Advice on a combi boiler please.

[Mc100]

Hi all

i have been searching the internet for advice on choosing the right boiler for my needs.

i was looking at Worcester Bosch as I have always been under the impression they are a quality boiler.

i live in a 6 year old, 3 storey 4 bedroom house in the Midlands and have 12 rads, with underfloor heating ran off the heating circuit in a ensuite that is 3 meters square and plan to install underfloor heating in a 3 x 3m conservatory next year. There are two ensuites and a bathroom in the property with one electric shower and two thermostatic showers.

After doing a quick search on Worcesters site they recommended a 440 cdi. Based on this I had a quote from a registered fitter for both this and the 42cdi.

i have done the calculation as recommended somewhere on this site idhee (rest of link removed as this site wouldn't let me post) and had a result of 10.3kw.

i am trying not to overkill with a boiler as I understand a boiler that is too big turns out to be less efficient. I am also led to believe that a combi boiler would not cope with two thermostatic showers running at the same time so a very good flow rate is not as important as I first thought.

i am struggling to understand why combi boilers seem to start at 24kw when my needs seem so much lower at 10.3kw and the two boilers Worcester recommend through their site are 30kw models and then when I have researched different sites it seems that it is the amount of rads determines the boiler size.

Any advice is greatly appreciated.

 

[anz]

Your heating needs are smaller.
It's the hot water you need the additional power for in a combi Boiler.

You'll need to make sure that the boiler you choose can modulate down to your heating needs.

For your needs I'd recommend an Unvented system with a system boiler.

 

[REDSAW]

if you think plastic is fantastic then go with a worcester!.

if you dont like paying for expensive parts for when it will break down then go for the likes of baxi/main/poterton or intergas.

 

[plumber0808]

What system have you got at the moment?

For those requirements I would suggest a system boiler and unvented cylinder to cope with the hot water demand

 

[Mc100]

I was thinking Worcester as I thought they were highly recommended. I had also contemplated vaillant, are they any good? It seems Worcester may not be as good as I thought then.

i have an Ideal combi at the moment.

Would a system boiler and unvented system be as economical? I thought hot water on demand was best as no waste, am I wrong in thinking this?

 

[plumber0808]

I'm surprised that the property has 2 en suites 1 bathroom and has a combi. Especially as its only 6 years old.

If there are 2 people in your house then a combi will work but anymore than that then you have the potential of 2 hot taps/showers being used at the same time. Combis are not made for that really.

 

[Mc100]

Aha. We have converted the loft and added an ensuite.

 

[Killy Bing]

Imho an unvented would be better suited to your hot water demand.
providing you have good pressure and flow rate on the mains however that should be fine as you already have a combi?

 

[Aquariusplumb]

I agree with the other comments and would go with an unvented cylinder and system boiler. If you were to get a combi definitely look around, Worcester Bosch not the best and expensive

 

[plumber0808]

It will depend on usage, I am fitting a new boiler in a similar sized house soon, a Intergas 36/30 combi as there are just 2 people in the house so no need for over kill.

 

[REDSAW]

but then the intergas can be changed to whatever system you like in the future without replacing it!

 

[plumber0808]

Not on this job, the water pressure/flow ain't very good

 

[anz]

I think the Unvented system will definitely be more economical for your needs.

 

[Mc100]

Thanks for the replies.

what size cylinder would you recommend and what make and which boiler?

i am thinking about what to do. If I were to just straight swap combi what would be recommended there?

 

[Billy Bob Bob]

go baxi duotec combi HEA bolt this to a shower on your hot water outlet then via 2 port valves into a unvented cylinder that way you get the performance of a combi on heating but a good hot water flow everywhere else,also you will get a 7 year warranty on the boiler
BTW i repair most boilers,Worcesters are not good ones IMHO,a duotec HEA will outlive a worcester and be more reliable long term

 

[Mc100]

O.k. So I think I am looking for an unveted cylinder as I always like to take the advice of those in the know.

could anybody recommend what size cylinder I should be looking for?

 

[doitmyself]

i have done the calculation as recommended somewhere on this site idhee (rest of link removed as this site wouldn't let me post) and had a result of 10.3kw.

That means you should look for a boiler which will modulate below 5kW for heating, which will be the amount of heat required when it is 10C outside.

The Baxi HE combi will only modulate down to 6.8kW to 9.6kW, depending on size, whereas the Duotec 2 can modulate down to 3.4kW to 5.7KW, depending on size.

