Replace Combi With Viessman System Boiler?

macman

Can someone please advise on the viability of the following please?
I have a 12 year old 24kW SIME combi which has never had particularly good flow, despite adequate water pressure here.
In the next year or so I'm thinking of replacing this with a conventional boiler and adding an unvented hot water cylinder.
House is a 3 bedroomed 2 storey semi with 10 rads, 1 bathroom on 1st floor and shower-room in ground floor extension. The boiler would have to stay in the same position, in a utility room also in the g/f extension.
Options for the tank position are more complicated. The original tank and immersion heater were in the 1st floor bathroom, this is no longer an option as the bathroom has been enlarged. Other options are 1) in the loft, 2) on the ground floor in a dead end passageway-latter has the advantage that the existing pipe runs are just the other side of the wall. I would build a cupboard across this passage to conceal the tank.
Are there any advantages to putting it in the loft other than neatness and no loss of space? Major disadvantage would be extending the pipe runs above the first floor though. Will the pressure be adequate for the shower with the tank on the same floor, given that its main's pressure fed? Is there any thing else I should be considering?
The system I'm looking at currently is a Viessman Vitodens 200W System boiler-what rating would be suitable please? 26, 30 or 35kW?
The tank would be a Viessman Vitocell 200-what capacity is appropriate-I'm thinking 180 or 210 litres?
Thank you!

Pincher

No plans for solar thermal, then?

I think the boring answer will be 26kW boiler and 180litre cylinder.

Personally, I think you should have a look at an externally mounted Rinnai 16e multipoint water heater. With good pressure, it can supply two showers simultaneously. I put in a 16i, internally mounted version, and the tenants love it to pieces. The external version makes running the gas pipe incredibly easy, does not disturb the decor. You mount it on the external wall outside the shower or bathroom, where the hot water pipe connection takes the hot water to the rest of the house. The only tricky bit is the electricity for the spark plug, but it's only 80W, so you can use the lighting circuit. The external version does not need a big hole for the flue, because it's outside already!

No cylinder, no pipes from the cylinder.

Having the water heating function taken away, you can actually get away with a 15kW condensing boiler. It's more efficient this way, because the radator return will be cold, so will pick up the heat from the exhaust in condensing mode better.

macman

I've already occupied the only appropriately angled roof space with 6 solar PV panels unfortunately.

bambibashercol

Have a look at this link to help size your boiler.
http://www.sedbuk.com/

Cardew

The difficulty with sizing a boiler is often that you might need a large boiler to heat the whole house, but for many people several rooms will not be used most of the time.

There must be loads of couples with 4/5 bed detached houses whose children have now left home and so several rooms are not used the majority of the time. Even a couple in a more modest 3 bed semi might only be having radiators on in half the rooms.

Size the boiler to cope with all rooms and, as I understand it, it won't get in condensing mode when coping with half load.

Canucklehead

Hi,

I might be inclined to go with a 200 combi. This will give 14.3 l/m dhw.
You might need to upgrade the gas though as it's eating3.5 m3 of gas per hour. (on dhw) More than your existing boiler (as a 24 K/W boiler.)

With weather comp will reduce c/h bills.

Be aware that if you fit an unvented you will need to run a discharge pipe from temp/press relief valve to drain or safe place and have a continuous fall on the pipe. G3 installer will advise you.


GSR.

macman

Thanks, if I go unvented then what size cylinder should I be looking at? 180, 210 litres?
Discharge pipe is not an issue, as if the tank was on the ground floor then it will sit against an outside wall with a drain less than a foot away. I can see no good reason for loft fitting (and the cost would be much higher surely).
I will look again at the combi option, but which output are you suggesting-only the 35kW Viessman combi quotes 14.3l/min flow, I was thinking that 30kW would be enough in other respects (12.3l/min)? Existing combi is rated at 11.2l/min.

Pincher

A smaller cylinder recovers quicker from cold because there is less water to heat up, so if you want the water quick after pressing the boost button, you should choose a smaller cylinder. I heat a full cylinder around 5am, and then the heat goes towards warming the house up from 6am. By 7am, the spare capacity goes towards keeping the cylinder hot as people wake up and wash.

