New boiler - Viessman or Vailliant

[u587162]

Dan,
Just visited my local plumbing merchant who had an ACV on demo/display.....on the face of it, seems to look good. Is it true what they say about the better performance i.e. heats up faster etc. How much more would this cost than say one of the other makes of a similar 250 litre size?

In one of your earlier posts to me about 3-4 months ago we were talking about these systems. It didn't occur to me at that time (wasn't explained) that mega flows work on the principle that water from the boiler heats the water inside the mega flow. Nor did I realise that mains pressure is what pumps the water around the house, I thought the storage tank system pumped it electrically and also though the system filled up with hot water directly from the boiler. How is it that this is more efficient compared to the old style cylinders that took a direct feed from the boiler?

 

[Dan Robinson]

You still don't seem to understand how it all works. But never mind for now.


The ACV is not more efficient, it is higher performing providing you have a huge heat input. Which usually is done at the detriment of heating efficiency. But as we haven't seen the house we can't judge.

I put one in recently as the house had huge hot water demands but the plant room was too small to house two cylinders.

This cylinder could absorb 70kw from the boiler, which had an input if 140kw.

Whether it is the best option for you depends on which boiler (and capacity) you have selected and the control system. As well as the head room.

Having too big a boiler makes the heating less efficient. Running the wrong boiler at maximum capacity on an ACV can kill the boiler prematurely.

In my workshop I have a boiler that was too big for the heating load of the house and half the rating of the ACV. The stress on the heat exchanger was too much.

 

[dilalio]

Eh? (In response to u587162)

Water in a megaflo (unvented) cylinder is heated the same way as a conventional (vented) cylinder... Indirectly via a coil inside - through which, hot water from the boiler circulates. The water in the cylinder never mixes with the water from the boiler but is warmed via heat exchange.

The difference between vented and unvented cylinders is that the former is replenished with water from a cold water storage tank above with a vent pipe to allow heated water to expand, whilst the later is replenished with water from the incoming main... Hence the need for unvented cylinders to be constructed to take more pressure and have safety devices to release pressure when required.

 

[Dan Robinson]

It is not very clear, but the OP is possibly running off a booster set and break tank.

If I were he/she, I would be quizzing the builders sub contractor more than an internet forum about this. He is on a fixed price contract and the builder is not going to be banging in ACV's on Viessmann 200's with Divicons and full monty controls.

He's going to end up with a Pair of Stelflows on a 4 series Bunny and basic S plan+. Probably with a grossly undersized pump.

When we do our big contracts, the builder is giving the price at the tender stage for the right system and he prices that in to his build cost. Or if it is a more basic build, he knows what we have done/charged in the past and takes it into consideration (usually after checking with us quickly over the phone with a quick discussion on how similar the current project is with project x or y).

 

[u587162]

Eh? (In response to u587162)

Water in a megaflo (unvented) cylinder is heated the same way as a conventional (vented) cylinder... Indirectly via a coil inside - through which, hot water from the boiler circulates. The water in the cylinder never mixes with the water from the boiler but is warmed via heat exchange.

The difference between vented and unvented cylinders is that the former is replenished with water from a cold water storage tank above with a vent pipe to allow heated water to expand, whilst the later is replenished with water from the incoming main... Hence the need for unvented cylinders to be constructed to take more pressure and have safety devices to release pressure when required.

Hi
Yes you have described what I was trying to perhaps (badly) say above. Put prior to visiting a plumbing merchant I wasn't aware of that.

At present there is no plumbing in the house - it has all been removed.

I'm using this forum mainly for my own learning because there are helpful people like you guys explaining this stuff and not to advise my contractor what someone else has said on a forum. ...although I might challenge them with choice of selection.