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#8290
Hi All,

I am looking to replace an Ideal Classic FF350 in a 4 bed detached house, 1 main bathroom with a bath and electric shower, also en-suite with a thermostatic shower that is currently pumped (knackered) from a DHW cylinder (essex flange weeping). F&E and CWS tanks in the loft, gravity fed system. Only wife and I, mostly take baths and unlikely to use the bath showers at the same time.

There are 13 rads, 3 of which are towel rails. 1 rad in an extension is tall vertical designer style rad, the rest are standard sized Stelrad dual or single convector rads. Upstairs Rad pipework is all copper 15mm and the drops to downstairs are 10mm flexi plastic. All power flushed last year, and looking to have done on new install.

Looking to replace with a combi to solve pump and cylinder problems, and some weaker flow to extension kitchen taps, not interested in an unvented cylinder, had combis in previous houses so aware of limitations.

Water pressure measured 3 times now from same outside tap at different times by different people. This has varied from 14-16 Litres a minute. Will assume lowest value, so 14L/min incoming mains flow.

Had several quotes - most engineers seem happy to fit the Ideal Vogue Max 40kw Combi (12yr warranty appeals, if you can claim on it) to give me the best potential hot water flow (16.4L/min). This seems logical to me that the boiler has more headroom for pushing more water flow than I have coming in (rather than less) but....

A couple of the engineers disagreed and said would be better going with a lower rated kw boiler of 30kw-35kw max. Advice here was 40kw boiler when running the hot tap (or thermo shower) due to flow rates being lower than boiler can deliver, would cause hot water to pulse hot and cold as the boiler would constantly cycle on and off,as the flow isn’t high enough to make use of it properly (and empty the heat exchanger). Trying to find out if this is a genuine concern or not.


A couple of other suggestions coming out of the quotes I was hoping for opinions:

* Fit a Spirotech MV2 de-aerator to remove air and microbubbles.
Seems like a good idea for a relatively minimal cost.
* Fit a 15L Expansion vessel (additional to one inside boiler).
Really unsure on this one. Only 1 engineer suggested it.

So with the above info in mind, what do you guys knowledgeable guys think?

Thank you for any help and advice you can give on this matter!
#8294
System boiler and unvented cylinder if pressures and flows allow.

Sounds like a large system with possible need for additional expansion. I'd be looking at the guy who quoted additional expansion vessel from the info you have given.
#8297
Thank you Simon.

I do see from the forums that unvented cylinders are generally preferred, and I do understand the reasons for that.

Being just a couple, and having had combis in previous houses over the years, we know their limitations and what to expect, and would prefer the relative simplicity of a combi really, not to mention avoiding the extra cost of an unvented cylinder that frankly would probably be overkill for 2 of us.

From my research and suggested on all quotes anyway was a magnetic system filter (believe the Vogue Max comes with one), limescale filter and shock arrestor (again cheap enough and seems like a good idea).

Most have said flush with chemicals, but only 1 specifically said a powerflush with a kamco machine, so would have to check that was a powerflush and not just a mains job if using the others.

In terms of the hot water requirements and the warning we had on the 40kw model, is that definitely likely to hamper us as I have been told by 1 engineer?.

My old Bristan Prism Thermo shower for example is specced to draw @ 1.5bar - 8.1 litres /min (rainfall type shower head and fixed column). At 2bar it can do 9.5 L/min so both of these are well under what even the 32kw could deliver.
Theres a strong chance I might replace the thermo shower for a different or faster flowing one, and possibly the electric shower in the main bathroom for a mixer in the future.

To a novice like me, it seems like going with a 40kw will give us a bit more scope for the future, and more DHW headroom (it can shift more than my mains can provide). I suppose it depends which quote I go with, as some have said to go with the 32kw anyways.

I was just wanted to make sure I wasnt setting myself up for a problem down the line.

I cant afford to have a new system fitted, and then have to replace it again in the future if it was undersized or oversized.

I want to make it the best it can be, get it sized right, and hopefully installed right, and then have it serviced annually as you are meant to, so that it can last as long and work as well as it can for its lifespan.
#8301
Have a look at the Vaillant ecotec 938 - just to throw a spanner and an extra point of view in the works - That performs well and has a good reputation.

As far as DHW (Domestic Hot Water) is concerned, you will only get out what you can put in but the rise in temperature will be higher on a lower flow rate so you need to weigh up just how far over the odds you need to go size wise if that makes sense?
They will all provide a similar MAX outlet temp and vastly over sizing it can cause issues.

Vaillant make their own version of the Spirotec filters (looks the same to me) which can give you an extra few years warranty.

I have never installed a vented filter. If your system is installed correctly, you shouldn't need one in my opinion.

Hope this helps.

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