Viessmann Vitodens 100-W Compact boiler and pump overrun

[mysteryman]

The best form of control is weather compensation, and this has to be on water priority, because it can't supply the cylinder at high temp and the rads at variable temp simultaneously. A W Plan valve is simpler - it only has a motor in the head; no microswitches or electronics.

If you fit a Compact without weather comp, it will have to be set for the maximum possible flow temp to heat the cyl, and this will prevent it from condensing in central heating mode. Weather comp sorts all of this out.

 

[MJN]

The best form of control is weather compensation, and this has to be on water priority, because it can't supply the cylinder at high temp and the rads at variable temp simultaneously.

Ah of course - that fact had somehow escaped me, and it is my intention to employ weather compensation. W-plan it is.

A W Plan valve is simpler - it only has a motor in the head; no microswitches or electronics.

I hadn't picked up on that nuance either - I didn't realise it was just a plain old valve... That'll teach me to make assumptions without doing my homework!

Thanks Mysteryman and everyone else for the advice - really appreciate it.

Mathew

 

[mysteryman]

I expect that is to enable it to be connected just like an old boiler which was when powered only when heat was required.

Tony

This is not correct. These boilers require a permanent live plus a switched live plus a pump live if overrun is wanted.

 

[MJN]

The best form of control is weather compensation, and this has to be on water priority, because it can't supply the cylinder at high temp and the rads at variable temp simultaneously. A W Plan valve is simpler - it only has a motor in the head; no microswitches or electronics.

Apologies for bringing this back up but having gone away to familiarise myself with with W-plan layout/wiring I'm sure I've stumbled across a major flaw which hopefully you can correct me on.

With this schematic for reference (I know you know this inside out but it'll help me keep up) my interpretation of the wiring is that even when there is no explicit call for HW (i.e. HW timer OFF) then if there is a call for CH the boiler's output will be sent to the cylinder until the cylinder stat is satisfied. Only then will the valve be energised and water allowed to pass into the CH circuit.

This is not really my desire - I'm happy to accept that during simultaneous CH and HW demand the HW takes priority, that's fine, but heating the HW when there is no explicit demand is not really what I'm after. However, if that's the way it goes then I can live with it.

More pertinently though, and this is where I am seeing a flaw, in the above scenario there is no explicit HW demand and hence in response to the CH demand the boiler will be running in weather compensation mode. If, say, this results in the boiler's output only being 50C then the cylinder stat is never going to be satisfied and hence the CH won't get a look-in.

This can't be right, can it?

Mathew

 

[Kev F]

Your not quite understanding it,

W plan is hot water priority, when a call for hw is made boiler ramps up and loads cylinder as quick as poss, it then drops back down to weather comp mode when hw stat is satisfied, weather comp will then control temp at much lower rate than hot water, how I do it is have the water come on 1 hour before heating, if a new cylinder I personally leave hw on all day say 6am-10pm as the heat loss on new cylinders are minimal. It works very well like this as most new cylinders load up in between 15-30 mins, of you have an old cylinder just set hw to come on an hour early.

 

[MJN]

But outside of the HW demand times if the cylinder temperature drops then the CH will not function and, this is the critical bit, if using weather compensation it will never function because the cylinder won't reach the target stat temperature until the HW timer next kicks in.

Mathew

 

[DP]

The manufacturers do not insist on using a pump over run connection.

Tony where does it say that in Viessmann MI. I looked in the MI but cannot see any reference to what you say. Am putting finishing touches on this boiler hence the question

Have fitted above boiler to three zone heating, so a bypass fitted. Unvented zone has a MV as it rightly should, so fitted a bypass here too. Cannot see how the system will not be in HW priority unless timer has two contacts hence my post on this boiler

MI certainly resembles a skeleton. Needs plenty meat and flesh to be a proper MI

 

[MJN]

I think I've managed to solve the issue of CH not functioning outside of the HW time periods when using weather compensation by using the changeover contacts of the HW timer.

The modified wiring is here if anyone could give it a once over?

