Inherent fault with combi boilers?

[johnkirk]

After spending a lot of time researching boilers I came down to a final choice between either the Worcester Bosch 38CDI Greenstar Classic, or the Vaillant ecoTec plus combi 837 for my mother’s 1-bathroom, 3-bedroom house with a shower and no bath, and just herself as the occupant.

But I was dismayed to be told by WB technical dept that the minimum flow rate through the boiler needs to be 3.5 – 4 lit/min before the boiler fires to warm the DHW. That’s a huge flow rate, relatively, isn’t it? Less than that and the water in the taps and shower come out cold.

The full flow rate for the DHW with these boilers is around 16 lit/min, so this means that the flow has to be at least a quarter of this rate just for the boiler to fire!

Does anyone know whether this is typical of combi boilers? I have just emailed Vaillant to ask what their equivalent min flow rate needs to be for the ecoTec plus 837, but I fear their answer is going to be similar to WB's.
I had thought a combi would be the ideal choice for the circumstances, but am reconsidering now if it means every time she wants hot water she must run the taps fast. Could this be a problem in practice, or am I worrying unnecessarily?

Any advice would be greatly appreciated.

 

[Dan Robinson]

Vaillant's minimum is 1.5 l/min, but in 14 years you are the first person I have heard complain about the minimum flow rate to activate hot water.

3.5 litres per minute through a shower means I wouldn't want to stand too close to you on the train.

 

[johnkirk]

Dan, I think you have misunderstood my post! I was querying the high min flow rate needed for the boiler to fire and produce hot water!

 

[Dan Robinson]

I know.

But if you are worried about a shower not activating the boiler at 3.5l/min then.....




In 14 years you are the first person I have heard complain about the minimum flow rate to activate hot water.

 

[johnkirk]

I was thinking more about the use of hot water for the sink/washbasin situations actually, when you may want the water to flow slowly for sustained periods of time, but at the same time be hot. Sometimes also needed when cleaning the shower tray, if not for showering, hence my reference to the shower hot water.

Anyway, ignoring Dan's red herring, I've now had a reply from Vaillant to confirm Vaillant's min water flow rate for producing hot water is 1.5 lit/min, which is far lower than WB's, so I think this will be the decider in favour of Vaillant!

 

[Dan Robinson]

My red herring?

Anyway - the Vaillant is a superior boiler to the Worcester so not a bad choice.

Also consider the Intergas at 2 l/min and the Atag A325EC - which is the most efficient combi for hot water production.

Although I confess to having no idea on the minimum activation level... However as it uses a preheat function, like the Vaillant and Intergas a small draw off will result in some hot water arriving at the outlets in most circumstances anyway.

 

[dcawkwell]

The flow rate may be because it is a big boiler.
Compare it to smaller boilers and the minimum flow rate may be smaller.

The big combi boilers seem to be a lot or nothing.
I expect the minimum flow rate is quoted at the rate that will keep the burner firing. Below this the burner will have to cut off and therefore
the flow temperature will fluctuate quite badly.
Combi boilers yuck!

 

[Agile]

What he means is that the boiler may well actually turn ON at a lower flow rate of say 1.5 li/min but because their minimum output power they may not be able to maintain such a low flow rate.

But the Vaillant will also be likely to be turning its burner on and off to maintain such a low flow rate.

Unless there are some special hot water requirements a lower powered boiler would usually be better.

Tony

 

[JohnD]

3.5 – 4 lit/min before the boiler fires to warm the DHW. That’s a huge flow rate, relatively, isn’t it?

No, it isn't.

 

[bazdaman]

Have you calculated what size boiler output you actually need? Or do you just think bigger is better

 

[Whitespirit66]

What system does your mother have now?

Why are you looking at boilers that are possibly more powerful than you need?

 

[Charnwood]

WB 38CDI GS - for a 3-bed, 1-bath, single occupancy house.

Overkill.

The Queen would be happy with the results if that were used to heat Buck House.

 

[johnkirk]

Thanks for your helpful responses.

Agile and bazdaman: The higher output boiler is selected not for the space heating, but to get a decent flow rate for the shower, especially when hot water is simultaneously being demanded elsewhere in the house.

Although the house currently has just one bathroom, a second one may be installed some time in the future, and for that reason alone doesn't it make sense to go for a boiler that has a higher DHW flow rate, given it's a combi?

In fact I was considering the WB 42CDI, which has an even higher DHW flow rate - WB recommended it for a 3 bedroom house that might have a 2nd bathroom sometime in the future, so in this sense I suppose bigger is better for the DHW, though irrelevant for the radiators.

Whitespirit66: the current system (just removed) was a 35-year-old floor standing Potterton Kingfisher boiler, that had an even older gravity HW cylinder with scantly jacket insulation and no thermostat on the cylinder.

Unfortunately, to keep costs down and gain valuable storage space from dispensing with the HW cylinder it seems a combi system has to be the way to go, unless someone knows better!

 

[Gasguru]

Just remember when a combi breaks down you have no hot water..perhaps for several days whilst parts are obtained (say over the Christmas break). Traditional systems have a nice immersion heater for backup.

 

[The831Bunny]

The most important question is whats the incoming water main flow rate. Any thing under 20 l/min and you are wasting your time with an 837

Ideally for your future planning of two bathrooms/showers you'll need 25l/min @ 1.5 bar minimum working pressure.