Suitability of combination boiler in a townhouse

[theomegamale]

Hi

I've done a bit of research on the subject but I was hoping to get another opinion to try and come to a conclusion on this...

I live in a 3 floor townhouse. There is currently an indirect system in place, but since the CWST, hot water cylinder and bathroom are all on the same top floor, there is no decent shower (flow is zero when you lift up the hose attachment we currently use). A good shower is a high priority for the wife, and rather than go with a twin impeller pump we're looking at fitting a combi instead.

My concerns with a combi are:
1) Do I have enough pressure? Two plumbers have read 2.5 bar at the kitchen stop cock. On paper that sounds like enough.
2) Currently, the rising main which fills the cold water tank will stop filling the tank if I turn on the kitchen sink cold tap on the ground floor. I'm concerned that if we fit a combi and everything is under mains pressure - will this mean that if someone is showering on the top floor and the washing machine kicks in, the shower will stop in the same way? Is it right to make the comparison as I have done?
3) Powerflushing: do I need it? It's a late-70s house, and I have nothing to suggest that the system has been "cleaned" in anyway. One plumber said I need it, the other didn't. Any further opinion valued.
4) I've been recommended the Potterton Performa HE30 by both plumbers. Any thoughts/comments? Does the combi "pump" the water in anyway i.e. perhaps to combat point #2?

Thanks, and sorry for the long post but the wife is nagging after a year of inaction so appreciate any advice received which will help me come to a decision!

 

[gas4you]

The 2.5Bar measured was probably static pressure and not working pressure when another tap was in use.

Each storey of a property will reduce the pressure by 0.3Bar.

As well as the pressure, you need to check the flow rate.

For a 30Kw combi, you will ideally need about 12-14 l/min and at leats 1.5 Bar pressure.

Less flow than that and a 30Kw is a waste of time and money, and would probably leave you with earache from a disappointed wife

 

[theomegamale]

Thanks gas4you

Yes you are right it would have been static pressure as we didn't run any other taps. So it sounds as though I have sufficient pressure. Or were you referring to working pressure/pressure elsewhere e.g. on the top floor?

Also in terms of checking the flow rate: can i just check how much pours out of the kitchen sink or do i need to check the flow rate on the top floor (e.g. say into the CWST currently?).

I don't understand why the CWST stops filling if i turn the kitchen tap on - does this suggest any problem with my rising main that I would also need to get checked?

 

[powell30]

Pressure and flow are 2 different things OP. To improve flow you need to upgrade the incoming mains. to a larger bore ie 25mm to 22mm or larger.....

 

[mickyg]

given the height of the property and your intended use, I think its essential you have the flow rate measured at the top floor.
and yes if the washing machine kicks in your shower will die..well sounds like it anyway.

 

[simond]

On the basis of your rudimentary check the combi solution will be rubbish. Your 'plumbers' must be idiots, frankly. Just as well you applied your own logic to their advice.

You either need to enhance the water supply by some means - the options which are regularly debated here - or keep a tank fed system.

 

[theomegamale]

Thanks to all those who replied.

I've now been round a neighbour's (same style house) who has a combi set up which seems to work very well when there is no other tap/appliance running. There is a noticeable reduction in flow (approaching a trickle in my opinion but probably still usable) when the kitchen tap is on. Not sure how they get over my apparent problem of poor flow on the top floor when the downstairs tap is on.

I'm leaning toward a twin impeller negative head pump and fitting a surrey flange to the hot cylinder... perhaps noisy at times (pump + we seem to get a whistling noise when the cwst refills), and means i don't get to the replace the boiler and thus have a thermostat (currently all our rads have TRVs which is the only way to adjust the temp) but it will mean a good shower (i hope!)

 

[mickyg]

Sounds like the better option. you can still fit a thermostat to your existing system if you like, usually very straight forward.

 

[chadfieldpropertyservices]

Check that your incoming stopcock is fully open, so many clients I come across like to have it just cracked open for some bizarre reason, if it was an old place on lead pipes I could understand low flow with 2.5 bar static pressure but a 1970's place sounds like there's something else to investigate.
In our last house, even with a big 35kw combi the thermostatic shower would stop completely in the winter if anyone turned a hot tap on in the house. combis are ok if you live on your own, they are cheap to install and thats about as far as it goes. considering that you have heating on for about 70% of the year, the heat "lost" from a hot water cylinder located in the heated part of the house is not really lost.
We have an unvented cylinder that is fed directly from the mains via a pressure reducing valve that limits to 3 bar, you can run the bath, fill the washing machine run the hot tap in the kitchen and the shower can still be used. you should be able to run it off your existing boiler BUT, whilst your static pressure is ok you will need to find out why the flow is restricted and probably run 22mm pipe from the stopcock to the tank.
Another cheaper option would be an electric shower.

 

[simond]

How would an electric shower help? It would use the same water main and therefore stop working when the kitchen tap was on.

 

[chadfieldpropertyservices]

In a 1970's house there is obviously something wrong if the static pressure is 2.5 bar but the header tank stops filling if you turn on the kitchen tap, there will be a restriction in the line somewhere, also, does the header tank really stop filling or does it just go quiet when the pressure to it is reduced slightly by the kitchen tap being turned on.

 

[theomegamale]

I'm certain it stops - I can get a hand into the top of the tank and feel the inlet from the rising main, and there is no water left running.

I'll probably just live with this "restriction" (whatever might be causing it). The pump I'm looking at (Monsoon 2.5bar universal twin impeller) won't have dry run protection so I guess we'll need to watch out if we're having multiple showers and running the w/m at the same time.

 

[simond]

In a 1970's house there is obviously something wrong if the static pressure is 2.5 bar but the header tank stops filling if you turn on the kitchen tap, there will be a restriction in the line somewhere, also, does the header tank really stop filling or does it just go quiet when the pressure to it is reduced slightly by the kitchen tap being turned on.

If you run a seaside guest house in Yorkshire and do a spot of kitchen fitting you may not be conversant with what is normal in London.

Recommending an electric shower would have been a costly and fruitless exercise if your advice had been heeded.

 

[Agile]

I have a 70s town house and it has a black plastic 25 mm supply pipe and the flow rate is about 22 li/m plus to an open pipe.

What needs to be measured in these cases is the dynamic flow rate, the flow rate which can be taken but still leave, say, 1 Bar in the supply pipework otherwise showers wil not work properly.

In your case a shower pump is the obvious solution!

Tony