Question about Boiler water heating temperature

Hattie627

Hi there. I am not up to speed on domestic energy saving and am trying to rectify this with the help of MSE and this forum. Sorry if this question shows a lack of understanding/knowledge.

I was reading one of the MSE articles about energy saving and it said that most combi boilers are set to produce hot water at a temperature which is higher than necessary (not talking about flow temperature through the radiators, different issue which I think I understand),

The recommendation seems to be to set the hot water temperature at 55C. If I recall correctly, this recommendation comes from Octopus Energy.

I was about to reduce the hot water temperature on my boiler when I remembered that I had recently had a Legionella Risk Assessment carried out on a flat which I rent out. An LRA has to be carried out by the landlord every 2 years (a Scottish landlord requirement).

The LRA seems to suggest that the hot water temperature from the shower head (maybe other water outlets as well, not sure) should be at least 60C for safety reasons.

Just wondering if 55C, while better from an energy point of view, is too low?

QrizB

I would suggest that the LRA is incorrect.

Legionella is a problem for stored hot water. Combi boilers don't store hot water. As far as I can tell, there is a near-zero legionella risk from a combi boiler.

Hattie627 said:

... I remembered that I had recently had a Legionella Risk Assessment carried out on a flat which I rent out.

If I had paid a professional for this assessment, I would be asking them questions.

QrizB

Just to add:

The official HSE advice here is:

Hot water should be stored at least at 60°C and distributed so that it reaches a temperature of 50°C (55°C in healthcare premises) within one minute at the outlets.

You shouldn't have any stored water with a combi boiler, and it only needs to be 50C at the outlet.

Hattie627

Many thanks for comments and reassurance.

I may have misunderstood the LRA. It is filled out on a "pro forma" with printed questions, and the assessment is done by an employee (not a heating or plumbing specialist person) of the lettings agency following the questions on the pro forma. (It's not a report, as such). Cost was £72 in October, and involves a visit to the property where there had been no changes to the heating or plumbing arrangements since the last check in 2021 (which concluded that the property had very low risk, as was the position in 2019). The letting agency instruct the LRA automatically every 2 years, and charge for it. I suppose I could ask for it not to be done in 2025 and just do the assessment myself using the form. (There's a fear in anything landlord-related of being judged a penny-pinching, irresponsible landlord and falling foul of the legal requirements).

FreeBear

QrizB said:

Just to add:

The official HSE advice here is:

Hot water should be stored at least at 60°C and distributed so that it reaches a temperature of 50°C (55°C in healthcare premises) within one minute at the outlets.

You shouldn't have any stored water with a combi boiler, and it only needs to be 50C at the outlet.

At the bottom of that page, HSE also point out that there is a risk of scalding (especially with vulnerable/elderly people) when the water temperature is over 44°C at the tap.

With my combi boiler, I have the DHW set to 45°C and have zero concerns about legionella. Also gives me a nice hot bath without looking like a lobster once I'm done having a soak.

Reed_Richards

The other issue you have to consider, both for yourself and as a landlord, is the rsik of scalding from water that is too hot.  Water at 60 C will produce 2nd and 3rd degree burns after 5 seconds of exposure: http://www.accuratebuilding.com/services/legal/charts/hot_water_burn_scalding_graph.pdf

The HSE advise that where there a vulnerable individuals then hot water temperature should not exceed 44 C https://www.hse.gov.uk/healthservices/scalding-burning.htm

markin

Reed_Richards said:

The other issue you have to consider, both for yourself and as a landlord, is the rsik of scalding from water that is too hot.  Water at 60 C will produce 2nd and 3rd degree burns after 5 seconds of exposure: http://www.accuratebuilding.com/services/legal/charts/hot_water_burn_scalding_graph.pdf

The HSE advise that where there a vulnerable individuals then hot water temperature should not exceed 44 C https://www.hse.gov.uk/healthservices/scalding-burning.htm

That's where a mixer tap with a safety button comes in that mix it to 45c, Would still mean its not 60 at the shower head though.

Some new builds can come with a mixer under the sink and when they fail you get very hot water.

HumberFlyer

We are a 40 degree everything household, washing the pots, showers, baths (combi boiler)

Rodders53

Some combis can and do store (a relatively small amount) of water in them though.  (Usually best to disable it just for energy saving reasons).  It's to give small amounts of HW quickly while the boiler is firing up and heating more...

