Hot water flow rate from Vaillant 937

[buggle52]

Hi there,

I've just had a Vaillant EcoTec 937 installed (the one with the storage tanks) and I'm not happy with the hot water flow rate. I'm wondering if anyone else has had similar concerns.

The combi part is supposed to provide a max 15.2l/min at 35 degree rise (or 65 depending on which brochure you look at), but the storage tanks should (based on my understanding of these things) allow the flow rate to reach a level closer to the cold water flow rate coming into the boiler, like an unvented system. In other words, even if it can't heat the water fast enough, the storage tanks should provide more until they're depleted (of hot water - they fill off the cold water directly, not through the heat exchanger).

So, the cold water flow rate to the boiler is over 25l/min, at 2.5 bar. I'm only getting 10-11l/min of hot water out, regardless of whether the storage tanks are being used or not, or regardless of the hot water temperature I set. This is the same from any outlet (upstairs or downstairs).

The plumbers have checked and rechecked the piping to the boiler, and we've had to Vaillant engineers out who can't find anything (superficially) wrong with the boiler.

I can't get my head around the idea that a storage tank can take water at 25l/min but only draw at 10l/min, even taking into consideration some expected drop from the boiler's internal pipework, that seems excessive.

Any ideas?

Thanks

 

[Dan Robinson]

If it is only running 10l/min, it is only taking 10l/min.

As for your flow problem. There will be a restriction somewhere. Have your plumbers by-pass the boiler with a loop of pipe. If the flow rate is back to the higher rate, then there is an issue in the boiler.

If the flow rate is still 10l/min, then your problem lies with the plumbing/fittings.

 

[Agile]

I expect that your "plumbers" measured a STATIC pressure of 2.5 Bar and an OPEN PIPE flow rate of 25 li/min.

Any proper boiler engineer knows that the only figure that mattters is the DYNAMIC flow rate!

It would be interesting how a Court would judge when a householder sues an installer who has made this most basic error!

Tony

 

[buggle52]

Thanks Dan, I'll speak to the plumbers about your suggestion.

Agile, the flow rate measurement certainly was open pipe, but I can't speak for how the pressure was taken. The boiler spec says it can operate at full capacity at 0.7 bar. Could there be such a big difference between static and dynamic pressure? The boiler is downstairs and the measurement was taken nearby.

 

[Dan Robinson]

A lot will come down to pipe size - that has jut as much n effect as the pressure.

 

[buggle52]

A lot will come down to pipe size - that has jut as much n effect as the pressure.

I agree, which is why I had a new 32mm water main installed (the flow rate on the old lead main was 14l/min), and have 22mm pipe from the stop valve to the boiler. The boiler itself only accepts 15mm so it's reduced just before.

 

[ALEC1]

This boiler benefits from a blending vale to increase the flow rate...store the water at 70 and blend it down to about 40-45c as lomg as the mains can provide the in put....

 

[buggle52]

This boiler benefits from a blending vale to increase the flow rate...store the water at 70 and blend it down to about 40-45c as lomg as the mains can provide the in put....

Forgive my ignorance; do you mean manually blending (i.e. using hot and cold taps) or is there a feature on the boiler that automatically blends 70c water to 40-45c? If there is, I couldn't find it on the controls (but they're not the most intuitive).

 

[ALEC1]

no its a mixing valve that costs around £120 which goes under the boiler..

 

[buggle52]

no its a mixing valve that costs around £120 which goes under the boiler..

Thanks - that makes sense! I'll speak to the plumber about this.

It might not explain why the boiler isn't functioning to the level advertised (IMO*), but it sounds like a pragmatic way of overcoming the issue.

* I know this kind of comment could stir up arguments about dynamic pressure and flow rates, but I'm looking at this from a layman's view!

 

[slup]

Surely Dan's earlier advice was the most sensible. Bypass the boiler completely and check the flow rate. If it's poor, you need to sort that out first.

 

[Agile]

It all sounds as if the dynamic flow rate is very poor. Thats particularly surprising with a 32mm supply pipe and 22mm to the boiler inlet.

Any competent plumber would have measured the open pipe flow rate at the house end of the 32mm pipe when it was installed.

The limitation may lie at the road connection! Do you know the pipe sizes/distances involved there?

Its all very well having a 32mm supply pipe ( length ? ) but if its only a 25mm pavement valve/meter then that will cause a restriction.

No point in buying a blending valve if the mains supply is limiting the flow rate!

Tony Glazier

 

[buggle52]

I actually had Three Valleys Water perform a measurement before I had the new supply pipe done. Here were the stats:-

- 25 metres head / 2.5 bar at property
- 40 l/min at the boundary
- 4" cast iron water main on the road
- 17m supply pipe (was lead, now 32mm)

I haven't seen any results from a flow test at the end of the 32mm, nor the 22mm directly, but the 25 l/min (open) measurement was taken from a 15mm pipe connected to the 22mm feeding the boiler (which is in turn connected to the 32mm of course). I've performed a simple timed-bucket test myself to confirm this result. I also performed a similar test on a hot water outlet directly below the boiler to confirm it's only giving out 10l/min.

I'm going to speak to the plumber about bypassing the boiler next time he's in, but I admit I'm confused as to how that can be done with the storage tanks being integrated into the boiler... unless this is just plugging the cold into the hot water pipes as they come out of the boiler? i.e. just to check the hot water pipes are clear?

Thanks for your continued suggestions guys.

 

[Dan Robinson]

buggle it will take ten minutes to lash up a simple rig.

The tanks will hold their contents. You need to stop thinking of then in terms of a storage cistern, and more like a very very fat section of pipe.

Plumber turns off water. chops out a few inches of pipe of the hot and cold pipes under the boiler. joins the two together and turns the water on.

Test flow rate.

If low = restriction in house plumbing

If High = Restriction in boiler.

Investigate and reinstate.

Total time = 30 mins not including the time to deal with the actual problem when found.

 

[buggle52]

Thanks Dan. I'll definitely be talking to the plumber next week about performing this test.