Vaillant EcoTec 838 and hourglass

[mtmcgavock]

My 25kW boiler is set to "auto". 25kW remains about the right size until I do something about some poor insulation. Should I leave it at auto or set it to 25kW? Is there a difference?

Leave on auto and the boiler will adapt it's load to the current demand.

[Dan_Robinson]

Leave on auto and the boiler will adapt it's load to the current demand.

Phahahahahahahahahahahahahahahahahahahahahahaha

[Dan_Robinson]

Sorry..... But every one I see in the wild needs d.0 setting manually to get it to fire properly/run sensibly.

[mtmcgavock]

For a combi, isn't boiler sizing more about DHW than CH? How many simultaneous showers, baths running, etc. you need to be able to cope with?

Yes, so the boiler in question might not be oversized at all depending on the hot water demand. But as Dan says combis are a big compromise, but if you don't have the space and want a decent shower then you need a large combi. In my opinion really with todays energy saving regs, large combis should be banned.

A system/heat only boiler for a standard sized house in 38kw is just ridiculous. I have a 4 bed house and mine only outputs 12kw and that's fine.

[mtmcgavock]

Sorry..... But every one I see in the wild needs d.0 setting manually to get it to fire properly/run sensibly.

Sorry but we fit loads of Vaillants, along with smart zoning and haven't had any issues with the above setting on Auto.

[Dan_Robinson]

Sorry but we fit loads of Vaillants, along with smart zoning and haven't had any issues with the above setting on Auto.

I assume that you fit them properly.... I suppose that those I inherit are by definition, from customers who are unhappy with the installer.

[dty]

A system/heat only boiler for a standard sized house in 38kw is just ridiculous. I have a 4 bed house and mine only outputs 12kw and that's fine.

I have 6 bedrooms, 7 reception rooms, a mostly uninsulated basement, 24 radiators and almost 4,000 sqft, and so far my 25kW (installed in May) has been OK. The last couple of days I've had some very long low-power burns, including 3.5 hours today when it was burning between 6kW and 8kW and condensing the whole time. Once I've properly sorted the insulation in the basement, and put decent thermal performance doors between the kitchen and the conservatory, I reckon my heating engineer's recommendation of 18kW would be just fine too. But right now, it's just a bit too small.

[bruce_miranda]

Vaillant have actually done well to even introduce the "auto". In my 438 there is no auto. Also the old 438 PCBs had a problem in that even if you derated the boiler at d.0 it would still ramp up to the max before starting to modulate downwards. Now with a new PCB the boiler only ramps up the value set in d.0 to begin with and then modulates down from there, often never needing to go back up to what is set in d.0.

[bruce_miranda]

3.5 hours today when it was burning between 6kW and 8kW and condensing the whole time.

How do you tell if the boiler is condensing or not?

[dty]

How do you tell if the boiler is condensing or not?

Flame on, return temp < 55. It's an approximation, I guess.

[mtmcgavock]

I assume that you fit them properly.... I suppose that those I inherit are by definition, from customers who are unhappy with the installer.

Yes we certainly do, for you to think otherwise is a bit of an insult. However the jobs that we see completed by other 'installers' I can see why you can jump to such a conclusion.

[bruce_miranda]

I have had my install looked at by at least 5 people, all of them said that the installation was top quality and couldn't be faulted. Everything was as per the book, correct size expansion vessels etc etc. Except that the boiler was too big for my needs. I guess there are good plumbers and good heat engineers. Add on to that the current need to be energy efficient too, and that's where the old plumbers needs to up their game and skill to keep their knowledge current.

[Dan_Robinson]

Vaillant have actually done well to even introduce the "auto". In my 438 there is no auto. Also the old 438 PCBs had a problem in that even if you derated the boiler at d.0 it would still ramp up to the max before starting to modulate downwards. Now with a new PCB the boiler only ramps up the value set in d.0 to begin with and then modulates down from there, often never needing to go back up to what is set in d.0.

It's hardly rocket science. A simple ramp logic based on flow and return temperature is all that's needed.

Flame on, return temp < 55. It's an approximation, I guess.

Broadly speaking yes, but that 55 degrees is not a fixed value. One also needs to consider how much of the HEX is below 55. I size my systems to run at 60 degree flow. If ufh only then 50. More or less 100%of the HEX surface area is condensing... Not just the first 1 or 2 coils of the HEX.

Yes we certainly do, for you to think otherwise is a bit of an insult. However the jobs that we see completed by other 'installers' I can see why you can jump to such a conclusion.

It was not meant as an insult by any means. One would assume a professional installer posting on this forum would know where he's at

[bruce_miranda]

I still don't understand why the boiler or the installation doesn't have a flow gauge, that too is such an important factor in efficiency. Some modern pumps have them though.

[dty]

I still don't understand why the boiler or the installation doesn't have a flow gauge, that too is such an important factor in efficiency. Some modern pumps have them though.

You can always calculate it from your dT and current power output.