Starting planning the renovations on a house we’ve bought and it needs a new heating system.
One plumber recommended a combi boiler (Worcester Bosch 38CDI) but my uncle (who does a lot of renovations) has recommended a system boiler (Worcester Bosch i30).
Based on the ‘bolier chooser’ on the WB website, it recommends a Greenstar 35CDi Classic..

Could someone explain the difference in simple terms?
It’s also been recommended to get a mains pressure water tank..

For reference, the house is a 4 bed, 2 bath (+ w/c) approx 1900sqft.
Ta

Combis just work and don’t take up cupboard space all over the house.

Separate tanks are more amenable to taking heat from hot water solar panels or immersion heaters (possibly from PV solar too), but really only if the whole system is designed for it.

System boiler you’ll need an unvented hot water cylinder somewhere. I guess for your size property one around 210L would be plenty.

Depends on your budget. An unvented system will cost more (around double in materials) than the 38CDi and will only be any good if your mains water pressure is up to it.

It comes down to the shower really. If you can live with it going cold occasionally when someone uses the other bathroom or runs the washing machine (or you can prevent them doing that while you are in there) then the combi will be simpler, cheaper and more efficient. If that is a deal breaker then you need a system with a hot water tank.

There are 3 females in the house! 2 of them are under 6 but, going forward, they’ll be wanting to shower/bath/preen themselves all at the same time 🙄

In which case….

Huuuge tank of water, massive boiler and solar panels, each bathroom with good extractor fans and heated mirrors…

😆

2 of the bedrooms have these in (which will be renovated and kept for the girls):

2 of the bedrooms have these in (which will be renovated and kept for the girls):

You’ll still be queuing for the bathroom for most of your 50s. 🙂

I’d go for a system boiler with a biggish unvented tank, if you’ve got somewhere to put it.

Mains pressure showering is great, too.

It’s like the 70s, 80s, 90s and 00s never happened 😯

More pics, what’s the kitchen like? – I wish i had some of ours when we moved it, i half expected Fanny Craddock to walk in any moment

Stevie I had the same choice.

We ended with combi (biggest bosch one – maybe what you said). House is 5 bed, 2 showers.

The main thing that swung it was that our water pressure was not very high, so maybe we could not have 2 showers at the same time running – we certainly can’t now.

Wish we had gone with pressurised system.

Also hot water seems to take forever to flow through – partly I think it is the pipe run and partly the combi affect.

There are 3 females in the house! 2 of them are under 6 but, going forward, they’ll be wanting to shower/bath/preen themselves all at the same time

I’d go for a system boiler with a biggish unvented tank, if you’ve got somewhere to put it.

If you haven’t got anywhere to put it, box off your half of the bedroom and put it there…… and then start making preparations to move into your shed.

TOJV, who also has 3 females in his house but substantially older than 6

Just renovated my gaff and went for a non vented system, very pleased with it so far. It’s great having mains pressure on every tap and water outlet. Loos fill up really fast compared to my old sclerotic vented gravity system.

It is more inefficent than a combi which just heats the water you use, but having a 250l tank full of hot water means I’ve never run out and able to have two mains pressure showers running at the same time with nae probs. Also if the boiler goes tits up you can still heat water via the immersion heater in the tank.

what’s the kitchen like?

if you’ve got somewhere to put it.

4.5′ x 5′ should do?

Gold bathroom fittings & bidet – check:

I have mostly solved all of these problems by having two seperate combi boilers feeding 3 bathrooms and two completely seperate heating systems.
Bizarre as this sounds it actually works pretty well.

….except for the gas bill. That’s a bitch.

You have a serving hatch…how posh are you.

System boiler mains pressure hot water cylinder

Put the cylinder in the loft and make better use of the existing airing cupboard

Also consider a new location for the boiler

4.5′ x 5′ should do?

Yep, you’d get one in there. You could put it in the loft but make sure your joists can support it! Also they do need repressurising every now and again which would be a bit more of a pain if you’re having to climb in and out of the loft to do it.

Against that is the sheer quantity of creams, salves, cosmetics and cleansers which will need that cupboard space in a few years’ time.

Love the wallpaper…

Unvented cylinder and a boiler sized to suit the loaf of the heating.
1900 sq ft is about 180 sq m so assuming 100 watts per m2 you should be able to use around 18 kw.
Best method would be to heat loss the property with a decent calculator (not one from a radiator company if possible) and work out size of boiler from that. And don’t forget to work out pipe sizes too as no point in having a big boiler if the heat can’t be distributed properly.
Advantage of cylinder is back up hot water supply in event of boiler failure, and probably better flow than a combo too, unless you have a huge combi which is often dearer than correctly sized boiler and cylinder.

