Combi in a 2/3 bathroom house?

[outmyway]

I'm considering changing my Kingfisher2 RS80 boiler and was thinking about a combi.

I had a Combi in my last house but found running more than one hot tap reduced the flow considerably ...

My friends have just had a Worcester Bosch Greenstar 42CDi fitted
http://www.worcester-bosch.co.uk/homeowner/products/lpg-boilers/greenstar-42cdi in there 2/3 bathroom house and say they dont have a problem running two showers.

Is this becuase its a high rated boiler, or because it has a 'Keep hot facility - instantly availablefor hot water' - is that a small storage pressure cylinder for hot water?

Or should I go for a pressuised water cylnder and new boiler?

 

[tripty]

Hi,

You need to check your incoming mains pressure and flow rate, if its very good and you get a *huge* combi then it might work.

Unvented cylinders are much better for 2+ bathroom houses, imho, but again you need sufficient pressure and flowrate. In terms of efficiency there isn't much between a combi and cylinder because the combi would have to be so huge compared to the boiler needed for the cylinder.

So, check your mains and then you know what systems you have to choose between.

Hope that helps

 

[Elkato531]

I recently put a WB 42CDi combi in a bugalow conversion which had a family bathroom and 2 en-suites and it works ok. But and its a big but, it had a new 25mm supply pipe, 6 bar standing pressure and a flow rate that swamped my flow cup. Even given all this, I would have still fitted an unvented cylinder if I'd had the space.

In my opinion this property was overstocked with bathrooms and it was unlikely that the system would be overstretched very often. If it had been a big house where 3 bathrooms would be getting full use I would not have gone for a combi.

 

[CharteredEngineer]

As far as Combi's go and big ones at that, I would only recommend one, Remeha Avanta 39C.

If like the other two posters have pointed out, you have the pressure and a decent flow rate, this is a very good piece of kit.

If you want to go down the unvented cylinder route, I would suggest you get something like a Viessman vitodens 200 and Vitocell Cylinder.

You will however be limited to 3 bar.

CE

 

[Agile]

I recently put a WB 42CDi combi in a bugalow conversion which had a family bathroom and 2 en-suites and it works ok.

How can it "work OK" when the boiler can only give 16 li/min which is only just over 5 li/min per bathroom?

I expect you mean that there is only one old couple there and they never use three showers at the same time.

Its not so much the number of showers as the number of people!

Tony

 

[outmyway]

Thanks for the replies ... your putting me off going down the Combi route!

A local plumbing firm recomended a Glowwork System boiler, where an unvented cylinder is built in with very quick heat up times, and has a combi backup ....... on paper it looks sensible ... they claim to have fitted them before ...
http://www.glow-worm.co.uk/products/system-store-boilers/ultrapower-sxi/

Currently my boiler is in the garage and the cylinder upstairs in the airing cuboard. I want to reclaim most of the airing cuboard space into the adjoining bathroom, but could have just an unvented cylinder in there ...

It seems simpler to put both in the garage where its easier to run a 22mm mains to, and use the existing 28mm coil feed from garage to airing cuboard o connect up the hot water from a new boiler/unvented in the garage into the existing hot water pipework ...

 

[Elkato531]

If you read my whole post Tony, I think its clear that the combi was a considered compromise based on the property, and likely usage and that, ideally I'd have fitted an unvented. I even said that in a larger house it would have been an unvented without question

In actual fact, once cold water is mixed for the showers, two running together give perfectly acceptable simultaneous performance and three running together are still better than many gravity showers I've come across. I also ran 22mm rising main up to the T for the boiler so that cold draw off for flushing toilets, washing machines etc would have minimal effect on DHW flow.

All things considered, it works "ok"

 

[muggles]

The Ultrapower is a good setup, provided you have the mains flow rate it'll out-perform any combi out there, go for the 170L and that should do you just fine.

 

[ianniann]

The "Keep hot facility" is simply a mechanism to keep the heat exchanger continuously hot so that hot water can start to be provided more quickly. It won't actually provide any larger amount of hot water. For that you simply need a more powerful boiler. Or of course a tank full of hot water

I think you should choose the solution that best suits your lifestyle. No point choosing a different approach because you think it might save you a tiny amount of gas, but then having to organise your lives around when hot water is available to you. If that's a combi boiler with unlimited water available but only at a rate determined by the size of the boiler, then go for it. If you need very high rates of hot water flow for short periods then a tank may be the best way. Have you looked into solutions that combine a combi boiler with a small-ish hot water tank?

There are combi boilers available that will provide almost any amount of hot water you might want. The 42CDi mentioned will manage two reasonable showers, although there won't be anything left over if someone else turns on a tap. Alternatively, with a suitably intelligent shower, you will be safe having a single shower even if someone decides to run a hot tap (or a cold tap!). Beyond that, Worcester have a range of Highflow boilers that can provide even more hot water than that. I assume that other boiler manufacturers have similar offerings. The question is whether you want to purchase such a powerful boiler when you only need about a tenth of that power to heat your home.

Also consider whether you want the expense and inconvenience of switching from the system layout you have now to one that involves pipes running in completely different directions, all for no real gain other than combi boilers being fashionable.

 

[outmyway]

I'm a bit confused Re. the Glowworm Ultrapower boilers ....

If the water is OFF and the built in cylinder cold - does it work as a combi?

Does it recharge the cylinder when the water water is ON via a coil or via a combi outlet into it?

 

[muggles]

IIRC there is no coil, it's a direct connection to the cylinder (so yes, in basic terms the cylinder is heated by the combi outlet). The cylinder part acts like a normal cylinder, IE it has to have hot water in it in order for you to get hot water at the taps. The boiler fires as soon as you draw off water, the same as a normal combi, but it's feeding hot water into the cylinder, and it's from here that the water is drawn off. If the cylinder is cold, you'll get cold water out of your hot tap.

 

[outmyway]

Thanks muggles - So potentially if the house has been empty for a few days, I would have to run off 170lt of cold water before seeing hot appear out of the taps? Or is the feed off from the tank near the hot fill feed?

It seems a good system for a house that has a lot people living in it, to keep the cylinder always topped up with hot water? But not so good for occasional hot water usage?

 

[outmyway]

I'm edging towards getting a Worcester Bosch Highflow 550CDi.

I've had combi's before in smaller houses and like the 'on tap' principle of hot water ...

Search around it seems the Highflow boilers will work well with 2 bathrooms and have ample for the CH.

 

[Agile]

You are talking about combis and unventeds but have you measured the dynamic flow rate into your house yet?

Tony

 

[outmyway]

Tony - No I or any of the two installers I have had around have measured the dynamic flow rate ...

Although my neighbours have a megaflow and the pressure is good for them, and I used to live houses away with a combi ....