Combi boiler hot water temperature

chubsta

Have just had a lovely shiney new combi boiler installed to replace my decades old system that used a hot water tank. I understand the central heating side of things as that is not really any different but just wondering how the hot water bit works.

On my old system it would heat the water up to whatever temperature I set it to and that's what it would be when it comes out of the tap. But how does a combi do this? If I have it set to 60 degrees how does it regulate this 'on the fly' as against having it set to 55 degrees? Surely it just heats the water up as it passes over the heat exchanger or does it hold it there until the right temperature is achieved?

Also, how to use it - it was originally set to 60 degrees, I have moved it down to 55 and it is still too hot to have on Max Temp at the shower - should I make the most of the hot water coming out of the boiler by not having it mixed with cooler water at the shower and therefore set it to even lower at the boiler and have it on max at the shower? The flow rate is fantastic so there are no issues there with whatever temperature I choose, in fact I can barely turn the shower on as it's so powerful!

I know we are probably only talking pennies per year but its my new toy and I would like to get everything working as efficiently as I possibly can...

sheenas

The Combi boilers in most cases does not store
water thu there are some that do. I have mine set at 55c and I would agree with you it could be set lower. A range of 50c to 60c is recommended in the uk. 

Emmia

And you can turn the shower temperature down too - no need to have it on max. 

One of the downsides of a combi, is if it stops working there is instantly no heating or hot water. This has happened a few times to us. If we had the room I'd look at having an immersion heater fitted. It turns out, no heating is fine for a few days, but I'm not someone who enjoys an ice cold shower at any time of year, and definitely not in the depths of winter. 

daveyjp

For the right temperature for you set it on its lowest temp, run the hot tap nearest the boiler and increase the heat until it is comfortable for you without adding cold.  Ours is currently on 46 degrees, in winter I increase it to about 50.

Scot_39

The 60C hse is for everything but lowest volume stored hot water.  And for non medical / care applications comes with a 50C tap temp target.

Many boiler manufacturers therefore recommend 50-60 for combi hw temps - but if Iirc one large uk selling brand said lower limit 47.

Legionella stops growing around 45 - but needs far higher to kill it.  60C kills it in minutes, 55C in 10s of minutes iirc.  But really shouldn't be an issue for combis anyway - even though it does grow in pipework too.

It's a bit of a trade off though for other reasons - as 47-50 might not do a great job on greasy dishes in a kitchen sink.


In many businesses and medical facilities and modern donestic bathrooms - since c2009 - for baths and bidets - thermostatic mixer valves are used to further drop the output temps to iirc a max of c48 C depending on application (think thats domestic bath max nhs / care etc lower) but many will be set lower as even 48C risks scalding and too hot to sit in anyways.  NHS and care home guidelines used to limit to below that like 44C for unassisted bathing 41C for showers etc.  Current targets?

So your likely always going to need to mix hot and some cold even at 47-50 for your shower.  And if gets much hotter than above nhs bathing limits then perhaps you need a tmv based mixer tap or one with a lower max limit.

Reed_Richards

chubsta said:

Have just had a lovely shiney new combi boiler installed to replace my decades old system that used a hot water tank. I understand the central heating side of things as that is not really any different but just wondering how the hot water bit works.

The boiler can modulate its output.  It monitors the water temperature at the outlet and turns the boiler output up or down to maintain that temperature.  The only problem that might possibly arise is that if you run too many hot taps at the same time, causing water to flow very quickly through the boiler, then it cannot provide enough power to maintain the temperature and the water out the taps runs a bit cool.  Or there might be a flow regulator so the water out the taps stays hot but the flow rate out of any one tap drops.  

FreeBear

chubsta said:  Also, how to use it - it was originally set to 60 degrees, I have moved it down to 55 and it is still too hot to have on Max Temp at the shower - should I make the most of the hot water coming out of the boiler by not having it mixed with cooler water at the shower and therefore set it to even lower at the boiler and have it on max at the shower? The flow rate is fantastic so there are no issues there with whatever temperature I choose, in fact I can barely turn the shower on as it's so powerful!

I know we are probably only talking pennies per year but its my new toy and I would like to get everything working as efficiently as I possibly can...

I have my combi (Viessmann 050) set to 45°C for DHW - Plenty hot enough for the bathroom. And as the incoming water is treated and in a sealed system, there is virtually zero risk of Legionella*.

One of the first things I did when the boiler was installed was to enable Eco mode - This stopped the boiler firing up at random intervals to keep the internals hot to ensure minimal delay in getting hot water. This instantly cut gas consumption at the expense of a slight delay in getting hot water out of the tap. Running at 45°C rather than the default 60°C boosts efficiency by maybe 2-3%. It also reduces the amount of scale build up in the P2P heat exchanger (important if you are in a hard water area). Thermal stresses are reduced which should ensure long term reliability.

45°C isn't really hot enough for washing up, so I'll boil a kettle for that task - Having done the calculations, it saves having to heat 7l in the boiler and the ~1.7l that would remain in the pipe between boiler & kitchen tap.

Do bear in mind that hot water will be a relatively small part of your total gas usage throughout the year. You can save quite a bit by running at a reduced flow temperature (but only if your radiators have been sized appropriately). A programmable thermostat that allows for different temperatures throughout the day/week could save up to 15%. With a thermostat that can talk to the boiler (OpenTherm, EMS, or similar) and modulate the output, there is potential to save another 5-10%.

*) Legionella is only a risk if you are storing water in an open tank at between 20°C and 45°C. And even then, it only becomes a problem if you inhale a fine spray of contaminated water. As there are only 200-400 cases reported annually in the UK (and approximately half of those are contracted overseas), the risk is extremely low. You have a better chance of being hit by a number 13 bus or winning a big prize on the lottery.

matelodave

The main thing to remember with combis is they aren't very efficient for short run-off's.

When you turn the tap on, the boiler purges, relights and send most of the energy out of the flue for the first minute or so until it settles down. They work best for long draws, like filling a bath, having a shower or at a pinch filling the sink to do the washing up.

Therefore, just turning on the hot tap to rinse a dish or your hands wastes quite a bit of energy, not only in the initial boiler firing sequence but in the amount of cold water that goes down the sink whilst you are waiting for the hot to flow and also in the hot water that sits in the pipework getting cold until the next time you turn on the hot tap.

So try to be a bit clever with your use of hot water and dont waste it on trivial stuff like rinsing a plate or even your hands - use the cold tap instead.

chubsta

Thanks for all the replies - some stuff to ponder!

EssexHebridean

Also and particularly if you are on metered water start thinking about creative ways to use the run off from running water for washing up - we always leave a watering can outside the door as with our boiler > kitchen tap being a long run we can fill that with the cold run-off before the hot water comes through! 

Ectophile

Turning down the temperature to something more reasonable and less scalding will make the boiler more efficient.  It will be able to modulate down rather than running at full blast.

Also, setting it to 60C may be wishful thinking in the depths of winter, when the ground is frozen and the incoming mains water is icy cold.  The boiler can only do the best it can in the short time that the cold water is passing through the heat exchanger.