Is this the right boiler for me?

Frog_Prince

This Is Weird asked: F-Prince, how often will both bathroom/showers be used at the same time? Have you completely discounted having a combi?
Not very often. I sometimes have people stay overnight, but that's usually at weekends, when I do not go to the office. I have not discounted a combi.

Freebear said: If you are set on a Viessmann boiler, I'd suggest contacting an engineer that has been trained on their products.
CH Group sent someone over this morning. My mains water pressure is 12-13 l/min, which ruled out a 111W for reasons I couldn't quite understand. There's no issue providing the long flue. They have quoted for a Viessmann 050 25kW.

ThisIsWeird

12-13lpm is your flow rate, and I have to say it isn't brilliant.
The actual pressure is a different thing, but equally important. Any idea what it is?

Frog_Prince

ThisIsWeird,

I'm sorry, my last post was lazily written. The engineer to whom Viessmann referred me had a small measuring device with him. He put it under the kitchen tap and let cold water flow into the device and through a graduated exit in its side. The water reached up to the "14" mark as it flowed out, I'm pretty sure he said "12 to 13" as he closed the tap. TBH, I was quite chuffed, as I had used the "measuring jug and second hand of your watch" method with the kitchen tap yesterday, and could only up with a figure of 9l/min.

Viessmann also gave my number to another firm, who are sending someone over on Friday morning. Will he be able to give me an accurate water pressure figure?

FreeBear

Frog_Prince said: CH Group sent someone over this morning. My mains water pressure is 12-13 l/min, which ruled out a 111W for reasons I couldn't quite understand. There's no issue providing the long flue. They have quoted for a Viessmann 050 25kW.

I have a ~10l/min flow rate from the tap in my bathroom. The Viessmann 050-W that I had installed is rated at 30KW. It currently runs at full power to heat the water. With a 12-13l/min flow, I have a feeling a 25KW boiler is going to be a disappointment. The 35KW version may be better suited - It will still modulate down to 3.2KW for heating, so no loss of economy there.

Side note - Kitchen taps, particularly mixer taps, often use micro-bore pipes and aerators. Both of these restrict water flow (I only get ~7l/min in my kitchen). The cold tap on your bath is likely to give a better indication.

ThisIsWeird

Frog_Prince said:

ThisIsWeird,

I'm sorry, my last post was lazily written. The engineer to whom Viessmann referred me had a small measuring device with him. He put it under the kitchen tap and let cold water flow into the device and through a graduated exit in its side. The water reached up to the "14" mark as it flowed out, I'm pretty sure he said "12 to 13" as he closed the tap. TBH, I was quite chuffed, as I had used the "measuring jug and second hand of your watch" method with the kitchen tap yesterday, and could only up with a figure of 9l/min.

Viessmann also gave my number to another firm, who are sending someone over on Friday morning. Will he be able to give me an accurate water pressure figure?

Not a problem, and what you said seems correct in any case :-)

That device would, indeed, be a flow meter, so the result of 12-13 lpm is quite likely correct. (Not sure how you got the figure so 'wrong', tho'! That's the layman's method I suggest on here - stick a pan under the cold kitchen tap (or garden tap might be better), and turn the cold on full for 10 seconds. Times that by 6 for lpm.)

12-13 lpm is an 'ok' figure, but not at all brilliant.

If you were to, say, fit a ~30kW combi, which has a +35oC DHW heating rate of around that figure - ~13lpm, then it would be capable of running at pretty much the max flow rate when needed, but with no mains flow in reserve to supply cold water, such as you'd expect a shower mixer to have to do. So, when blending water in a shower, the hot would be backed off by the cold amount, so the overall rate would still 'only' be around 13lpm coming out your 'head (that's a pretty nice shower, tho'). And turning on any other tap in the house - hot or cold - would be bound to have a noticeable effect on this. So, unless you are running only a hot tap - say for filling up the kitchen sink - then a 30kW combi will be unlikely to be maxing out. I see no rationale in considering a combi larger than this - unless your mains supply is actually better than 13lpm.

The same applies to your current Potterton with its hot storage cylinder, or using a mains-pressurised hot cylinder. These will also only be able to supply a ~13lpm rate maximum as they are driven by the mains - mains in = water out. (Unless they also have a pressure vessel fitted to them to give this a limited boost).

Ah, your Potterton isn't the same as the one I'm familiar with. Yours has a Thermal Store which instantly heats the DHW via a heating coil. The 155X model has a max DHW flow rate of 18lpm, but obviously yours wouldn't be able to deliver this because your mains seemingly cannot deliver more than ~13lpm.

Q: You have a bathroom with bath/shower, and a separate en-suite with a shower? Are both showers supplied by the Potterton? And have you ever had both running at the same time? When someone is in one shower, and a toilet is flushed, does the shower flow rate drop?

Ok, that's 'flow rate'. Now 'pressure'! That's a different, tho' oft-connected, beast. The pressure is the 'force' behind the water flow - not how much water, but how strongly it is being delivered. And it matters, because a low pressure will make the flow rate collapse if more than one outlet is opened at a time. A strong pressure will help to maintain that rate, regardless of how much of the flow is being used. ~13lpm appears to be your max, so you want to hold on to that as much as possible. A plumber should also be able to measure your mains pressure - I'd hope for at least 2bar.

'Albermarle' said on a previous post, "My own experience is that even with an unvented hot water tank under mains pressure, it is not guaranteed you can have two or three showers running at the same time without issues. Although maybe my system is not set up quite right." And that is correct. A mains-pressurised DHW supply is only as good as the mains supply. That applies both to combi boilers and unvented stored hot cylinders; they are as good as their mains supply.

I realise that the above doesn't give you answers or solutions or recommendations. :-( It's just 'info'.

"Powermax 140 and 155x are combination boilers
in which the gas burner heats the contents of a
built-in thermal store. This heated water is used
as ‘primary’ water and circulated, as required, to
the radiators. Domestic hot water (‘secondary’
water) is heated by a heat exchanger and
delivered via a thermostatic blending valve which
prevents the hot water exceeding 60°C. The flow
rate is governed to a maximum of 12 litres per
minute (140) and 18 litres per min (155x.)"