What temperature should my hot water be set at?

[Radian]

From The Heating Hub:

How does a condensing boiler work?

To understand how a condensing boiler works, and more importantly to explain why they are not as efficient as they should be in 99% of households, we first have to understand how a non-condensing boiler works.

 

All central heating systems work by sending hot water out into the radiators in one direction (called the flow) and receiving back after it has travelled around the radiators (called the return). A non-condensing heating system is set up to deliver heat to the radiators at 80°C. As the water ‘flows’ through the radiators about 20°C of heat is given off to the room. Water ‘returns’ to the boiler at 60°C.

Condensing boilers need to operate at lower temperatures to non-condensing boilers in order to recover the lost heat. The process is called condensing because the heat needs to turn back into water before it leaves the flue via the second heat exchanger. When the temperature of the water - as it returns to the boiler - is less than 54°C, the boiler will run in what is known as ‘condensing mode’. This is when the boiler starts to recovers its lost heat.

 

Even this is a sliding scale however. At 54°C the boiler is 87% efficient (in perfect testing conditions). The boiler does not reach 90%+ efficiencies until the return temperatures are around 45°C. This means water must exit the boiler at a maximum of 65°C, impart 20°C of heat to the room via the radiators, and return at 45°C. This makes radiator balancing very important to ensure enough heat is given off to the room to lower the return temperature sufficiently.

 

The problem is, most boilers are still set up to output at 80°C and return at 60°C. Some are reduced to 70°C because this is deemed efficient by many boiler manufacturers but as most systems are not balanced the return temperature can still be 60°C. The boiler still works but it does not condense and it certainly does not reach 90%+ efficiencies.

 

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The above is very focussed on space heating rather than DHW but it makes it plain that operating a boiler in condensing mode isn't possible if heating DHW to 54 or above. I believe this limits their efficiency to 78% at a maximum (the maximum for a non-condensing boiler).

[SteamyTea]

1 hour ago, Radian said:

about 20°C of heat is given off to the room

I know you did not write this, but there is someone who has mixed up temperature, power and energy.

 

Here is a bit about higher and lower heating values.

https://www.engineeringtoolbox.com/fuels-higher-calorific-values-d_169.html

Not a great difference for natural gas, 14.5 kWh/kg higher heating value and 13.1 kWh/kg lower heating value.

Should be possible to measure the flu gas for the RH and temperature and then work backwards.

What would be really posh is a feedback system that adjusts the return temperature to keep it all in the optimum place.

Edited September 9, 2022 by SteamyTea

[Radian]

36 minutes ago, SteamyTea said:

I know you did not write this, but there is someone who has mixed up temperature, power and energy.

I suspect it's why they put singe quote marks around 'flows' - after all water does actually flow through the radiator! Possibly it's the power dissipation resulting in a 20^oC drop across the radiator that's the bit they instinctively know but fail to express succinctly. You're welcome to put it into better words 😃

 

42 minutes ago, SteamyTea said:

What would be really posh is a feedback system that adjusts the return temperature to keep it all in the optimum place.

I think the boiler might be doing this with it's modulation - the gas rate does drop as the return temperature increases - all the while the flow temperature remains close to set point. Only when the flow temperature exceeds the set point by a few degrees does it shut off completely for a bit.

[JohnMo]

6 hours ago, markc said:

 I have found my boiler works best, shower heats up quickly and my bills appeared to reduce when I upped the water temp to 70.

If this is a combi I have found the same.  I upped my temp to 65 and have a significant drop in gas usage.  Tried at 48 and up, so 65 has been the best for gas usage.

[Miek]

A combi operates in condensing mode for DHW since the water comes in cold, ideal. Also no standing losses from a tank. 

 

I tried operating my old gas combi heating circuit at 55 degrees (returning at roughly 45) to try and get it to run efficiently but the rads were not really big enough to get sensible heat out. It did work OK, if one was prepared to run the boiler for longer periods.

 

 

[markc]

3 hours ago, JohnMo said:

If this is a combi I have found the same.  I upped my temp to 65 and have a significant drop in gas usage.  Tried at 48 and up, so 65 has been the best for gas usage.

Yes it’s a combi, old railway property, cellar, no cavity walls etc. so boiler is big to give enough heat for radiators but I also found turning water temp down guzzled gas and figured the lower temp was causing  cycling instead of keeping burner running 

Edited September 9, 2022 by markc

[JohnMo]

May try upping to 70 also

[SteamyTea]

It is odd as I have heard on the radio (I don't have a TV) several times recently, that turning down the temperature at the boiler will save energy.

Some people may be in for a shock, especially ones on quarterly billing, the period between October 1^st and January 1^st could be painful.

Edited September 9, 2022 by SteamyTea

[markc]

1 hour ago, JohnMo said:

May try upping to 70 also

Just tried turning mine down a bit with shower running at its usual setting, still ok at 65 ish but lower and it’s on and off with burner surging which will definitely decrease efficiency 

[JohnMo]

I had heard the same, they have been saying the same for years.  I have preheated water going in to the boiler via a solar PV heated cylinder, so was using the combi, topping up DHW to 48, and still using bout 8-9kWh of gas a day.  I read a bit about Alpha Boilers Superflow (preheat cylinder on a combi) and the instructions made a big thing about making sure the DHW was turned up to the max, so gave it a go.  Changes made early June.

 

May monthly gas usage 269kWh (no CH heating, just DHW)

June 105kWh, used loads extra messing with settings over a couple of days as a trial.  Should have used around 70kWh for the month.

July 64kWh

August 104kWh, less solar output going spare for preheated water.

 

So seems to have made a quite big impact, £10-14 per month lowering of DHW costs.