But like I was suggesting before, why have a cylinder at all?
Assuming a 180 litre cylinder cost £400, how much is the extra plumbing? And the blocking off the dead end corridor?
And it doesn't have to mean combis.

The Rinnai 24e has 24 ltr/min raised by 25°C, 4.4kW - 45.2kW max.
It's about £700, but you absolutely do not need a cylinder at 24litres/minute! Unlike a cylinder, you will never run out of hot water. Unlike the 16e, the 24e is condensing!, and since the input is cold mains water, it is condensing all the time! This is more efficient than using a Band A condensing boiler, which only condenses some of the time. And you don't lose heat throughoput the day like a cylinder.

Cardew

Pincher wrote: »

This is more efficient than using a Band A condensing boiler, which only condenses some of the time. And you don't lose heat throughoput the day like a cylinder.

This of course is the argument also put forward in favour of a combi.

IMO however the heating losses from a tank need to be put in perspective.

Modern tanks are tested to a British standard checking the loss in 24 hours with water at 65C, and typically that is 2kWh.

A tank heated on a timer will never have water at 65C all day so losses will be much less - 1kWh? - 1.5kWh? So 400-500kWh a year say £15?

Also, as you mention in your post, that heat is not lost most of the year as it warms the fabric of the house, and the reason why most tanks are in an airing cupboard.

Pincher

Cardew wrote: »

This of course is the argument also put forward in favour of a combi.

A combi is just a water heater squeezed into a boiler.
It's just that the schizophrenic nature of the beast makes it easy to mis-diagnose when faults occur. Of course, if you had a decent heating engineer, it's not really a problem: IF YOU HAD ONE.

I have been on a steep learning curve this year,
and looking back, I should have:

1. Put in a 15kW condensing boiler dedicated to central heating. Not even a two port valve. The only concession to complexity is weather compensation. The 30kW boiler will not condense all the time.

2. Instead of spending £1,100 on a 250 litre Megaflo cylinder, put in a £800 26i, which does 19.6 ltr/min raised by 33°C, 4.3kW - 54kW max. I only got the bloody 250litre because the plumber pointed at a table and said for the size of house that's the one I should have. It takes longer to recover when you hit boost, and we DON'T NEED that much hot water, but we still have to heat the whole cylinder to get that "quick" shower!

Cardew wrote: »

IMO however the heating losses from a tank need to be put in perspective.

Modern tanks are tested to a British standard checking the loss in 24 hours with water at 65C, and typically that is 2kWh.

A tank heated on a timer will never have water at 65C all day so losses will be much less - 1kWh? - 1.5kWh? So 400-500kWh a year say £15?

3p per kWh? You are obviously not on FixOnline 8.

Cardew wrote: »

Also, as you mention in your post, that heat is not lost most of the year as it warms the fabric of the house, and the reason why most tanks are in an airing cupboard.

Sadly, the 250 litre cylinder is in an outside boiler room,
because I didn't want any possible leaks into the dining room.
The old 210 litre cylinder lost heat like a sunday roast, and dried clothes really well.

Canucklehead

macman wrote: »

Thanks, if I go unvented then what size cylinder should I be looking at? 180, 210 litres?
Discharge pipe is not an issue, as if the tank was on the ground floor then it will sit against an outside wall with a drain less than a foot away. I can see no good reason for loft fitting (and the cost would be much higher surely).
I will look again at the combi option, but which output are you suggesting-only the 35kW Viessman combi quotes 14.3l/min flow, I was thinking that 30kW would be enough in other respects (12.3l/min)? Existing combi is rated at 11.2l/min.

Hi,

I would go with 180 litres. I got the impression the cylinder location was internal, but as it's not then that seems be a good place to fit it.

I thought a higher output combi would suit your needs as the most economical method of getting heat and hot water.

Do you get 11.2 from your 24kw combi? 9.8l/m is the best you'll get from most.


GSR