Vaillant Ecofit Pure Modulation Question 18kW

[Mattg4321]

Leaving aside any comments re why I want to know/is this a good boiler at least initially... Can anyone explain why the figures for nominal heat output and modulation don't seem to tally up with eachother. Interested as much as anything and can't find any info. 

 

 

[Mattg4321]

[JohnMo]

Is an 18kW boiler a good boiler is the first question?

 

Depends on you heat loss, if it's only 6kW it's too big. Min modulation is about that, so is too big.

 

Open vent boiler, do you need that or a pressurised system?

 

Then is a the specific boiler good, should be ok, in mist instances.

1 hour ago, Mattg4321 said:

nominal heat output and modulation don't seem to tally up with eachother

Modulation of any heat source will vary with output temperature. It's pretty much true with any boiler or heat pump. Flow a different temp you get a different output. Some boilers will have a hard lower limit other won't.

[SimonD]

2 hours ago, Mattg4321 said:

Leaving aside any comments re why I want to know/is this a good boiler at least initially... Can anyone explain why the figures for nominal heat output and modulation don't seem to tally up with eachother. Interested as much as anything and can't find any info. 

 

 

 

As @JohnMo suggests it's down to flow temps and latent heat when condensing. 

 

1:5 modulation ratio ain't that good. Better go with a Viessmann if modulation is important to you. Personally I'd also lean to a system boiler unless you've got particular system issues that make it impractical.

[marshian]

Just now, SimonD said:

 

As @JohnMo suggests it's down to flow temps and latent heat when condensing. 

 

1:5 modulation ratio ain't that good. Better go with a Viessmann if modulation is important to you. Personally I'd also lean to a system boiler unless you've got particular system issues that make it impractical.

 

The min outputs are very much aligned to flow temps you are running

 

My Viessmann 100-W 16 kW Heat only (Open Vented) is rated at 2.9 at 80/60 but that increases to 3.2 at 50/30

 

I'm running it at max 32/25 at -2.5 OAT - it's range rated to absolute min for CH and my data gathering tells me that clearly it's running at 4.0 output under those temps which is perfect for me because house heat loss is a smidge under 4.0kW

 

So in a nutshell the modulation ratio is actually 4.0 but it doesn't really matter to me as it's sized to meet the heat loss of the house with a max of 16 kW which I need to heat HW tank in a reasonable time frame.

 

Ignore the yellow highlighting I used the image elsewhere (Highlighting the importance of knowing what your boiler needs to function properly)

 

 

 

[JohnMo]

11 minutes ago, SimonD said:

Better go with a Viessmann if modulation is important to you.

Adding to what @SimonD says above - no matter what boiler you go with, also steer clear of boiler that will only do S and Y plan and get one that does  priority domestic hot water (PDHW). This will allow you to run heating a more low and slow and balls out doing cylinder heating - so two different flow temperatures for heating and DHW.

 

While your at it, if replacing the cylinder do a heat pump cylinder, the large coil in a heat pump cylinder give really quick reheats of the cylinder compared to a normal gas boiler cylinder or do a much lower flow temperature from boiler for best efficiency.

 

A decent system boiler will also give you variable circulation pump speed so boiler can better adapt automatically to changes in flow requirements.

[Mattg4321]

Thanks guys, I think that explains it.

 

It was mainly through interest. Nothing is really wrong.

 

I'm not planning to change the boiler as it's not very old. System is sealed with expansion vessel and pump in airing cupboard - so it effectively works as a system boiler, just the EV and pump are outside the boiler, which is actually preferable imo.

 

I will be swapping out the existing controls for Vaillant and running PDHW when I get a spare minute