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I assume you need the vaillant control (eg. VRC 470 as per your other thread) as well as the VR65/66? Otherwise, how do you lower your flow temp?

You are trying to rule out the following causes: 1. Expansion of HW back up the cold pipe when the tank is being heated 2. Thermal transfer from another pipe to the incoming CW feed to the combination valve (this could be any of the heating circuits) Try turning each one off in isolation and...

But the flow rate does not seem to be fixed - that's my point. At peak times, the flow rate is markedly lower, even though the (working) pressure remains the same.

I've given a lot of thought (+reading!) to understand the flow rate coming into my house and I'm not sure I fully get it. I have a 32mm incoming mdpe with 28mm/22mm running to various showers around the house (relatively high flow rain type shower heads). Depending on the time of day, having 2...

What happens to the water temperature whilst the pump overrun is running? Why do you believe what is happening is insufficient? My boiler (Vaillant 438) had approximately 3m of pipe between flow/return through an ABV (had, as I now have a LLH, but the pipe run is still relatively short). The...

Can you isolate your HW supply? By doing so, you should get no water from a HW tap (once the residual water in the pipe has drained out). If you do, a mixer valve somewhere may be allowing cold and hot to mix (usually debris holding the NRV open).

That's the route I went down, after advice on this forum and lots of struggling. Meant I could buy 2 standard domestic pumps, rather than use a much more expensive commercial pump, and the whole system just behaves better overall (especially as I have multiple zones).

We dont really know what the OPs actual symptoms are, as there's been no information given about what the boiler flow/return temps are when firing and what S codes are displayed. Is it inadequate flow? Is there an ABV? How is it set?

You're right the number of rads is immaterial, but it usually indicates a large system, which equates to longer pipe runs. The only thing which is important is the highest resistance - aka the index circuit. This combined with the resistance of the boilers hex (which is very high on these...

The index circuit is the one with the radiator with the highest resistance (usually the furthest, but not always). The heat exchanger in the 637 has it's own resistance, which increases with flow rate. The pump needs to be adequately sized to overcome the internal resistance of the hex + the...

It's possible the pump integrated in the 637 is undersized to overcome the resistance of the hex + your evidently large system. Do you know what the required head for your index circuit is?

Could someone point me to advice and terminology for horizontal valley gutters between 2 hipped roofs. Here's an image I found online: https://1stassociated.co.uk/article_images/Roof%20Problems%20Valley%20Gutters_clip_image033.jpg Reason I am asking is 2 fold: 1. I already have one of these...

Could be reverse circulation. The last tee on the return leg should be the HW cylinder. Google "last tee reverse circulation". Alternatively, maybe the motorised valve is letting water by even when closed. Should be able to quickly check this yourself.

It's possible, but if the office is always the last to heat up, it'd be the same as just having a single thermostat in the office. Without zoning the actual pipework, the heating will stay on everywhere until the office reaches the stat temperature. The manual TRV's will help to stop the other...

If you have a good installer, then surely go on his/her recommendation? Each installer has their own preference and, from experience, I'd recommend the installer install what they are well versed in, rather than getting them to install something they've never come across before. As for "nest...

Push fit installed properly is no more prone to leaks than badly soldered copper. Most new builds are done in plastic now and they're absolutely fine.

I wasnt able to find an online price for the replacement membrane. I'm inclined to think the GWS has better marketing behind it and essentially is the same as the Reliance (they talk about patents related to better design - but does it translate to better longevity in reality?). Someone must...

Does anyone have experience of either of these? Reliance – Aquasystem 500 Litre Potable Expansion Vessel XVES050150 £660 GWS Challenger 450L Accumulator Tank (same as Stuart Turner Mainsboost 450SV?) £690 Both are similarly priced. The reliance has a replaceable membrane. The GWS has a 5...

22 mins for 500L. Wow. Thanks.

What kind of showers? I have a similar setup to you with a 38KW heat only boiler, separate EVs and pump (pump sizing calculated with the help of D_Hailsham - Thanks!). U/s, d/s and HW zones. I'll share some of my experiences: 1. With multiple zones and a large system, I found a Low Loss Header...