How does lowering the water temperature save money?

QrizB

ripplyuk said:

Thanks for the replies. I don’t think this is a condensing boiler. Does that make any difference? I asked the engineer when they installed it and he said it’s not a condensing boiler, it’s a ‘conventional’ boiler. It was only installed last year but this is a council house so the fittings tend to be at least 20yrs behind everyone else.

If it was installed last year, it'll be a condensing boiler. Conventional just means it's intended for use with a hot water tank; it's not a combi boiler.

ripplyuk said:

It’s in my back yard and pumps out steam constantly. I assume a condenser boiler would condense the steam instead? 

There will be less steam if it's condensing properly. Lots of steam means it's working inefficiently.

matelodave

Any boiler installed since 2005 has to be condensing. Dont confuse condensing with combi. A combi doesn't have a hot water tank but all boilers, whether combi or not must be condensing.

If its sending out a plume of water vapour then its probably condensing but you can improve its efficiency by getting the return temperature as low as possible so the boiler extracts the maximum amount of heat from the exhaust.

for a description of how it works have a look here  - https://www.heatgeek.com/condensing-boilers-efficiency/

Reed_Richards

I think you are confusing two different temperatures, the temperature you heat your hot water cylinder to and the temperature of the water from the boiler which you use to heat the cylinder.

If you make the temperature of the water from your boiler too low it can indeed take a very long time to get your cylinder up to temperature.  So turn up that temperature; you may be able to control this independently of the water temperature used for your heating system.  On the other hand you can save money by only making the target cylinder water temperature to be as hot as you need but no hotter.  Hopefully you can set this temperature somewhere on your heating controller.   

Scot_39

You need a hotter flow though the indirect heating coils in the tank - to heat the water in the tank that you use - at a given rate.

IIRC basic physics - and the very simplest of pipe heat exchange laws - the energy loss / desired flow in this case - the effective power - is essentially linear with temperature differential.

I can imagine heating engineers will have full models looking at the more complex dynamics in a cylinder - and may be able to come up with the real dynamics.

But using the simple pipe linear analogy - say you set the boiler at 65C - and you start with a cold fill at 10C

Thats a 55C temperature differential - to drive the heat flow - across the coil surface.

By the time you get up to near HSE recommended 60C - your down at 5C differential.

The rate of heat flow from a simple pipe - the actual figure for a coil in the tank ? - is less than a 10th of it was when tank cold (delta T 5 vs 55).

So the rate of increase slows - possibly dramatically slows in the simple linear pipe law - with an increase in water temperature.

Even if target say 50-55C - HW target - youd still be looking at 10-15 differential - and so 20% of the effective cold power - using the simple pipe analogy.

It's quite believable that it would take several hours to heat the whole tank to a temperature only a few degrees below the boiler flow temperature.

And for GCH radiators - manufacturers often rate their radiators at a delta T of 50C (a quote from Stelrad site who do use 50C) - drop the temps - a la swap to more / esp ASHP efficiency - less  power so longer heating periods - or bigger radiator surface areas required.

Any chance bath upgraded with a TMV (Thermstatic valve) at same time as boiler work ?

But remember the 50 or 60C or whatever in the tank - doesnt mean you get that at the tap - or even want it for a bath or shower especially if kids or frail / elderly involved.

So as well as care applications - all modern builds have mandatory TMVs on bath taps - their trade association recommends 41-44C for bath tap applications.

For those used to topping up with really hot water for a long hot soak - that max tap temp - will be a notable change.

FreeBear

Scot_39 said: It's quite believable that it would take several hours to heat the whole tank to a temperature only a few degrees below the boiler flow temperature.

When I had a hot water tank (with a Baxi back boiler), it would take around 45 minutes to heat from cold up to around 50°C. That would give me enough water for a hot bath.

matelodave said:

Beeblebr0x said:

FreeBear said:

I heat my water to a maximum of 45°C (combi boiler). Simple maths tells me that it requires less energy than it would to heat to 60°C or hotter. Heating to a lower temperature reduces the amount of scale build up (a big issue if you are in a hard water area), and also reduces the effects of corrosion.

In terms of boiler efficiency, even with heating water to 45°C, the boiler is running outside of condensing mode, so it doesn't affect efficiency that much. At most, a percentage point or two.

45°C wouldn't feel hot enough to do our washing up.

I find that water at 45 degrees is more than hot enough for a shower and there's not much chance of scalding yourself

We use a dishwasher for most of our washing up and if I need water hotter than 45 degrees I boil up as much as I need in a kettle.
2 litres of very hot water costs a lot less that heating a whole hot water cylinder full or running lots of hot water through all the plumbing for a combi.

With my combi, I need to heat 7l of water inside the boiler, and then another 2-3l or so in the pipes before I get hot water out of the kitchen tap. Even with electricity at four times the price of gas, it is cheaper to boil the kettle for washing up.