Viessmann system boiler 100W or 200W

[MNW67]

The installer recommeded fitting a 19kw boiler to take into account the possibility of a new unvented cylinder in the future. My Mears Calculator tells me the (14) radiators are rated at a total of 15kw plus 2kw for the existing cylinder so that would suggest a 16 kw boiler may well suffice but his preference for a 19kw may well be prudent. I gather it can be range rated and it does have that very low modulation figure.

I had a Viessmann installer around recently and I was also very impressed.

Basing your boiler size on the radiator output is not the best way to do it. Often the actual heat loss from your house will be less than half the combined radiator output. It might be irrelevant in practice, because all Viessmann's 100 models seem to modulate down to the same output. But what you really need is a heat loss calculation. Did the installer do one? They are supposed to. It's very unusual for a new boiler requirement to be higher than one from 30 years ago. Roughly what size and type of house is it?

 

[Mr.B]

He didn't do one. He accepted the calculations I did using the Mears Calculator.

It's a four bed detached house built in 1996. Walls insulated and loft insulated.to a pretty good depth. Double glazed throughout.

It's a warm house and there's only two of us so not all the bedroom rads are on and only half the rads in the house get used and none of them are blazing hot (TRVs nipped down)
I think he might have been influenced by the possibility of fitting a 210 litre unvented cylinder at some future point and, as mentioned, he said he can range rate it.

 

[desktop987456]

Basing your boiler size on the radiator output is not the best way to do it

Couldn't agree more.
I have a double storey 4 bed + 4 baths + Lounge Upstairs (650sq.ft with a fancoil unit) and a Lounge downstairs. We heat our bedrooms (1200mm rads) , baths and Lounge (1200mm rads) mostly during day time. The heatloss of my property (which I did it myself in Excel) turned out to be just under 12 kw.

My T50 Rads (10 total + fan coil) came out to be 17 kw. I went with a 13kw Viessmann heat only boiler OV with external pump and is being used only for Central Heating (no DHW). I have set the boiler's flow temp@50 and room stat@19 and it has heated the house fine this winter. My house is not as insulated as yours (no double glazed windows) with all single glazed windows.

I am not in UK but January being the coldest month the temperature remains in single digit when boiler is in operation.
+1 for Viessmann on simiplicity and design. It is very quiet, flawless and most importantly economical. At some point I will also get weather compensation fitted to it. I have read that Tado works really well with Viessmann though I have no experience of it.

I had space constraint where I wanted to install the boiler. The actual static head required for boiler to work is 1m as opposed to what is stated in manual, I got confirmation from Viessmann UK and is working brilliantly at 1m head. You can search for my posts on OV installation here. Your installation will be done by professional so it should be least of your worries.

This year I plan to connect a hot water cylinder with the boiler. I have read that 3kw should be plenty for DHW. I am weighing my options whether I should go for PDHW or S plan. More on that as I make progress with it during the year.

Hope this helps.

 

[Notch7]

But it's not a heat only is it? Vogues are either System or Combi and I've said I want a heat only. But see below for my acceptance of the second paragraph regarding pressurizing. .... I'm sure you're right in what you say there.

BUT, if I was to go pressurized you might well say "get a Vogue System boiler" and you'd be partly right except I prefer to have separate pump and expansion vessel (rightly or wrongly

It does seem a bit odd to me that Ideal don’t just do stainless hex across the board.

I wasn’t particularly recommending Ideal, only pointing they have the criteria you want in a system boiler

My main doubt concerns pressurizing the system and the possible leaks thereafter. He said it's very rare, and it may well be, but it still scares me.

It’s a valid concern, but I just replaced a 45 year old open vented heating system, I had to take up lots of floorboards to do radiator changes and some of the old central heating pipework wasn’t great, but I’ve the new system in place for a couple of weeks and it holds fine at 1.2 bar

If you have soldered copper pipework in conventional 22mm and 15mm, I would say go for a fully sealed pumped system.

Open vented is more likely to allow air into the system, which leads to corrosion and sludge, I think modern boilers are better on fully pumped.

If you are considering an unvented cylinder then the hot water side would be pressurised to over 2 bar, but heating is only pressurised to just over 1 bar.

But the debate regarding when to replace is acknowledged

I replaced a 45 year old Stelrad Ideal boiler, my old plumber who retired a few years ago used to say: its a very simple boiler with hardly any parts to go wrong and a cast iron hex, he was right but the cast iron hex eventually started leaking, hence our boiler breaking down in mid Jan when temps outside were -3c.

If you do replace, I recommend you make sure you go for a system setup that allows the boiler to run at low flow temperatures to make best use of efficiency. In that respect, Viesseman, Intergas, Vokera and Vaillant all have that designed into the boiler.

I used to have the heating set for shortish time morning and evening on a system with typical flow around 70

Now I have the new boiler setup timed from 5.30am to midday, then 4.00pm to 11.00pm and I’m finding the weather comp is setting the flow at around 44c return is around 38 - 40, but the house stays at an even temp, the rads are just slightly above warm and the house feels comfortably warm. Early indications show gas usage is around £2.80 per day (admittedly weather is hovering around 7-11 deg mostly).