 

[Dotty]

O.k. So I think I am looking for an unveted cylinder as I always like to take the advice of those in the know.

could anybody recommend what size cylinder I should be looking for?

You really need someone in to size the property properly!

 

[Mc100]

As Baxi boilers seem to be the ones that are recommended I have been looking at them now in some detail and came across the Neta Tec 33 ga. is this the same as the Duo Tec but in a smaller casing as the outputs seem to be the same?

 

[Centralheatking]

In no way do you require a combination boiler your house and demands are far too large
a conventional system with a cylinder is the only system for you.

All you do going down the combi route is end up with a 40-50kw combi to heat up your large hot water demands that will not deliver what you want because the mains water supply will not do it anyway. Its a common mistake.

Just fit a conventional system with an unvented hot water store


centralheatking

Hi all

i have been searching the internet for advice on choosing the right boiler for my needs.

i was looking at Worcester Bosch as I have always been under the impression they are a quality boiler.

i live in a 6 year old, 3 storey 4 bedroom house in the Midlands and have 12 rads, with underfloor heating ran off the heating circuit in a ensuite that is 3 meters square and plan to install underfloor heating in a 3 x 3m conservatory next year. There are two ensuites and a bathroom in the property with one electric shower and two thermostatic showers.

After doing a quick search on Worcesters site they recommended a 440 cdi. Based on this I had a quote from a registered fitter for both this and the 42cdi.

i have done the calculation as recommended somewhere on this site idhee (rest of link removed as this site wouldn't let me post) and had a result of 10.3kw.

i am trying not to overkill with a boiler as I understand a boiler that is too big turns out to be less efficient. I am also led to believe that a combi boiler would not cope with two thermostatic showers running at the same time so a very good flow rate is not as important as I first thought.

i am struggling to understand why combi boilers seem to start at 24kw when my needs seem so much lower at 10.3kw and the two boilers Worcester recommend through their site are 30kw models and then when I have researched different sites it seems that it is the amount of rads determines the boiler size.

Any advice is greatly appreciated.

 

[doitmyself]

large hot water demands that will not deliver what you want because the mains water supply will not do it anyway.

Just fit a conventional system with an unvented hot water store

If the incoming mains supply is not sufficient for a combi boiler, then it also unsuitable for a mains-fed unvented cylinder. Both rely on the mains pressure and flow rate to deliver water to the taps. The only difference is that the combi has to heat the water as it is used.

 

[acecharly]

If you have a store of hot water then the idea here is that your water pressure does not matter because while the hot water is not being used the cylinder is filling to a point where your likely to reach your peak demand. Having an engineer calculate your hot water demand would mean a cylinder of appropriate size will be installed to ensure that you dont run out. Efficiency wise unvented cylinders are good many will heat up in 20 minutes and will stay warm for up to 2 days.

 

[doitmyself]

If you have a store of hot water then the idea here is that your water pressure does not matter because while the hot water is not being used the cylinder is filling to a point where your likely to reach your peak demand. Having an engineer calculate your hot water demand would mean a cylinder of appropriate size will be installed to ensure that you don't run out.

A conventional unvented HW cylinder relies on the pressure provided by the cold water tank in the loft to provide the flow of hot water from the taps. An unvented cylinder relies on the mains pressure to do the same job. So if the mains pressure is not good enough, you will not get an adequate flow from the tap.

All the unvented cylinder does is to allow you to store a quantity of water at a high temperature, which means you do not have to heat the water instantly so the boiler can have a smaller output compared to a combi.

Yes, the cylinder has to be sized correctly to ensure that there is an adequate supply of hot water.

 

[Centralheatking]

So after all this we are back to the traditional conventional boiler set up - hey

2 tanks in roof space - f& e & cold water storage and a cylinder which will be sized
to serve the property heated by a suitable sized boiler. ok some fancy controls
regarding zones etc to comply

So its really back to the early 80's for larger buildings -with new boilers etc

centralheatking

 

[doitmyself]

So after all this we are back to the traditional conventional boiler set up - hey

Only if it's not possible to get sufficient flow and pressure from the mains, e.g by upgrading the incoming pipe.

Vaillant, for example say:

To obtain optimum cylinder output, there must be a suitable cold mains inlet, i.e. the measured static line pressure must be at least 2.0 bar. A corresponding flow rate of at least 20 to 25 l/min must be available. ... The cylinder still works satisfactorily with a pressure below 2 bar but the flow rate is reduced. ... If the pressure is below 1 bar, you should not install an unvented cylinder.