Building Regs in E&W (probably Scotland and NI as well?) require thermostatic bath tap mixers now due to the scalding risk (this may only apply to new build homes though, but is sensible anywhere).  Showers usually have thermostatic mixers with the same settings?
AFAIK the set temp mixed should be 38C and an over-ride to a bit hotter with an absolute max permitted of 48C out of the bath tap (or shower head).

Legionella in a domestic setting is relatively unlikely.

https://forums.moneysavingexpert.com/search?domain=all_content&query=legionella&scope=site&source=community is a search on the term for this site which is worth reading through to avoid the same arguments and includes Landlord insights.  You may try a search on Legionnaires as well.

NB Your tenants will be the ones paying to heat the water to the temperature you set.  They may decide to over-ride your setting to save energy.

JSHarris

We're another 40°C hot water household.  Our water heater (a stored thermal energy unit) instantly heats the incoming cold water to (a fixed) 55°C, very like a combi boiler.  I fitted a thermostatic mixer valve on the outlet of that unit to bring the hot water temperature down to 40°C. 

This temperature is fine, and was chosen after I'd measured the hot water temperature we normally used, with mixer taps and the shower mixer.  Turned out that we tended to set the hot water at the point of use to between 38° and 40°C, so there didn't seem any point in distributing water at anything above that.  Not sure how much energy it saves, in practice, but there's no noticeable downside to doing this AFAICS.

Scot_39

The Scottish gov site where it gives the 2 year LRA requirement for landlords links to this specific page for landlords on HSE site

https://www.hse.gov.uk/legionnaires/legionella-landlords-responsibilities.htm

Note the duty to inform tennants not to tamper etc - eg by turning down boiler temps (that specific one suspect not aplicable to OP) - even re cleaning shower heads regualrly.

HSE Legionella protection includes protection of the impact of the distribution system - not just the boiler.  

Thats in part why one of the standard tests for risk is 50C at the outlet on that link above for domestic systems fed by non tank based systems - for landlords on HSE site - to get a low risk assessment.

Looking specifically at legionella (not the only bacterial risk) one site quotes 90% legionella killed in 2 mins at 60C - but takes 2 hours to kill 90% at a lower 50C. At 38-40C it's still breading - US CDC says can do so from 20C in some scenarios but from 25 upto c45C more widely - and growth peaks at c35C iirc.
 

Theres an even higher requirement to distribute water so that it reaches 55C in some care and health facilities.

And I believe - but not clear from that HSE site link - that's now interpreted as 50C at the hot input to the local mixer valve of shower or tap - minimising low temp pipe runs.

The HSE standard for stored volumes - hot water tanks - is 60C min - and not sure it makes any real distinction between mains fed or gravity fed / vented - but may have to be higher in larger sites to cover pipe losses - or only higher in winter etc.

Re mods to above system

on the landlord link above and section on low risk scenarios

"where hot water is fed from instantaneous heaters or low volume water heaters (supplying outlets at 50 °C)"

The water in the pipe - if has any legionella bacteria in it - will now also be a potential breading volume.

Whereas the boiler output at 55 would actually be killing any legionella present in the pipe run at times - it no longer will.

Industrial sites have to do even more stringent tests - and keep records for HSE inspection - iirc that includes checking boiler and flow temps and regular actual legionella bacteria level testing (certainly the last factory site I worked in had people in periodically to do so). 

In reality I suspect mains fed closed systems are probably a lot less of an issue than open vented systems - but the HSE still includes them as a low risk factor - not explicitly saying a zero risk factor.

I still have an open vented / gravity (loft cold tank) hot water system - no way I'd want to drop the immersion temp on that sort of system - below it's current level - a good few degrees over 60 - my HW taps run dangerously warm - easily in high 50s at times.  Too hot to safely keep (skin) contact with.

Checking Scot gov website there were seemingly 44 recorded cases last year in a population of over 5 million - which was up on Covid and precovid levels - but no breakdown of likely domestic vs industrial sources. 

https://publichealthscotland.scot/news/2023/august/slight-increase-in-legionnaires-disease-incidence-in-scotland-in-2022/

(Industrial coooling towers, condensors and ventillation systems always seem to be suspects when get clusters - but there was a report years ago when elderly gent in 60s died - and although not definitely the source - the local council did report having found legionella in his undrained in the sun according to some - garden hose ).  Which if nothing else shows risk just not in hot water systems.