Expert advice needed on Valliant combi boiler

[Razor900]

Good thing about a thatched roof, the loft space remains cool.

Until it catches fire

 

[muggles]

Sorrry Muggles dont know the answer to your question !!!

Best to find that out before choosing a boiler. There's no point having the 843 if your incoming mains isn't up to running it.

 

[muggles]

Will the Valliant 843 give us more heating output ??

It will, but unless your house is a mansion (in which case a combi is totally unsuitable) you won't need the extra power

 

[John D v2.0]

looking at the vaillant green iq one, the modulation ratio is quite good, down to 10% before it has to turn off. Is it better or worse to run a system at a low percentage all the time? other than the capital cost.

 

[dilalio]

Until it catches fire

Well at least that should kill all the bacteria!

 

[dilalio]

I was though, referring also to your beloved cold water cisterns sitting in nice warm lofts this time of year.

Which reminds me... the look on that boy’s face (and that of his mother) last week, when I observed him drinking water from a tank-fed cold tap and told them some reasons why it might be a practice they should refrain from, in the future... pipework alone was older than my grandmother and no lids on the tanks either

 

[Combicert]

In this instance

Hi All
I am changing my boiler as previous one old and struggled to cope with heating all the rads.
I have had a couple of quotes and decided to go for a valiiant combi however my dilemma is which one should I go for. One of the guys suggested the 43KW the other one says 38KW.

Feel free to shoot me down guys.

Worcester bosch training used a general rule of thumb of 1.5kw per rad plus 10% for fittings for heating side (not set in stone - but to help with calcs..)

Assuming correct incoming pressure boilers should be sized on hw output. I have a 42cdi in my house (19rads) and it heats them wonderfully... obviously having been range rated correctly to ensure it’s condensing (etc etc...). When heating on a normal cycle it’s burning about 28kw -30kw

As for 2 showers - a combi will struggle to run both at the same time.

Hope this helps OP. I personally dont think it would make much difference which boiler you went for, but if it were me i’d plump for the 43kw as it allow for any future expansion... although i dare say the 38kw will suffice.

Also - i’d add another expansion vessel... i have t read the other posts in any great detail but expect its come up from other knowledgable installers.

 

[John D v2.0]

obviously having been range rated correctly to ensure it’s condensing

I've seen this suggested a few times but never understood. Given we used to fit 10kw non condensing boilers, the heat input can't make a difference. Why not just set the flow or return temperature correctly to ensure condensing?

 

[Combicert]

I've seen this suggested a few times but never understood. Given we used to fit 10kw non condensing boilers, the heat input can't make a difference. Why not just set the flow or return temperature correctly to ensure condensing?

Six of one and half a dozen...

Yup - I take your point. Whenever i install its just something I do. Force of habit and a result of doing these worcester courses i reckon. I only ever really do worcesters on installs to be honest... i don’t mind doing ideal/baxi et al - but i guess it’s personal preference.

Amazing how many installers just whack a boiler in and walk away without doing a thing.

Hey, as long as it works and your methods are within the rules i guess it just doesn’t matter.

 

[bernardgreen]

Why not just set the flow or return temperature correctly to ensure condensing?

Because the flow temperature necessary to keep the return temperature below the dew point is invariably too low for the radiators to output enough heat.

At least that is the case when the new boiler is installed on an existing system that was designed to operate with a high flow temperature. Lower flow temperatures require larger surface areas on the radiators.

Hey, as long as it works and your methods are within the rules i guess it just doesn’t matter

Maybe it doesn't matter to the installer but it does matter to the people living in the property

 

[John D v2.0]

Because the flow temperature necessary to keep the return temperature below the dew point is invariably too low for the radiators to output enough heat.

At least that is the case when the new boiler is installed on an existing system that was designed to operate with a high flow tempe

Agree with all that, but i don't see how range rating would help condensing either way.

I assumed range rating is just to stop the boiler putting more heat into a system than it could possibly dissipate, making the boiler turn on and off, better to operate at a sensible level all the time.

 

[Combicert]

Agree with all that, but i don't see how range rating would help condensing either way.

I assumed range rating is just to stop the boiler putting more heat into a system than it could possibly dissipate, making the boiler turn on and off, better to operate at a sensible level all the time.

Surely by ensuring the amount of heat going into the system is correct - and the return on the CH is below the dew point then the boiler will condense - and be running optimally.

During winter and cold snaps many people’s condense pipe’s freeze up because their boiler is condesnsing correctly. Turn the heat right up on the ch and the boiler wont condense = no condense freeze.

More exonomical and better performance

Works for me anyway...

Someone i know likened it to a car... stick the car on a motorway and the engine is gently bobbing along with little effort... do a lot of city driving and the engine is stopping, starting, high revs, low revs...

The heating curve on a correctly programmed boiler will be much smoother and will perform to manufacturers specs.

That’s my understanding anyway... perhaps i’m wrong.

 

[John D v2.0]

Surely by ensuring the amount of heat going into the system is correct - and the return on the CH is below the dew point then the boiler will condense - and be running optimally.

Agree, spot on, the return temperature is the critical thing.ie the power input based on the current conditions of the system, air temperature etc.

During winter and cold snaps many people’s condense pipe’s freeze up because their boiler is condesnsing correctly. Turn the heat right up on the ch and the boiler wont condense = no condense freeze.

Agree but not relevant.

I see your car analogy but any analogy between cars and boilers isn't going to be instructive.
If you really want one though, that would be like saying you'll get better economy by putting some wood under the throttle to limit to half way. You can still brake at the last minute and eventually get to 70mph which is less efficient than 40mph
Better to limit the car to the average speed needed to get there in time and drive without deviation. And avoid breaking. that is basically weather compensation for boilers.