Filling a bath with hot water or not?

Bluefairie56

I have a gas condensing boiler and I am not on a water meter. I read that it’s best to turn water temperature down so you don’t put hot water in then have to put cold in to get the right temperature you want but my thinking is that surely it’s better to put a less amount of hot in then top it up to get what you want. Which is cheapest does anyone know? And please don’t say a shower. A bath is my one and only vice lol

Reed_Richards

Combi boiler or one with a hot water cylinder?

Bluefairie56

sorry forgot to say. Combi

Qyburn

More efficient to fill with water at the correct temperature, than with very hot water plus cold. The hotter the hot water the more heat loss from pipes, as well as possibly meaning the boiler doesn't condense.

FreeBear

Qyburn said: More efficient to fill with water at the correct temperature, than with very hot water plus cold. The hotter the hot water the more heat loss from pipes, as well as possibly meaning the boiler doesn't condense.

If you are in a hard water area, keeping the DHW temperature low reduces the amount of scale building up inside the boiler. There is also a slight gain in boiler efficiency, but only by a percentage point or two.

mavenmim

Water takes a lot of energy to heat; the hotter it gets the more energy it takes to get it to that temperature and keep it there, and the more heat loss occurs in the pipes. So the most efficient systems keep water at about 40 degrees, rather than any higher. That must mean it is cheaper to heat all the water to the correct temperature than some to a higher temperature and some not at all.
Our new multi building heat pump means we have showers that can top up cooler water, and tanks that can heat enough for a bath, all are kept cooler than old fashioned systems, but to reduce the risk of legionnaires disease they have to heat any water in the tank periodically hot enough to kill that off.

Bendo

Having the water heated only to the correct temperature is all well and good of you have a short bath. If you liketo spend an hour in the bath though,  good look adding additional water to heat it up.

FreeBear

Bendo said:

Having the water heated only to the correct temperature is all well and good of you have a short bath. If you liketo spend an hour in the bath though,  good look adding additional water to heat it up.

One advantage of having a combi boiler with a smart control - If you want to increase the DHW temperature, it is just a click away on an app.

p00hsticks

Bluefairie56 said:

 my thinking is that surely it’s better to put a less amount of hot in then top it up to get what you want. Which is cheapest does anyone know? 

I think which method is cheaper is perhaps less important than which is safer.
You shouldn't just run water hotter than you can stand into a bath - always run some cold with it. There have been some horrifical accidents where people have got into baths containing too hot water - either scalding themselves (e.g. Princes Margaret) or causing them to black out and subsequently drown. 

daveyjp

If its a combi set the hot water to a temperature where no cold is needed.  Probably around 45-50 degrees depending on the season.

Scot_39

p00hsticks said:

Bluefairie56 said:

 my thinking is that surely it’s better to put a less amount of hot in then top it up to get what you want. Which is cheapest does anyone know? 

I think which method is cheaper is perhaps less important than which is safer.
You shouldn't just run water hotter than you can stand into a bath - always run some cold with it. There have been some horrifical accidents where people have got into baths containing too hot water - either scalding themselves (e.g. Princes Margaret) or causing them to black out and subsequently drown. 

In care and NHS settings - TMV's are iirc now mandatory in many facilities - for that very reason.

The really old NHS guidance was as low as 44 for baths, 41 for sinks/ showers iirc - since TMVs mandated - these may have been adjusted.

And TMVs may even be set lower for a margin against regs.

And similarly in all modern build homes for baths (and bidets - temp?) - set to iirc water below 48C max.

The OP may or may not already have one fitted on his bath - mandatory in new builds since c2010 - making their boiler temp question perhaps less relevant - as may not be able to fill and mix say 60 hot / 10 ambient to get 35 at 50:50 anyway.

But the physics is simple enough for the water in the bath - regardless of volume mix (you either heat more of the water to a lower temp - or the hot water you have het - heats the cold ) - it's the average temp that costs - but secondary savings - any boiler efficiency - and larger pipe losses at higher flow temperatures - could be factors.

But the OP may want to check for TMV fitted  - some bathroom resellers will advise fit even on refits - post regs change (initially they were just new build - but might have changed - and if had kids or elderly - you might want them in any case - we did as mum in mid 60s when redid her bathroom). They arent that expensive in scheme of things.