Conventional setup operates as follows:

1. HW demand (time + temp) = Valve sprung to HW, boiler runs at full output (not weather compensated)
2. CH demand (time + temp) and HW demand (time + temp) = HW gets priority (valve sprung to HW), boiler runs at full output (not weather compensated)
3. CH demand (time + temp) and no timed HW demand (but cylinder stat satisfied) = CH gets flow (valve activated to CH through satisfied cylinder stat), boiler runs in weather compensated mode
4. CH demand (time + temp) and no timed HW demand (and cylinder stat not satisfied) = HW gets flow (valve spring to HW) but boiler runs in weather compensation mode and may never satisfy cylinder stat hence CH won't ever get to function [This is the problem scenario]

The revised setup operates as follows:

1. HW demand (time + temp) = Valve sprung to HW, boiler runs at full output (not weather compensated)
2. CH demand (time + temp) and HW demand (time + temp) = HW gets priority (valve sprung to HW), boiler runs at full output (not weather compensated)
3. CH demand (time + temp) and no timed HW demand (but cylinder stat satisfied) = CH gets flow (valve activated to CH), boiler runs in weather compensated mode
4. CH demand (time + temp) and no timed HW demand (and cylinder stat not satisfied) = CH gets flow (valve activated to CH), boiler runs in weather compensated mode

The key benefit is proper function of scenario 4 i.e. even if the cylinder stat is not satisfied outside of timed HW demand then the CH can still function, even in weather compensated mode. An additional benefit is that the valve does not remain energised when there is no CH demand and the HW temperature demand has been satisfied (but is still within the HW time period).

Mathew

 

[MJN]

MI certainly resembles a skeleton. Needs plenty meat and flesh to be a proper MI

You're not kidding. I happened to have a quick browse of the WB1A MI's and they were far more comprehensive.

Mathew

 

[Agile]

The manufacturers do not insist on using a pump over run connection.

Tony where does it say that in Viessmann MI. I looked in the MI but cannot see any reference to what you say. Am putting finishing touches on this boiler hence the question

You have to deduce that because it does not say that you HAVE to use pump over run.

I would always connect it with over run though.

Tony

 

[transam]

Dunno abou the MI's but the Viessmann brochure page 8 states

compact heat only boiler " No pump overun required "

 

[DP]

Tony, vboiler comes fitted with 15/70 Grundos pump. It does not matter what one has to assume or deduce- system is assembled to do what it needs to do out of the box.

Transam, spoke to Viessmann when i fitted the boiler. They suggested that bypass be fitted. Perhaps when using Viessman UV cylinder, setup is different so bypass not needed as there would be no MV on the heating circuit. Book also (if I am correct) says for S or Y plan do not use WC control, but my boiler change is S plan and guy at technical said WC should be fitted.

MJN. 5 connections under the boiler (disregard condensate discharge and PRV). Middle is gas. Connections either side of that go to cylinder. I would think there will need to be a 2 port valve here, so fitted a bypass on that (also as mentioned above) and suggested same by Viessman. Two outer connections go to radiators so no need for MV here or bypass.

Boiler switches to HW mode when 240 volts applied to auxillary Viessman box (interface), low voltage side, same box connected to plugs marked 5. This tells the boiler to switch flow to 'HW' connections. Application of loop across 1 and L fires the boiler for CH or HW. To remove priority function, have been doing plenty head scratching, I think a programmer with two switches would be needed.

I am sure someone who is familiar with this boiler will correct me if I am wrong.

Tony wrote.

I would always connect it with over run though.

How would you do that on this boiler?

 

[Kev F]

God you really don't get it, if hw is off and stat is calling it won't fire, if ch is calling and hw isn't it will fire, heating is wired so when demand on heat as long as hw is satisfied/or off on timer it will fire, when hw is called for on timer it will override heating hence hw priority,

Just because hw is off and timer but hw is cold it will stil function on heating as 3 port won't get signal to cyl stat until hw timer is on, it's a very basic wiring function, check out the Honeywell diagrams, it's really not complicated, I've done quite a few

 

[Kev F]

Is this a 4 pipe system boiler or heat only?

 

[Agile]

Tony, vboiler comes fitted with 15/70 Grundos pump. It does not matter what one has to assume or deduce- system is assembled to do what it needs to do out of the box.

Tony wrote.

I would always connect it with over run though.

How would you do that on this boiler?

You seem to be discussing a system boiler.

This thread seems to be about heat only boilers where the installer does have the option to connect the pump to the boiler or not.

Tony