New boiler advice needed

[aceuk]

I am replacing my boiler.
I first thought about having a unvented system as I wanted 2 showers fitted(one over the bath & one in en-suite).
I was told that my pressure was ok(20L min) but flow rate was not .But this could be solved by fitting an accumulator.
I had another installer tell me that he has a Vaillant 831 combi and can run a bath and shower at the same time with minimal reduction in flow,and suggested I have a 837 which should be fine.

does anyone have an Vaillant 837 combi ?
I would'nt mind having a combi if it did what I wanted also a lot cheap to install.

 

[simond]

You won't be able to run a bath and a shower off a combi such as 831.

With an 837 it is almost possible if you do not open the bath full bore.

With your flow rate at 20l/m the accumulator is going to add value with a 937; with an 837 it may not be worth the investment.

Have you looked at the 937 unit, Vaillant have now cured the problem with these, it was the PCB.

 

[aceuk]

My main concern with having a combi was the reduce flow when you run water else where.What the installer was saying to me was that whilst it will reduce it will still be good enough to have a good shower .
I suppose the chances of running a bath + shower or 2 showers at the same time are slim but should it happen I wanted something that will still work.
Will running cold tap(washing machine/dishwasher) affect performance.

Also isn't the 937 the same as a 837 with a small storage container.The size of the container is so small i didn't think it was worth having.

Without getting to technical could you tell way the accumulator will be ok with 937 and not 837 when they are basically the same .

One more thing if the output of the 837 is 15L/m(approx) will i get that at my shower.

Thanx

 

[gas4you]

I always advise my customers that any combi will only supply one hot outlet at a time, even the 837.

The 937 is more than just an 837 with a tank behind it.

Glad they've sorted the 937 Simon.

 

[Agile]

Without getting to technical could you tell way the accumulator will be ok with 937 and not 837 when they are basically the same .

Thanx

I think you may have been told that because the flow rate of the 837 is lower and merely boosting the supply flow rate is not going to improve the DHW because it is limited by the boiler.

The 937 stores some hot water so will give an increased flow rate for a LIMITED time! So the increased flow rate from a small accumulator will enable a higher flow rate to be achieved for a short time. But as the storage is not that big it will not need a very large accumulator.

I would generally recommend an unvented cylinder rather than a storage combi unless the property is very small or the DHW usage is unusual.

Tony

 

[simond]

The boost provided in the 937 is delivered for the first 200 litres of hot.

Obviously, most of us will mix a proportion of cold in with it.

Therefore, in excess of 200litres of accumulated boost will be required to get the max out of the 937 in a heavy use scenario.

Therefore the accumulator would ideally be the largest size available, 500 litres/260 litres of usable water.

Those knocking the 937 may not be aware that less than 10 minutes of latency is all that is required for the next 200 litre drawoff in the next 10 mins.

If you manage to use all 200 litres of available flow rate in one 10 minute go, it reverts to the performance of an 837, which is not a bad second best.

 

[Agile]

The boost provided in the 937 is delivered for the first 200 litres of hot.

Therefore, in excess of 200litres of accumulated boost will be required to get the max out of the 937 in a heavy use scenario.

Can you explain, based on physics etc, how such a small storage volume ( 20li + 20 li ? ) can provide 20 li/min output for very long based on the underlying flow rate capability of the boiler?

Tony

 

[grahamderek]

looks like some people actually believe manufacturers claims.
going out to buy some Lynx; boy, will I pull tonight!

 

[aceuk]

Ok.
Thanx for the info on the 937.

Can you tell me if I have this right.
The total output from you hot & cold can not be more than your mains input.
Assuming I have 20L/m coming in and the 837 gives me 15L/m.
So if I turn my shower on full (just hot) I will get 15L/m.
If I then turn on the cold I can add 5L/m without affecting the hot supply.
If I was to add 10L/m of cold I assume the hot would decrease to 10L/m.

If the above is correct.
I could get 10L/m from 2 showers at the same time.
Do you not think getting 10L/m from a shower is a good proformance.

 

[Agile]

Thats pretty much correct from a simplistic point of view.

Thats why I always recommend measuring the dynamic flow rate and not the open pipe flow rate which virtually all plumbers do.

In the real world to get a proper shower you need a pressure of 0.5 to 1.0 Bar to push the shower water through the small jets to give a powerful and invigorating shower rather than just water falling from the shower head under gravity!

You really need to measure the flow rate whilst retaining a pressure in the pipework as thats the realistic maximum flow rate which is useable on a shower. Of course more is available to just taps.

Tony

 

[aceuk]

What is dynamic flow rate and how do you measure it.
Also how do you measure the flow rate whilst retaining a pressure in the pipework .

 

[Agile]

Dynamic flow rate is the flow rate whilst retaining a pressure in the pipework as thats the realistic maximum flow rate which is useable on a shower.

Tony

 

[aceuk]

But how do you measure the flow rate whilst retaining a pressure in the pipework

 

[gas4you]

The boost provided in the 937 is delivered for the first 200 litres of hot.

Therefore, in excess of 200litres of accumulated boost will be required to get the max out of the 937 in a heavy use scenario.

Can you explain, based on physics etc, how such a small storage volume ( 20li + 20 li ? ) can provide 20 li/min output for very long based on the underlying flow rate capability of the boiler?

Tony

It is actually 2 x 7.5 litre stores. From my basic understanding of it, (not physics) it is blended with the instant HW being heated by the 837 part of the boiler, rather than just using the 15 litres first as in a WB highflow.

Depending on the demand for HW, it uses one or both stores, controlled by the extra shift load pump.

Hopefully Simon will be better positioned to give more accurate and fuller operating details.