Replacement Boiler - W/B 24Ri or 30CDi

[wetwipe]

Hi folks, newbie here.
We are installing a replacement boiler for our aged but reliable Myson Marathon 600B (17.3Kw output). We have decided on Worcester Bosch, mainly on reputation, but cannot decide which size. Our plumber reckons that the 24Ri (24Kw output) will be okay, but I'm thinking of the 30CDi (30Kw output). We live in a three storey Victorian end terrace, with 13 radiators and 1 bathroom, and no kids, just the two of us. The current Myson, which I thought was well undersized for the property, has worked well, but it struggles to heat the h/w tank and the house on winter mornings, and is then generally on its limit with heating our single glazed, suspended floored, solid walled house. On the face of it the 24Ri might be okay, but the 30CDi is only about £110 more, and for some reason weighs much more (39.5Kg against 22.6 Kg) and seems to have more electrickery inside. We also need to install at least one more radiator on our chilly ground floor, so would I be gaining performance and options with the 30CDi, or going a step too far. Your informed comments would be welcome. Thanks.

 

[Dan]

Welcome to Plumbing Forum | Plumbing Advice | Plumbers Forums | A forum for plumbers advice in the UK!

Hi wetwipe and welcome to UKPlumbersForums.co.uk!

 

[fuzzy]

The boiler only needs about 3kw to heat your hot water so 30kW is ample. do you have an s or y plan? do you have a 3 port or 2x2 port valves?

heating wise, you could always do the full house method which is free online. googl whole house method and with a bit of searching should find it. put in your approx measurements and it will tell u what output your boiler needs to be

 

[Puddle]

Hi folks, newbie here.
We are installing a replacement boiler for our aged but reliable Myson Marathon 600B (17.3Kw output). We have decided on Worcester Bosch, mainly on reputation, but cannot decide which size. Our plumber reckons that the 24Ri (24Kw output) will be okay, but I'm thinking of the 30CDi (30Kw output). We live in a three storey Victorian end terrace, with 13 radiators and 1 bathroom, and no kids, just the two of us. The current Myson, which I thought was well undersized for the property, has worked well, but it struggles to heat the h/w tank and the house on winter mornings, and is then generally on its limit with heating our single glazed, suspended floored, solid walled house. On the face of it the 24Ri might be okay, but the 30CDi is only about £110 more, and for some reason weighs much more (39.5Kg against 22.6 Kg) and seems to have more electrickery inside. We also need to install at least one more radiator on our chilly ground floor, so would I be gaining performance and options with the 30CDi, or going a step too far. Your informed comments would be welcome. Thanks.

With todays boilers as said before ,there modulation allows slight over sizing as it allows greater range rating,(30CDI ranges from 7.7 k/w to 30 k/w)you are loosing on purchase price but as stated not much.However it would be nice for more people/engineers to work out what the actual heat requirements are for a property rather than saying,that should do you
IMHO

 

[fuzzy]

With todays boilers as said before ,there modulation allows slight over sizing as it allows greater range rating,(30CDI ranges from 7.7 k/w to 30 k/w)you are loosing on purchase price but as stated not much.However it would be nice for more people/engineers to work out what the actual heat requirements are for a property rather than saying,that should do you
IMHO

i agree puddle, it will take 10 minutes to do the whole house method

would like to know if its s or y

 

[wetwipe]

Thanks wetwipe & puzzle,

The system has a 3 port valve, so I guess it's Y plan. I've just done the whole house method, took about 15 minutes and not entirely accurate due to the complicated form of our old Victorian house. The outcome is a boiler output of 25.5 kW, including only 2kW for h/w and with no additional allowance for old sash windows or draughty suspended ground floor. At this rate it looks like the 30CDi is the only option. But I have to say that I'm no great fan of generalised calculations, they can be quite misleading, as any calculation works best for elements with more accurately assessed parameters, such as heat loss in newbuild housing (and loadings on steel beams!). Still, referring back to my original question, do either of you know why the 30CDi weighs 39kG compared to 23kG of the 24Ri, and is it a good choice? Thanks

Oops, meant to say thanks Fuzzy and Puddle! It's been a hard day, and it's still only Monday!

