combi boiler, mixer shower, conflicting temperature requirements...

timmer3

i'm considering getting in a gas condensing combi boiler to replace an old gas boiler + hot water cylinder + attic vented setup. the model i'm looking at is the worcester bosch 42 CDi, i've done a lot of research and think i've picked out a very energy efficient and capable boiler.
trying to plan out the bathroom shower set up as well, i understand from reading around here that electric showers are a convenient but expensive way to get hot water. and since the boiler produces DHW on demand i figured it makes sense use that supply and go for a mixer shower.
but... the DHW from the boiler is set at let's say 40c for normal bath/sink/basin use. and the same temperature would supply the shower, meaning that you would not be mixing in any cold water for an average hot shower, and i guess the pressure will not be any good then?
i looked at a Triton Avon thermostatic mixer shower and it specifies incoming hot water at 49c minimum, which i guess is above normal for sinks/taps. bit of a conundrum. i haven't come across any boilers that have dual hot water outputs, one 'hotter' for showers and one 'hot' for taps.
i love the space saving that the new boiler will give, to lose the cylinder and attic tank, not to mention drastically simplified plumbing, but i want to make the right decisions and i need a bit of help with this one!

the house is a small 3 bed semi with one bathroom. very low hot water demands as i'm the only person in the house and usually shower in the gym anyway, but want to build to scale for a family in the future. i know you can get boilers with a built in hot water reservoir of 40 litres or whatever, but i think the same problem will apply in terms of output temperature for a mixer shower conflicting with taps.

thanks for any help!

Alun

I always thought 60 degrees was the recommended temperature for DHW due to Legionnaire's Disease concerns? Even a normal combi boiler will have a small storage tank built in to it as a buffer.

timmer3

i am far from qualified to offer an opinion but i did read a paper today about legionnaires and the consensus was that it applied to electrically heated large water cylinders. i would have thought these small volume instantly heated water systems which will get recycled several times a day are not susceptible to it.

also, according to this site, 60c will give a child 3rd degree burns in one second. i wouldn't want my taps coming out that hot!

timmer3

just reading over that paper again on legionnaires, they recommended all DHW should be supplied at 49 to taps for gas and oil heated water. so i guess that answers my question. 49c is the right setting for the boiler and it will meet the shower incoming hot water specification as well. and it won't scald anyone.
thanks!

Pete67

One disadvantage of Combi boilers is that you generally don't have any backup if the boiler breaks down. With a hot water cylinder, you can always use an immersion heater to generate hot water, not an option with a combi.

Also, combis are more complex and require more maintenance, especially if you live in a hard water area, as the heat exchanger and diverter valves are more likely to scale up.

I have seen designs where a combi is used in conjunction with a hot water cylinder. The combi supplies instantaneous hot water to the kitchen only, and heats a hot water storage cylinder in the traditional manner for showers and bathrooms. Quite a good approach, as long as the hot water cylinder is really well insulated.

Just something to think about.

The first thing you need to establish is have you sufficient water pressure on your in coming mains water to supply a combi, the one you mentioned requires 17.2 l/min to give you a 35c temperture rise, also your gas supply to the boiler will have to be increased due to the massive amount of gas the boiler will need.
My idea of a good heating system is one that is as simple as possible which would mean a system boiler, cylinder and immersion, but there is nothing wrong with a good quality Combi, i do think there has to be a good reason to fit one, normally space saving. You can fit a electric shower so you have a back up, one more thing with combis you control the temperature at the tap by the flow of water, so the faster the water the cooler and the slower the hotter, Gary.

timmer3

pete, gary, thanks a lot for the tips. the incoming mains flow rate is certainly above average, not sure exactly what that is, but it's amazing compared to my last place! you can't turn the kitchen tap on fully without splashing everything in sight. i had the mains connection replaced when i moved in, and the council upgraded the pipes to the estate so i think it's as good as i will be able to get it. i don't have a way to measure the pressure but as a layman i would say it is very good as well.
the site for the boiler is right next to the gas main and water main so that will make the installation easier. the combi i'm looking at is the highest spec one they sell and i've read very good reviews about it. from reading around you would think boilers break down all the time and that you should always have a backup hot water system. to me that's a bit like having a backup car though. all that extra plumbing and kit just in case it breaks down for a few days in the winter. i'll grin and bear it if it happens i'm willing to bet if i keep it serviced and make sure it is installed correctly it will last. the problems i've read seem mostly related to dodgy installations, like people who don't flush their old rads and end up clogging up their boiler. i'll be replacing all the pipework and rads anyway.

gary the combi i'm looking at has a CH temperature setting and a DHW temperature setting, so i gather it will heat to exactly that and give whatever pressure is available. maybe it's a new feature on combi boilers.

does anyone have experience comparing the running cost of a shower supplied by a combi boiler vs an electric shower? i know it will depend on the price of gas and electricity etc., but maybe someone has figures to hand? i read an interesting discussion here from the UK, anecdotally people were saying the combi is cheaper to run and generally better performing, but i'd be interested to hear a local perspective.

thanks again for the advice.

311

Make sure that the flow rate of the boiler is not less than your shower.

There are shower heads that are designed to feel like they have a higher flow rate. Economical type ones.

timmer3

timmer3 wrote: »

Gary the combi i'm looking at has a CH temperature setting and a DHW temperature setting, so i gather it will heat to exactly that and give whatever pressure is available. maybe it's a new feature on combi boilers

Nope, you will notice the control knob for the hot water shows 1 to max, not a temperature, you could say 1 is 40c and max is 60c but this is still dependent on in coming cold water temperature and flow rate of cold water, your boiler will give you a temperature rise of 40c on the incoming mains temperature at a flow speed of 15.1 l/min and a temperature rise of 35c on the incoming mains temperature at a flow speed of 17.2 l/min, so long story short the faster the water goes threw the boiler the lower the temperature at the tap.
In respect of why boilers break down, i would put it down to installer error 90% of the time, so you're right if you fit and wire the boiler to manufactures instructions you will reduce the chances of a breakdown.
If i was in your position and it was easy to fit a electric shower, i would, this wouldn't be a cost factor, it's because i always plan for the worst(and i have a wife that refuses to wash from a kettle) and even all singing all dancing funky boilers break down, serviced or not and if you have to wait 3 or 4 weeks for a repair(which can happen) then a second source of hot water can be a god send, if as you said it's not a issue then just go for Worcester a very nice boiler, Gary

timmer3

hi gary. i see exactly what you mean about the water temperature, thanks for clarifying that.
i'm persuaded now to go for an electric shower as backup. it makes sense since i'm getting the house rewired anyway. the thing is i only have one bathroom so i'll have a shower at either end of the tub which will look a bit strange! i'll try and pick out a discreet unit, those triton so-called "digital" units look alright, at least the big box is hidden away.

311

The baxi Combi boiler is actually quite good at the moment, installed a few and it's won corgi awards for boiler of the year 2008.It's called Duotec.
They have a good price compared to the worcester.

Worcester are by far the best condensing boiler ,but the baxi is actually ahead in combination.

http://www.discountedheating.co.uk/shop/acatalog/Baxi_Duo-tec_Condensing_Combi_Boiler.html