Replace combi or install unvented system

[CamT]

Hi,

I'm hoping someone can help with my dilemma about replacing the current CH system.

The property is a 3 bed victorian semi in London, with a loft conversion giving 4 bedrooms. We have 1 bathroom (with bath and overhead shower) plus a shower room in the loft.

The previous owner has fiited a 24kw combi bolier which is on its last legs and needs to be replaced (and moved).

At the outside tap, water pressure is 2.5 bar and the flow is about 18L/min (bucket method).

It looks like we still have the original lead mains and 15mm pipe in the cellar.

So my questiion is whether to replace the combi with a more powerful vaillant version or go for the more expensive option of a system boiler with unvented cylinder (say 210 litres). I appreciate that with the low flow and pressure, neither option is going to give great performance but I need to replace the current bolier with something.

One suggestion is to go for the unvented cylinder option now, which would give us flexibility to improve performance by replacing the mains feed at some point in the future. In preparation for this the plumber will replace the internal 15mm feed with 22mm.

I've looked at the Vaillant 937 as an option, but think its probably too big, although if that's the best option we could probably squeeze it in.

Any thoughts or suggestions would be appreciated!

Thanks.

 

[shambolic]

Has this not been posted in several slightly different forms in the last week!
do a search for they answers

 

[doitall]

If you go for the unvented cylinder, maybe in the cellar, you will get a much better system, and more to the point, you can size the boiler much smaller to suit the heating load, which will considerably cheaper to buy and run.

However I would avoid Vaillant, their service dept sucks.

 

[bengasman]

Are you going to upgrade your incoming mains? At 18 l/min you won't see much return for the extra 1000 quid for the unvented cylinder.

 

[CamT]

If you go for the unvented cylinder, maybe in the cellar, you will get a much better system, and more to the point, you can size the boiler much smaller to suit the heating load, which will considerably cheaper to buy and run.

However I would avoid Vaillant, their service dept sucks.

Thanks for your reply. Good suggestion re siting the cylinder in the cellar, I hadn't considered that.

 

[doitall]

Quite often we used to turn the cellar into a plant room, and have a large sectional storage tank, and pump set, feeding the whole house and cylinder. Maybe a bit OTT for your needs, but London pressure is pitiful in most places, so we used to boost it to 3bar plus.

Another advantage is you can also add a water softener, and just run a raw water supply to the kitchen.

 

[CamT]

Are you going to upgrade your incoming mains? At 18 l/min you won't see much return for the extra 1000 quid for the unvented cylinder.

Yes, we do intend to do that at some point, but probably not straight away.

As long as we are no worse off, flow wise, for the next few months - I'm leaning towards the the unvented solution as a longer term investment.

Also they are in the process of upgrading all the water mains around here so fingers crossed that may improve the situation.

Thanks for your feedback.

 

[bengasman]

Also they are in the process of upgrading all the water mains around here so fingers crossed that may improve the situation.

Sadly, when the supplier renews the old street mains, they won't do your lead pipe.
If you dig a neat 1 metre deep trench, and stick a 35 mm blue pipe in there, ready for them to hook up, the will probably do it without charging you.

 

[morrismini]

18 litres a minute and the water mains to be upgraded? 1.5 baths?

The best and cheapest solution is a Remeha Avantaplus 39C combi. Do a search on it on this forum. Brilliant reports, quality and not expensive. It is selling like hot cakes.

In a borderline situation like yours, replace the maintap with a new full bore one. It is amazing how much flow and old maintap can rob. Run 22mm all the way to the combi inlet. Tee off at the maintap and run the cold taps from that. Don't put it all down one 15mm pipe. Restricters on the toilets and washing machines.

It should be fine and when they upgrade the mains supply it will sing.