Vaillant Ecotech Plus No Heating when hot water is on

[fixitflav]

No, the compensation has nothing what so ever to do with the mid-position, no relationship at all.

We're all agreed that if there is weather compensation you cannot have CH and HW simultaneously. So if you have weather compensation retrofitted to a system with a mid-position valve, something must tell the valve it cannot go to mid position. If it's a new system including weather compensation, it would be illogical to fit a mid-position valve as as its functionality is not used, and an either/or valve is (probably) cheaper and more reliable.

The compensation just helps the boiler decide how much heat it needs to produce and when, to just meet your set room temperature, with no wasteful overshoot.

That's right, and it's why you cannot have CH and HW simultaneously, as the reduced boiler flow temperature could be too low for HW.

Rather than burning flat out until the stat is satisfied, it approaches the set temperature knowing to ramp the output gradually down as the set temperature is approached.

Once at temperature, it just gently maintains that temperature, with no wild swings or creaking of pipes as the heating cuts in and out. All because the boiler has feedback rather than the boiler being simply on or off.

This is somewhat off the subject. Weather compensation also works with on/off control of the boiler, just that the control-stat setting is variable. Modulation has some benefits, but I'm not convinced as great as claimed. I have a on/off boiler, and in a heating cycle (room-stat calling) it never reaches control-stat setting, so unless modulation started at quite a low temperature, it wouldn't modulate anyway. It goes on/off several times in a HW cycle. And I don't get any creaking.

Yes, the only thing you cannot have, is both at exactly the same time. The boiler serves and satisfies the HW needs first, then the CH last.

What I meant was - can you have CH without HW being called for by the programmer, and the cylinder stat satisfied? You can on Y-plan (without weather compensation, maybe not with) but not on W-plan, which I have.

 

[Harry Bloomfield]

It goes on/off several times in a HW cycle. And I don't get any creaking.

Nor did/do I, when it is attending to heating the HW, because the pipework is much shorter. I do get the creaking though, when the heating first comes on because those pipes are all over the house, once the temperature has settled, no more creaking.

I am seeing some economy, but a big point is the temperature stability, there is no obvious heating on, heating off once it gets the place to temperature - no overshoot, no undershoot.

What I meant was - can you have CH without HW being called for by the programmer, and the cylinder stat satisfied?

I thought I had made that point clear - yes, HW and CH are entirely independent. I just cannot have CH, when there is a demand for HW - otherwise, if it tried to heat HW when CH was needed, the much cooler flow for CH would cool the cylinder down.

 

[fixitflav]

Nor did/do I, when it is attending to heating the HW, because the pipework is much shorter.

I don't get it in CH mode either, though I must admit in an previous house there were expansion noises on boiler start.

No, the compensation has nothing what so ever to do with the mid-position, no relationship at all.

We're all agreed that if there is weather compensation you cannot have CH and HW simultaneously. So if you have weather compensation retrofitted to a system with a mid-position valve, something must tell the valve it cannot go to mid position.

Any comment?

yes, HW and CH are entirely independent.

OK thanks, that's clear now, it wasn't before.

 

[Harry Bloomfield]

fixitflav said: ↑
We're all agreed that if there is weather compensation you cannot have CH and HW simultaneously. So if you have weather compensation retrofitted to a system with a mid-position valve, something must tell the valve it cannot go to mid position.

Any comment?

It doesn't need to be told not to go to mid position, the VR 66 simply doesn't instruct the valve to go to the mid position. The VR 66 sits in the airing cupboard, connected to the boiler via EBUS and the boiler connects to the VRC 470f programmer / indoor sensor and the outdoor sensor wirelessly.

 

[fixitflav]

It doesn't need to be told not to go to mid position, the VR 66 simply doesn't instruct the valve to go to the mid position. The VR 66 sits in the airing cupboard, connected to the boiler via EBUS and the boiler connects to the VRC 470f programmer / indoor sensor and the outdoor sensor wirelessly.

OK, maybe it's semantics whether being told not to go to mid position is different from not being instructed to go to the mid position! I just meant that with Y-plan without weather compensation the valve goes to mid position when CH and HW are both calling, but that can't happen with weather compensation, so something must prevent it.
Anyway, I think we've kicked this to death, thanks for the discussion