If you are changing a working functional boiler albeit old, It’s worth getting the extra benefit of improved comfort levels from low flow temps as part of the upgrade. But you will probably need existing controls changed you probably have S or Y plan



The only thing I’m finding is the boiler can’t modulate down low enough, maybe because of the boiler, partly due to rads not being big enough to operate at really low flow temps.

 

[Mr.B]

Very much appreciate the above replies.

I must apologise for not reiterating, it's a microbore system. 22mm copper from boiler to the manifold sited underfloor in the first floor landing, and 10mm soldered copper to each rad. Although I've not had the first floor chipboard sheets up so am unsure if every feed to every rad leaves the manifold in 10mm or if they've used any 15mm , but certainly all the drops in the walls (dot and dab plasterboard) are in 10mm.
Like I say, individual drops down to the downstairs rads as well as individual 10mm feeds to the upstairs rads are definitely all 10mm.

 

[MNW67]

It does seem a bit odd to me that Ideal don’t just do stainless hex across the board.

Vaillant stopped doing stainless steel open vent as well a few years ago. I think now it is just Viessmann and Alpha. I think @Gasguru posted something which might help explain it earlier in this thread. I think he was saying that the SS HX used by several manufacturers (Vaillant, Ideal, Baxi?) is very tight and easily gets blocked. I can see that might mean it is not a good idea for open vent which apparently is more prone to sludge. Also, I remember another poster also said there was a potential issue with some SS HX with venting and that meant they weren't suitable for open vent. Admittedly, I didn't actually understand that point! The Viessmann SS HX claims to be more free flowing.

 

[Mr.B]

Something made me think that the Viessmann HEX design was a continuous coil of round tubing (a very poor description,I'm sure) and appeared to be free flowing. I may well be wrong. I'm not committed as we speak and am all ears.

 

[Notch7]

Vaillant stopped doing stainless steel open vent as well a few years ago. I think now it is just Viessmann and Alpha. I think @Gasguru posted something which might help explain it earlier in this thread. I think he was saying that the SS HX used by several manufacturers (Vaillant, Ideal, Baxi?) is very tight and easily gets blocked. I can see that might mean it is not a good idea for open vent which apparently is more prone to sludge. Also, I remember another poster also said there was a potential issue with some SS HX with venting and that meant they weren't suitable for open vent. Admittedly, I didn't actually understand that point! The Viessmann SS HX claims to be more free flowing.

that makes a lot of sense

open vented is rather old technology really, fully pumped, pressurised does make sense

 

[Notch7]

Very much appreciate the above replies.

I must apologise for not reiterating, it's a microbore system. 22mm copper from boiler to the manifold sited underfloor in the first floor landing, and 10mm soldered copper to each rad. Although I've not had the first floor chipboard sheets up so am unsure if every feed to every rad leaves the manifold in 10mm or if they've used any 15mm , but certainly all the drops in the walls (dot and dab plasterboard) are in 10mm.
Like I say, individual drops down to the downstairs rads as well as individual 10mm feeds to the upstairs rads are definitely all 10mm.

I can understand your concern, although Ive no idea if 10mm microbore heating systems are more prone to leaks

it does sound like its properly configured so probably has just 1 radiator connected to the manifold in 10mm

I always wonder if there was ever any cost benefit to microbore.

 

[MNW67]

Something made me think that the Viessmann HEX design was a continuous coil of round tubing (a very poor description,I'm sure) and appeared to be free flowing. I may well be wrong. I'm not committed as we speak and am all ears.

That's certainly what it looks like. Whether it's more complex inside is above my pay grade!

I think all SS HX are coils. And then a radial burner, attached to the combustion chamber door, sits inside them.

 

[Mr.B]

Pain in the behind. Although easy to manipulate.
For the first ten years of it's life the system was neglected and sludged up but for the past 18 Ive cleaned it and dosed it up and the water is now clear.

 

[Mr.B]

All received re. S/S HEX, ta.
The water here is super soft and free from limescale.

 

[MNW67]

All received re. S/S HEX, ta.
The water here is super soft and free from limescale.

Your Viessmann guy will be able to explain it a lot better than me. I'm not trying to put you off. They definitely claim to use a wider coil.

I am also planning to stay with open vent and that is why I quite like the look of Intergas. Their heat exchanger is unique and has wide copper waterways. But two things put me off a bit. The modulation on the open vent model is a bit rubbish. Also, there is only a relatively small installer base.

 

[Mr.B]

The two things that put me off too.
Although I was dead set on one a while ago and even went to the lengths of making up a timber back board with fire proof facing sheet on which to mount the boiler and would have allowed the pipework to pass behind the boiler.

 

[desktop987456]

I am a home user and not a pro and would like to clarify as a user that I did check with Viessmann UK and they confirmed and it is written in MI as well that OV boiler can be used in a pressurised or seal system too.