Vaillant EcoTec 838 and hourglass

[KamSandhu44]

Hi all,

Since having EvoHome installed, I have noticed a hourglass on my Vaillant EcoTec 838 popup a few times. I believe this is the anti cycling behaviour.

Is this something I need to worry about?

Kam.

[bruce_miranda]

Anti Cycling is OK, Short Cycling is bad. Anti Cycling is designed to avoid short cycling.

[Dan_Robinson]

This is a side effect of old burner technology and small heating zones/loads.

One of the reasons some of us have been pushing for bigger turndown ratios or more sturdy boilers.

[dty]

This is a side effect of old burner technology and small heating zones/loads.

One of the reasons some of us have been pushing for bigger turndown ratios or more sturdy boilers.

38kW boiler is pretty big. What's the minimum modulation on that?

My EcoFit Pure 625 (which goes down to 6kW) has managed to do several low-power multi-hour burns today, including 4 hours from 5:30 to 9:30 this morning. Very happy with that.

[Dan_Robinson]

A 38kW leakoTec will go down to 7kW I think. And is huge for a domestic property.

Many properties only need one or two kW's for many weeks of the year especially with room level zoning and /or compensated controls.

[bruce_miranda]

I have a 438 and it was the only thing I let my plumber decide what to put in 8 years ago and it is still the only thing I am not happy about. He just put in the biggest boiler available and his reasons were based on my current need and possible future extensions.
It has caused me no end of grief. New gas mains, new pump, new 32mm flow and returns, the list was endless. I now have been running it at 28Kw for the past 5 years just to keep it running. Even then it's the minimum burner that I am not happy with. I think with smart controls and zones even the heat loss calculations done for the whole house are outdated. Those houses are never being heated to the same temperature through out, so the max you need is a lot less than you think and suddenly the min you need matters. Also, I believe UFH and radiators should be calculated differently but the experts disagree. They say heat loss is heat loss, and I get that. Here again in UFH I think the boiler should be on low for longer rather than shorter bursts of higher than required temperatures.

My plumber just played it safe, I guess he knew big boilers can be wound down but if the boiler is too small for the house then the customer will be very unhappy. Having said that, the boiler has been extremely reliable and has just worked flawlessly.
p.s. 438 min burner is 6.5Kw as reported by my boiler. The default anticycling time is 20 mins.

[dty]

Even if you are heating the whole house to the same temperature, different radiator sizes, heat losses, etc. make it unlikely that all zones will be at 100% simultaneously. I replaced a 30kW boiler with a 25kW. My heating engineer wanted to put in an 18kW. When I pointed out that it wouldn’t cope with the morning spike, he said to just move my DHW heat up earlier. Boom - no more need for 30kW! I’m finding that my larger rooms with wall stats are coming on very early these last few mornings, but as they start to ramp down then smaller rooms come on, etc. This is helped by staggered target times too which reflect our usage. So I agree, smaller boilers are better able to cope if they’re paired with zoned controls, etc. and minimum modulation output starts to become important for efficiency.

[Dan_Robinson]

I call it the "Spinal Tap" method of sizing. Boils my pi**.

[paulockenden]

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?

And even with stored hot water (although to a lesser extent) based on how quickly you want the temp in the HW tank to recover after running a huge bath.

I've been told by several heating engineers that unless you have a massive house with just a single bathroom, DHW is the key factor when sizing the boiler.

[KamSandhu44]

Thanks for all the info.

So much misinformation flying around, when I changed me boiler last year, I was told to get the biggest boiler I could. If only I knew then, what I know now.

When I had EvoHome installed last week, the installerer changed the output to 15kW.

[bruce_miranda]

Wouldn't it be nice if the boiler output if lowered at the top would lower at the bottom end too. Sadly it doesn't! So you now have a boiler that modulates between ~7 to ~15kW, so welcome to the Anti Cycling championships.

[Dan_Robinson]

Wouldn't it be nice if the boiler output if lowered at the top would lower at the bottom end too. Sadly it doesn't! So you now have a boiler that modulates between ~7 to ~15kW, so welcome to the Anti Cycling championships.

Yes, but in the case of Faillant and their bizarre auto range rating, which only works on correctly sized/piped/pumped/separated systems, fixing d.0 stops the thing ramping up too quickly then dropping its trousers because the delta across the HEX is too great.

[KamSandhu44]

I understood zero of the last two posts.

Could someone please translate to noob speak?

Os my boiler going to die?

[dty]

Yes, but in the case of Faillant and their bizarre auto range rating, which only works on correctly sized/piped/pumped/separated systems, fixing d.0 stops the thing ramping up too quickly then dropping its trousers because the delta across the HEX is too great.

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?

[Dan_Robinson]

Wouldn't it be nice if the boiler output if lowered at the top would lower at the bottom end too. Sadly it doesn't!

It's a rather complicated thing getting burners to operate over a large range. I researched it for a while, even tried to modify my old boiler to try and get a lower minimum. Ironically, I made it slightly more powerful and, I think slightly more efficient at high outputs. Did bugger all for the low end though.

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?

And even with stored hot water (although to a lesser extent) based on how quickly you want the temp in the HW tank to recover after running a huge bath.

I've been told by several heating engineers that unless you have a massive house with just a single bathroom, DHW is the key factor when sizing the boiler.

Combination boilers are a massive compromise. Reheat times of cylinders is dependent more on the tank than the boiler. One I'm fitting today has the same official rating as the boiler. However this rating is when the cylinder is cold. As the water temp rises, the rating is effectively lowered, so the boiler modulates.

This setup has the ability to send 50+kw to hot water, but it will only do it for short periods of extreme demand.


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Heating wise, the boilers are slightly undersized.