Valiant vs Viessman Heat Only Boiler

[JackK]

Replacing a floor standing boiler which is 20 years old in South London - I have an engineer trying to push for the valiant eco tec 430

Personally and what my research shows is that a Viessman is preferable - As I will be spending my life savings on this boiler exchange I would be keen to hear your thoughts - My system has cast iron rads so water will be dirty with time

Will a stainless steel heat exchanger be more forgiving to a dirty older system (which will be flushed)

 

[muggles]

Stainless is more forgiving of poor water quality. Viessmann don't recommend using any inhibitor though, so be aware of that. Do you actually need a 30kW boiler? Be aware that, since June 2022, it has been a requirement of Building Regulations to carry out a full room by room heat loss survey prior to replacing a boiler in order to determine the correct boiler size for the property. The installer needs to have measured the dimensions of each room and each window in order to be able to do this. Any installation which hasn't had this done won't comply with BR.

 

[Johntheo5]

Stainless is more forgiving of poor water quality. Viessmann don't recommend using any inhibitor though, so be aware of that. Do you actually need a 30kW boiler? Be aware that, since June 2022, it has been a requirement of Building Regulations to carry out a full room by room heat loss survey prior to replacing a boiler in order to determine the correct boiler size for the property. The installer needs to have measured the dimensions of each room and each window in order to be able to do this. Any installation which hasn't had this done won't comply with BR.

Do Viessmann still use rubber hoses on their boilers?, I think that was the reason for the no inhihitor recommendations, most people wouldn't be too happy with no inhibitor in their systems.

 

[JackK]

Its the viessman 100w I am going for and its a five bed detached house of 200 square metres - Probably 15 radiators in total.

I have been told I will need a Grundfos 25 100 pump which is £700 on purchase alone so this is a expensive quote for just a new boiler (cylinder will stay as cant afford a new one)

 

[fixitflav]

Its the viessman 100w I am going for and its a five bed detached house of 200 square metres - Probably 15 radiators in total.

I have been told I will need a Grundfos 25 100 pump which is £700 on purchase alone so this is a expensive quote for just a new boiler (cylinder will stay as cant afford a new one)

Sounds like another case of an installer trying to push you down the combi route. If you go that way, you wouldn't normally keep the HW cylinder.
But that may not be your best choice.
What output is your existing boiler? Needs a survey, but I'd be surprised if you need more than 20kW for CH.
What pump do you have now? Any reason to think it's not adequate?
I would consider replacing the boiler with a heat-only, keep the rest as is. You haven't said what controls you have but maybe an update needed.

 

[JackK]

Its a heat only boiler replacement that is being done and the cylinder is not getting changed - I do not want a combi boiler installed and the pipework is old so I do not want a sealed system

The pump I have is a DAB green pump - Its quite old - The radiators in the house currently work and the hot water is also ok

I am changing it as it is really old and dont want to have a boiler pack up in the winter

 

[Johntheo5]

I would think that a Grundfos UPS2 8M pump would be more than adequate for your needs, the 35kw vitodens 100 shows a residual pump head of 2.5M from a 6.8M pump at a flow of 1300LPH or 21.7LPM, which I assume means the Hx looses 4.3M (but not sure), if so the 8M UPS2 will still have 3.7M available to circulate through the rads at this 21.7LPM, if total rad output is say 25kw then the rad dT will be 16.5C and probably a lot less so that 8M pump may not even have to run on speed 3.

 

[fixitflav]

Its a heat only boiler replacement that is being done and the cylinder is not getting changed

OK sorry, I was misreading the viessman 100w data.
You could keep the existing pump and change it later if there's a need

 

[muggles]

Do Viessmann still use rubber hoses on their boilers?, I think that was the reason for the no inhihitor recommendations, most people wouldn't be too happy with no inhibitor in their systems.

No inhibitor is no problem if you treat the water to VDI 2035 standards

 

[muggles]

Its the viessman 100w I am going for and its a five bed detached house of 200 square metres - Probably 15 radiators in total.

I have been told I will need a Grundfos 25 100 pump which is £700 on purchase alone so this is a expensive quote for just a new boiler (cylinder will stay as cant afford a new one)

Sorry but there's absolutely no way you need a 30kW boiler in a house that size unless it has no insulation and single glazed sash windows with no draught proofing. It'll just be massively inefficient to run. You don't need a pump that size either - that pump is designed to push 60kW around a very high resistance system, and will cope with loads in excess of 200kW on lower resistance systems.

Here's a handy "quick check" guide to what size boiler you actually need...

30kW for a 200m² property is 150w/m² which is beyond the worst end of the chart

 

[fixitflav]

the 35kw vitodens 100 shows a residual pump head of 2.5M from a 6.8M pump

Residual head - doesn't that relate to a system (or combi) boiler with built-in pump? i.e. the head after subtracting boiler internal loss from the pump head.
We're told the Vitodens 100w is heat-only (but looking on the internet comes up with a combi, hence my earlier assumption).

 

[Johntheo5]

I would think thats what it means although I've also seen similar curves somewhere showing that as remaining head!, It would be far easier if they just gave the boiler internal losses at a number of flowrates, or even one, as head is proportional to flow squared.

 

[fixitflav]

It would be far easier if they just gave the boiler internal losses at a number of flowrates, or even one, as head is proportional to flow squared.

Agreed! The subject was discussed at some length in this thread https://www.diynot.com/diy/threads/gas-boiler-heat-exchanger-pressure-loss.599317/post-5336363

 

[vulcancontinental]

I would think thats what it means although I've also seen similar curves somewhere showing that as remaining head!, It would be far easier if they just gave the boiler internal losses at a number of flowrates, or even one, as head is proportional to flow squared.

My preference is the opposite, an installation will have one max flow rate for the design so a look along flow rate axis I’ll show available residual head.

 

[vulcancontinental]

Replacing a floor standing boiler which is 20 years old in South London - I have an engineer trying to push for the valiant eco tec 430

Personally and what my research shows is that a Viessman is preferable - As I will be spending my life savings on this boiler exchange I would be keen to hear your thoughts - My system has cast iron rads so water will be dirty with time

Will a stainless steel heat exchanger be more forgiving to a dirty older system (which will be flushed)

Why would there be dirty water in a boiler replacement, the system should be thoroughly cleaned.

Get another heating engineer to survey regarding boiler and pump.