Valliant 937 suitable? No one knows!...

[Salmoners]

Can someone shed light on the following? The plumbers I have talked to so far do not have the answers...

I know that combis are rated e.g. as capable of raising 20l/min by 35 degrees, and it is always said that there is no point having a boiler that is rated higher than the water flow available (i.e. in this case at least 20l/min). But surely, having a boiler that is more highly rated than the flow would allow the water to be heated hotter, which would effectively allow more useful water out (e.g. when the raise needs to be more than 35 degrees, which it often is). i.e. Is it possible to fit a larger combi and get more of a temperature lift from lower flow (which would help e.g. for 2 showers at same time in winter)?

Also, can someone clarify whether the Valliant 937 is really a 20l/min or 15l/min boiler in terms of input flow. It has a tank (which is an attraction for various reasons), which if I understand correctly is heated 'offline' (like a normal system tank), and can then be mixed with the directly heated (normal combi style) water to provide an output effectively of 20l/min until the tank runs out, then is rated at 15l/min. Am I correct in thinking that a house flow rate of 15l/min is then really what is required, not 20l/min as usually suggested?

The house I am interested in putting the boiler in has a flow of 16l/min and pressure 2-3 bar. It has one bathroom at present, but we intend to add an ensuite. We are shower, not bath, people.

Would appreciate any answers...
Best wishes,
Salmoner.

 

[kevindgas]

didn't read the whole post but you say you have 1 bathroom and intend to put in a 2nd shower room
from that i would say the 837 would be perfectly suitable and it does look like a washing machine bolted to your wall!!

 

[muggles]

Your research is generally correct, however beware the 937, it's suffered from software issues which I'm not sure have entirely been resolved yet. Would be worth waiting for one of the Bunnymen on here to comment on this before you make a decision

 

[Agile]

You are heading for a problem!

A flow rate of 20 li/min from an open tap is NOT suitable for getting 20 li/min out o a shower.

You need a DYNAMIC flow rate of 20 li/min @ say 1.0 Bar.

Everything coming out of the taps must be supplied by your mains water flow rate.

The 937 will only supply the higher flow rate until the store is depleted. That could be in about 10 minutes!

The main purpose of a 937 is to fill a bath quickly. Trying to run two showers will work if the supply is up to it but only for the limited time.

A better system is a hot water storage tank!

Tony

 

[Beerlover]

The 937 ecotec will only give 19l/min if a blending valve is fitted to the boiler (not supplied) otherwise the output is just below 15 l/min @42 degree rise.

Another thing to consider is, the installer must be qualified to fit Unvented Hot Water Systems (known as G3), and a blending valve must also be fitted to the hot tap of any baths in the property.

Worth considering is the Glowworm Ultracom2 35 Store - again G3 & blending valve needed.

 

[jetsetwilly2000]

@Agile, what do you mean by dynamic flow rate?

I'm puzzled by the messages I see everywhere that combis can't manage two showers but unvented tanks can do it easily.

Our electric shower at home puts out about 5l/min at about 45deg (so probably a 30-35deg rise in july), ok so it's not a great shower, but surely a 937 could easily manage twice that?

Another way I look at it is, a 28kW combi burner should be more than capable of supplying two showers, as electric showers are typically 8kW. Are electric heat exchangers somehow more efficient?

Finally, an unvented tank stores hot water, great, but surely it can only deliver it at mains pressure? Thus, although the water might be hotter than a combi can manage, you still have to use mains pressure to get it (and any mixed-in cold water) out to the shower head, and therefore it's confusing to read that combis are limited by mains pressure but unvented tank systems are not.

What am I missing?

 

[Agile]

Willy, you need to search the forum to see what dynamic flow rate means. ( Also called working flow rate by shower makers )

If you are content with a 5 li/min shower then a 28 kW combi can run two.

But normal ( not electric ) showers take about 10-14 li/min.

An unvented cylinder is restricted in exactly the same way as a combi by the dynamic flow rate available.

Its storage temperature is only 60 C which is the same as the max provided by a combi.

Where its NOT limited is the ability to deliver an unlimited flow rate if the mains supply is capable. Thats until its depleted of course.

Tony Glazier

 

[Biggles..]

Another thing to consider is, the installer must be qualified to fit Unvented Hot Water Systems (known as G3), and a blending valve must also be fitted to the hot tap of any baths in the property.

why must he be G3.

 

[Agile]

If the volume is over 15 li ( historically ) then many people interpret that as needing G3, including some boiler makers too!

 

[lightning]

Because its over 15 litres unvented needs a G3 ticket

 

[kevindgas]

I'm puzzled by the messages I see everywhere that combis can't manage two showers but unvented tanks can do it easily.

a 35,37 or 40kw combi will cope with 2 showers in most situations, i have several customers with vaillant ecotec 837's that run a bath and have a shower at the same time and at least 2 that have 2 showers

 

[Biggles..]

thought the store was 2X 7.5 litre cylinders, 15litres makes it exemot from G3.
Also the appliance doesnt have a TPRV so its manufacture( if your saying its under G3 regulations) is in violation of these regulations and fails its CE mark.

Also does the manufacturer state in its instructions that it needs a G3 installer.?

does it state it needs a blending valve in its instructions.

does it state 204L over 10mins at deltaT30K.

 

[Bunnyman]

Sorry beer lover No g3 required for the 937! But use the 837 it's a better boiler without the store

Untitled

• Bunnyman
• 26 Mar 2010
• 499

 

[Agile]

Because its over 15 litres unvented needs a G3 ticket

Keey your eye on that statement in case it becomes out of date!

 

[Beerlover]

Sorry guys but you are all wrong.

Renewed my G3 a couple of months ago, the scope of G3 now covers all stored hot water. That is both vented and unvented hot water. There is no longer any minimum amount!

Regulation G3: Unvented hot water storage systems - all systems

3.19 Water heaters with a capacity of 15 Litres or less that have the appropriate safety devices for temperature and pressure will generally satisfy the requirements set out in G3(3)

Also the other change is that G3 requires the hot water supply to any fixed bath must be so designed and installed to incorporate measures to ensure that the temperature of the water that can be delivered to the bath shall not exceed 48 ° C

This applies equally to vented systems as well as unvented systems.