Advice on Storage combis needed...

[barthompson]

I currently have a 24kW combi boiler for my 3 bedroom / 2 bath house. It is 12 years old and becoming increasingly problematic so I have decided to replace it. One issue I am trying to improve is the flow rate. I currently get just under 8 l/m hot water. However, I only get 10 l/m cold water so I think the flow rate coming into the house wouldn't actually support a much more powerful standard combi boiler.

Having read several threads on here, it seems that a combi with storage or an accumulator (my current understanding is that they are two terms for the same thing) would be able to vastly increase the flow rate internally to up to (say) 20 l/m but only for a certain amount of time until the storage tanks/accumulators empty.

I have seen discussed the Vaillant 937, the W-B Highflow and I have also found a Glow-worm Ultracom2 35 store.

In terms of requirements, I would like to be able to provide a shower with upwards of 15 l/m (minimum) for 10 mins (minimum). Which of these boilers is best? What are the pros/cons etc...

Any other thoughts welcome.

Many thanks in advance

 

[bolshy]

You need to get your cold mains flow sorted out first. These boilers are basically a combi boiler with a well insulated but small thermal store in the boiler, which heats mains cold via a heat exchanger supplied by the thermal store.
Unfortunately, you will only get a decent DHW flow out if you put in a decent cold flow rate.
You need to check your cold flow rate from a bath tap or the like, kitchen mixers and monobloc taps tend to restrict the flow too much. Or try a garden tap with the nozzle removed if you have one.

 

[barthompson]

Ah right...my previous post re cold water flow rate was taken from the kitchen tap. I have just tried the external garden tap and that had filled my 12 litre bucket in 30 seconds so incoming cold water flow rate looks ok actually.

[EDIT] I have just tested cold water flow rate from my shower and it was c.13 l/m. Does this likely mean the flow is being restricted by pipe work to the shower or the shower itself or both?

To the extent it is relevant it is a grohe thermostatic shower.

 

[bolshy]

If you want better DHW flow rate, then any one of those highflow boilers will support your needs. They are designed in general to support 20 litres per minute of DHW at 30 ° C rise for 20 minutes.
You have to remember your shower will be drawing hot and cold water at the same time. What's the flowrate when it's set to a warm temperature?

 

[dcawkwell]

Ah right...my previous post re cold water flow rate was taken from the kitchen tap. I have just tried the external garden tap and that had filled my 12 litre bucket in 30 seconds so incoming cold water flow rate looks ok actually.

[EDIT] I have just tested cold water flow rate from my shower and it was c.13 l/m. Does this likely mean the flow is being restricted by pipe work to the shower or the shower itself or both?

To the extent it is relevant it is a grohe thermostatic shower.

A storage combi or high powered combi will give you a better delivery.
A ravenheat 42kW will give 17 litres per minutes and is quite cheap you get 3 year warranty that can be upgraded to 5. Solid brass construction inside.

 

[barthompson]

What's the flowrate when it's set to a warm temperature?

I have just tested the warm water flow rate from the shower using the temperature setting I would use for a shower. It was about 9 l/m.

 

[bolshy]

Not sure I'd want a Ravenheat though. Landlords' and government grant favourites in the past. I know they're trying to improve their reputation, but I don't fit any of their stuff, their track record doesn't warrant it as yet.
IME their older models failed too often and parts were too expensive to justify repair.....possibly the failures were in many cases down to poor installation and neglect.
Can't comment on their warranty support.
Some installers prefer certain makes, but it's a careful and correct installation that makes the difference, no matter the boiler, and a long warranty with decent support behind it.

 

[barthompson]

If my shower is only giving me 13 l/m cold water - is this a sign that something (be it the piping or the shower) will limit the flow rate even the most powerful boiler will give me?

 

[bolshy]

I have just tested the warm water flow rate from the shower using the temperature setting I would use for a shower. It was about 9 l/m.

Could be a number of things, but a 24kW combi will only give 9 or 10 litres/min of hot water at 35 ° c rise when new. You could check your inlet filters on the mixer shower, but I suspect the boiler isn't delivering sufficient hot water at a good temperature. Incoming mains temp is around 10 degrees lower at this time of year.
Is the shower ok in the summer/autumn?

 

[barthompson]

Is the shower ok in the summer/autumn?

I havent noticed an increase in the summer. The shower was installed a year or two ago so I wouldnt *expect* anything blocked in the shower like filters.

It sounds like I should get a plumber out to see where is limiting my flow rate (boiler vs. pipework vs. shower) and see what he advises.

Thank you for the advice re: getting the right installation/support vs. brand - very sensible and helpful.

 

[transam]

Ravenheat combi store flue heat recovery storage set up ?? worth a look , (maybe ?)