Boiler gas consumption: how much more to heat water to 75 degrees C than to 55 degrees?

Combi/condenser boiler, solar-heated water supply to boiler inlet
averaging 35 degrees C (i.e. can be 20 degrees on overcast winter's
day, can quickly reach 65 degrees on a sunny morning).
Is there any way to estimate how much more gas is consumed in raising
the temperature from 35 degrees to 75 than if raising it to just 55
The reason I'm asking is that our condenser boiler always heats the
water to 75 degrees, despite its temperature control being set to 55
degrees or lower. It's not suppowed to kick in at all if the
solar-heated water feeder tank reaches 55 degrees, but it does.
The explanation we've been given is that the water pressure (20 litres
per min at the cold water tap) drops to 6.5 litres per minute at the
hot water tap, i.e. when it's passed through the solar-heated hot
water tank. The engineer says this means the water passes through the
boiler too slowly and so gets too hot. I can't quite believe this, as
surely when the hot water tap isn't turned fully on it'd have much the
same effect, i.e. reduced flow?
As far as I can tell, whatever environmental and cost-saving
advantages there may be in this system are being offset by wasting
what I assume is a serious amount of gas over-heating the water.
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Or in a system with no heat losses then forget the further maths. You will use twice as much gas. ie it takes (roughly) the same amount of energy to heat water by 20 degrees whatever the start temperature, so you use as much gas as from 35 to 55 as you do for 55 to 75.
Litres per minute is not a pressure reading, it is a flow rate.
It is a most unusual setup whatever you have. Certainly something I have never seen
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Okeydokey! Thank you.
Yes, of course... it was in my head ('cos I wasn't paying attention) that pressure causes flow rate which ain't necessarily so.
However, that emphasises my point: should you expect wateer from a half-turned-on hot water tap to come out much hotter than if you turned it on full? Isn't the boiler supposed to regulate the temperature better than that?
Well, it's a commercially available system. I've just written to the installers about this overheating waste.
There's a couple of solar collectors on the roof: piping filled with a heat-absorbent fluid running through a series of glass vacuum-'filled' sleeves, pumped through a heat-exchanger in a hot water tank in the loft. On a sunny day it could boil the water, and renders the boiler unnecessary; on an overcast winter's day it raises it to around 25 degrees, which helps.
But if every time we use the boiler it's using far more gas than necessary, then that's crap. It sounds like during the winter we might as well not have the system at all (i.e. raising the temp from 5 to 50 degrees is about the same as from 25 to 75), and during the summer when we should be only occasionally heating the water about 10 degrees or so (i.e. when it's at 40 degrees in the tank) we're still hoiking it upwards by 35 degrees to a pointless 75.
I think we've been mis-sold this system.
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I would have thought so.
I have heard of the soler combi set ups, I just have never seen one.
It could be a faulty boiler control.
Good luck at getting it sorted.
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I think you're missing one important point.
When you're heating the water to 75c you're going to mix more cold into it than 55c hot water to have a shower or a bath.
So although it's using more energy per litre to heat, you're using fewer litres of hot water.
Point is, the solar heating will be reducing gas consumption, though as Adam has stated, it sounds like something is wrong with the boiler controls.
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This is something I've occasionally wondered about fpr years: is it cheaper to produce 10 litres of water by heating it all to (say) 50 degrees, or by heating some of it to (say) 70 degrees and topping it up with cold water to produce the required amount. I can appreciate your argument, and it sounds reasonable, but it sounds like pressing the accelerator and brake simultaneously to achieve the desired speed...
But if I want a shower at 45 degrees (say), and the tank is heated to 35, I only want it raised 10 degrees by the boiler: the flow is adequate for my showering needs and I shouldn't need to add cold water just to increase the flow. However, if the boiler is heating the water to 75 degrees, that's a substantial waste.
Yes, at times, but wasting gas at others, so I suspect the net gain is far from the advertised rate, so reducing the environmental benefit and prolonging the period when outlay turns to saving.
I suspect the boiler is faulty, but Ferroli's engineer says not.
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