 

[fuzzy]

i presume it weighs more beacuse of the higher rating, it may also be a different set up as opposed to simply bigger. I dont know off hand maybe another search on the web and a detail drawing will show wether they are the same or not.

You can complete heat loss calcs for each room, this will take a while and why they brought in the whole house method. Most plumbes ignore it altogther and just gues, it was brought in to stop that. If you have a older house it may be more difficult to predict but in my experience its quite accurate.
try the long method if u prefer

 

[wetwipe]

Thanks fuzzy,
Do you have any experience of the relative efficiency of condensing boilers in older properties, particularly where the radiators are not oversized, consequently running at higher temperatures, which I believe reduces the time that a boiler runs on condensing cycle. Is there any way to combat this, apart from installing larger rads, and we've already changed most of our single rads into doubles. On the heat calculation front, I guess that I'm a little concerned that the little 17 kW Myson has almost coped with our needs, and now I'm planning to replace it with a 30kW boiler, and I keep reading that condensing boilers should not be oversized. I thought that the boiler swap decision would be easy - but it ain't.

 

[oldplumber]

Personally, I wold forget worcester as they cost to much and concentrate on ensuring I had an efficient system, ie unvented cylinder all nicely insulated, additional zones and room srats to reduce heating costs, a decent programmer and put in a 28 kw remeha avanta plus boiler, which is cheaper and as good as a worcestr re guarentees, and a lot smaller and easier to work on.

Thats what will be going into my 4 story victorian pile when the current saunier duval turns its toes up, and its a 30kw combi running 2 bathrooms and 17 rads(and copes quite well,surprisingly)

 

[Blackcatgas]

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[wetwipe]

Thanks o/p,
Remeha comments found online all appear very good. Never heard of them before, and can't find any prices online for the 30v. Still concerned about the sizing of boiler and effect of average to undersized rads in leaky old pile on efficiency of boiler. I appreciate your comments on re-vamping our installation, but at the moment it's just a replacement boiler for us.

 

[oldplumber]

you shouldnt need such a large boiler if you want efficiency. Get a heating engineer to do aproper assessment of your property and not just a whole house survey to see what rads sizes are required and ultimately the correct boiler size which is where youll save money both in the short term and long term. If you just pay the guy for his time youll probably recoup the cost in spades in the long term. I turned down a job recently as the customer insisted he needed a 40kw boiler and wouldnt accept anything less, and this was only a 3 bed house with 2 bathsrooms!!, by turn down I mean I quoted £5000 to install the boiler, if he wanted it that badly, I might consider putting it in.

 

[fuzzy]

Thanks fuzzy,
Do you have any experience of the relative efficiency of condensing boilers in older properties, particularly where the radiators are not oversized, consequently running at higher temperatures, which I believe reduces the time that a boiler runs on condensing cycle. Is there any way to combat this, apart from installing larger rads, and we've already changed most of our single rads into doubles. On the heat calculation front, I guess that I'm a little concerned that the little 17 kW Myson has almost coped with our needs, and now I'm planning to replace it with a 30kW boiler, and I keep reading that condensing boilers should not be oversized. I thought that the boiler swap decision would be easy - but it ain't.

a common miscoception is that condensing boilers need larger rads to condence, THEY DONT!
The boiler will condence in relation to the return temp which needs to be about 55C. The drop across the system should be about 12C and therefore your flow should be set at about 65/67C. The rads would only need to be oversized if you wanted the rooms to heat up quicker, they will work but what most people wrongly thin is that the flow shuld be 75/80C and the rads are bigger to dump the heat so it returns cool enough to condense, WRONG