Just how much modulation on Vaillant?

[Vet Tech]

Modern condensing boilers are supposedly designed to modulate the heat output. Am I correct in thinking that this modulation depends on the temperature of the return water temperature and/or the setting on the cylinder thermostat. My query is just how accurate and efficient is this control process? Is it a stepped process or truly linear.? For example, say its a summer's day and the CH is not on so all the boiler has to do is keep the hot water cylinder at the right temperature. You are all out that day so there's no real demand for hot water. When the timer swithes it on, the boiler presumably fires up and gradually brings the water temperature back up to the temperature of the thermo stat on the cylinder. Can it modulate the gas consumption down progressivley within fine limits eg 5% or is it a fairly rough comprimise eg down to 25% until switching off when the thermostat limit is reached. Incidentally, I have a Vaillant EcoMax Pro.

VT

[Dave Plowman (News)]

It should keep the boiler running at its highest efficiency until the desired temperature is achieved then switch off.

[geoff]

In message , "Dave Plowman (News)" writes

Well, no

Think of the hysteresis

Modulation is primarily to keep the hot water constant. Being now "office bound" and not actually doing any testing myself anymore - I don't know

Vaillant have used both methods in the past, I think the EcoMax is stepped (crudely so, IIRC)

[Dave Plowman (News)]

I didn't think we were talking about a combi.

[David]

I have a Vaillant ecoTEC+ 937, when it fires, the burner is on the lowest that it can go it gradually ramps up to full power and when it is approaching the set flow temperature it modulates all the way down, you can tell tell by the noise and the pump speed which automatically switches between speed 1 and 2. Turn the CH knob higher and it will ramp up the output again until the set flow temperature is reached.

With Vaillant controls the modulation depends on the set room temperature

[geoff]

In message , "Dave Plowman (News)" writes

[Dave Plowman (News)]

[David]

The ecomax pro is a regular boiler, the ecoTEC pro is a basic combi boiler.

[geoff]

From the link

"Vaillant ecoMAX combi boiler

The ecoMAX range of condensing boilers ..."

I thought it was a combi too

[Vet Tech]

No, not a combi. It's atraditional system with a bl**dy big cylinder. It was a direct replacement for an old Netaheat system.

VT

[Ed Sirett]

The external controls are either on or off. The boiler however can modulate over a large range about 4:1. This is truly linear, and is done by changing the speed of the premix fan.

The Vaillant is not as good as some models. Mainly all the models that are 31kW or less are the same model. The only difference is how fast the control firmware will allow the fan to run. So on the 31kW the fan can go from 150-600 rpm but the top speed of the fan for the lesser models is less and the depth of modulation is reduced. Also the max power in heating mode is reduced further so that the max heating power of the 824 is only 19kW, but the minimum is still the same.

The fan can't go any slower as there is a minimum gas rate below which instability of the flame might occur. If the boiler needs to output less than ~8kW then it has to cut right out.

[Vet Tech]

Much as I'd like to, I've not seen it cut out completely yet. The flame 'icon' in the LCD display reduces in size to about half but it still sounds as though the burner is still producing heat. Or might it just be the fan on its own? Given that the pump over-run would continue for 5 minutes after the flame shut down, then there would inevitably be a modest reduction in the return water temperature which in turn would fire up the burner again. Hence causing a continuous loop. Is this a valid scenario?

Incidentally its an EcoMax Pro 18E which appears to be discontinued now. It had the circuit board and the drain tank replaced replaced after a year. Apart from that it's been OK VT

[Ed Sirett]

What does parameter d.33 say?