Combi KW Rating (High or low?) - Fit De-aerator and Extra Expansion Vessel?

[Plumb0Jumb0]

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!

 

[Stu-B]

Forget the combi, go smaller boiler and unvented hot water cylinder, much better hot water delivery as it’s stored at 65C rather than a comb which will raise the temp 35C at full flow, so in winter when the water is colder coming into the house you’ll need to slow the flow rate to get hot water performance...
If you want the 12yyears warranty go for 26kw vogue max system, calcs permitting of course

 

[SimonG]

Thought I'd already replied to this.

System boiler and unvented cylinder.

Go for the guy wanting the extra expansion vessel.

 

[SJB060685]

The OP said above he is not interested in an unvented cylinder guys.
The point of hot water pulsating is a genuine one but depends on the flow rates out of individual outlets, although testing this with your current setup is not possible.
Fitting a de aerator is a good idea as as you mentioned above it removes dissolved oxygen and air trapped in the system which leads to corrosion.
An additional expansion vessel requirement comes down to system water volume and operating temperatures. More often than not the vessel supplied with the boiler is man enough but if the system volume is larger than normal then one should do some quick maths to see if one was required.

 

[Gasmk1]

Viessman 111 and fit filter and de-aerator also check may need extra expansion vessel due to size of system

 

[Plumb0Jumb0]

Thank you for the responses all.

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, and SJB above?. 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. 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).

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. 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.
[automerge]1597349312[/automerge]
Could anyone point me towards any formulas/documentation/web links to calculate whether or not an expansion vessel really is required?

 

[SJB060685]

The following list details the volume of water contained in a one metre length of copper pipe.
.8mm O/D = 0.036 litres
.10mm O/D = 0.055 litres
.15mm O/D = 0.145 litres
.22mm O/D = 0.320 litres
.28mm O/D = 0.539 litres

This is taken from my OFTEC books.

Bare in mind you would also have to take into account any fittings, valves and emitters.
If you can get a rough idea of system total volume I or anyone else will be able to advise you on correct size.

 

[Gasmk1]

As I have said look at the viessmann 111 if You want combi with flow rates Vitodens 111-W - high-performance storage combi boiler - https://www.viessmann.co.uk/products/gas-boilers/gas-condensing-boilers/vitodens/vitodens-111-w

 

[Plumb0Jumb0]

The following list details the volume of water contained in a one metre length of copper pipe.
.8mm O/D = 0.036 litres
.10mm O/D = 0.055 litres
.15mm O/D = 0.145 litres
.22mm O/D = 0.320 litres
.28mm O/D = 0.539 litres

This is taken from my OFTEC books.

Bare in mind you would also have to take into account any fittings, valves and emitters.
If you can get a rough idea of system total volume I or anyone else will be able to advise you on correct size.

Thanks for that info SJB. It would be a bit of guesswork in a lot of places as most of the pipework is hidden in the walls, or under floor and I don’t really know what the pipe runs are. It’s the weetabix boards as well so I won’t be taking them up !. I’ll see what I can cobble together. When you say emitters, does that mean rads or something else ?

Also thanks for the pointers Gasmk1 on the Veisseman. I have certainly read good things about them and a BG guy said they are decent but parts can be a right faff with them. I haven’t seen too many installers for them round here. Also assume that mod generally needs more room to accommodate the intg tank?.

This install is to go into a kitchen cupboard in the util room and I’m not keen on relocating it really and want to free up some room from existing tanks.

 

[SJB060685]

Thanks for that info SJB. It would be a bit of guesswork in a lot of places as most of the pipework is hidden in the walls, or under floor and I don’t really know what the pipe runs are. It’s the weetabix boards as well so I won’t be taking them up !. I’ll see what I can cobble together. When you say emitters, does that mean rads or something else ?

Also thanks for the pointers Gasmk1 on the Veisseman. I have certainly read good things about them and a BG guy said they are decent but parts can be a right faff with them. I haven’t seen too many installers for them round here. Also assume that mod generally needs more room to accommodate the intg tank?.

This install is to go into a kitchen cupboard in the util room and I’m not keen on relocating it really and want to free up some room from existing tanks.

Radiators are emitters yes my friend. On complete new design and install the engineers should be able aware of the system volume but in your situation it would not be easy to accurately calculate the system volume unfortunately.

 

[SimonG]

The additional expansion vessel doesn't have to be fitted at the boiler.