was significantly preheated and would result in serious fluctuations in the output temperature.

Tony

 

[BigBurner]

When plastics are on the cold inlet pre-heating is not allowed. If no plastics then it is OK. It also needs a restrictor that can be opened to increase the flowrate if that is what is wanted.

Pre-heating when using solar is quite common in other countries and Alpha and others do this.

Alpha do the Flowsmart, a combination of thermal store and standard combi, which give excellent flowrates. You can turn off the store in summer if only showers are being used. I think it also has electric store backup too. Best check. Alpha are now pushing this Flowsmart hard.

http://www.alpha-boilers.co.uk/products/?id=7

Using a small vented heat bank can do the same and cheaper too.

 

[BigBurner]

Similar plastic components are used on the CH parts of the boiler so that should not present any problem.

Most manufacturers who prohibit preheating seem to be scared of legionella. The recommendation is that mains water should not be stored between 20° and 50° for this reason.

Most manufacturers who allow preheating try to avoid the legionella problem by storing preheated water at 55°+ and using a blending valve immediately before the boiler. That can enable a solar preheat to be used with a combi boiler.

Oil and other non modulating boilers would be forced to turn on/off if the inlet water was significantly preheated and would result in serious fluctuations in the output temperature.

Tony

Alpha use a thermal store to avoid storing water and heat it instantly - legionella is then reduced to a minimum, or eliminated.

 

[BigBurner]

If you put "pre heated" mains water through a combi@ say 55deg then the boiler just would not fire up.

What would be the logic in doing that?

You and your mate sound a bit confused me thinks

Look at how the Alpha Flowsmart works. The water is stored at 70-80C and enters the combi at 30-35C via a blending valve. This means the thermal store and combi woirk together to give high flowrates and extended DHW flows. The combi will never run out of water. It reduces flow when the store is exhausted. The store can be off in summer and still give an element of pre-heat too. If the stored water is at 16 to 20C and the mains is 12C you will get some preheat for nothing.

 

[Goldspoon]

Oil and other non modulating boilers would be forced to turn on/off if the inlet water was significantly preheated and would result in serious fluctuations in the output temperature.

Tony

Can you explain this further? I know this is my lack of understanding here but if the water leaves the boiler at a temp above what the HW is set at then surely the boiler just doesn't light the flames? What is this fluctation scenario?

 

[Goldspoon]

Most manufacturers who prohibit preheating seem to be scared of legionella. The recommendation is that mains water should not be stored between 20° and 50° for this reason.

Tony

But the Legionella dies when water goes into combi and is heated to +50 is it not so never meets with a person?

(and how do you get the little degree symbol

 

[westeleven]

..

 

[ianniann]

I know this is my lack of understanding here but if the water leaves the boiler at a temp above what the HW is set at then surely the boiler just doesn't light the flames? What is this fluctation scenario?

Some boilers can't modulate at all. They either heat flat out or not at all. If the water comes in at 5C-15C then heating it flat out is fine, but if it comes in at 40C-50C then heating it flat out would make it too hot. The thermostat would detect that and turn the burners off. Then it would get too cold, the thermostat would detect that and turn the burners back on again. Modern boilers will typically have controls to prevent this happening more often than perhaps every minute and your hot water temperature will vary wildly.

Assuming a slightly better boiler that can modulate the burner levels considerably then it may be able to comfortably heat water at 20C to 50C with a steady low level of output, but might not be able to modulate low enough to heat from 40C to 50C. Then you are back to on/off again.