I’m not sure the relevance of this, but I was on a training course for friction stir welding a few years back. There was a guy there whom was learning about it for the new line of CDI boilers. I got chatting to him in a nerdy way, and the amount of design and engineering was quite amazing.

So maybe the lesson here is, take a look at the newer CDI range if you like the idea of technology??

[video]https://youtu.be/eyV3rKNd3x8[/video]

Definitely go for an unvented system and boiler if you have the space and the water pressure. Though be prepared to ask for a bigger hot water tank and boiler output than first recommended. If you have several folks trying to get loads of hot water at the same time or after each other a regularly-sized tank and boiler might not be able to keep up. That could lead to difficult questions in the future like ‘Why was there no hot water for me this morning? you said if we spent all that money on the heating and water it’d be fine. yet I had to have a lukewarm shower this morning after the children’

Yeah, they’ve got their own hour on the radio, what more do they wan….

Oh wait…

When I renovated, I installed a pressurised hot water tank. Don’t make do with a combi if you have a big family, take the opportunity now to install a proper system that can heat and provide hot water at the same time. Otherwise you will have to make do with the following compromises

– heating or hot water, not good on really cold mornings
– One tap solution
– Impact on pressure of appliances such as washing machine

We have a combi boiler with an integrated small water tank so no loss of hot water when another outlet is turned on, just reduced flow.

I couldn’t go back to a how water tank. In essence you are heating water for no reason, and you have to plan when you want hot water for.

Our combi can run 2 showers at once without any problems. However the water pressure in our area is very good.

We have a combi boiler with an integrated small water tank so no loss of hot water when another outlet is turned on, just reduced flow.

Called a ‘storage combi’ I think. We fitted one in the new house and it’s a great piece of kit which seems to deliver on the promise of having the best features of both a combi and a system boiler. Mains pressure water that can cope with multiple showers and taps being used without having a big store.

Ours is a Viessmann 222F

I think the tank is about 60L (as opposed to 300 or more being typical on a trad system). It has a fast heat exchanger so as soon as you start using water it’s reloading the tank. In the event that the tank empties completely you still get hot water (which you don’t with a traditional boiler) but in theory you lose some flow. We’ve had a load of people hammering the showers in succession a few times and never been aware of that happening though.

Those boilers do have good flow but no back up should the boiler go wrong.
And they are very expensive too.

Gold bathroom fittings & bidet – check:

That one almost made me vomit……

There are pro’s and con’s to combi boilers.

The pro’s are they take up less space, you only heat the water you need as you use and (due to no cylinder) cheaper to install and run.

The con mainly is that when (not if, but when) it fails you have no hot water or heating. The other con is that flow is limited to through put of the boiler. Filling a bath with ‘hot’ water only can take a while. From personal experience I can tell you that the boiler always fails at the worst possible time and not being able to wash/shower is worse than lack of heating… (there is usually an available alternative heating source; electric heater, gas fire or stove/fire).

Lack of hot water at point of outlet for ages is due to the distance from the outlet from the heat source/cylinder. You have to empty all that cold water lying in the pipes between them first… So locate the cylinder as close to the point(s) of use as practical.

Size of a system boiler is determined by the design building heat loss + hot water cylinder load.

The size of a combi is the hot water output you want (the instantaneous load of water from 10-45 deg C is pretty much always greater than the building heat loss….).

Me, if it’s not a flat I’d go system boiler & cylinder (with immersion back up) every single time. These days the standing losses from an insulated cylinder isn’t that great (worse is the make up water for the hot you draw off…).

You also get a nice warm airing cupboard with a HW cylinder….

In which case….

Huuuge tank of water, massive boiler and solar panels, each bathroom with good extractor fans and heated mirrors…As the father of three teenage girls I can vouch for the accuracy of this!

These days the standing losses from an insulated cylinder isn’t that great

Right now we’re losing a measured 5c from a 210L unvented Megaflo over a period of 17 hours (60c – 55c) with no water drawn off the tank. I think the majority of that loss is through the pipework leaving the top of the tank.

footflaps – Member

You also get a nice warm airing cupboard with a HW cylinder..

but a much bigger cupboard if you don’t have a cylinder at all….

Right now we’re losing a measured 5c from a 210L unvented Megaflo over a period of 17 hours (60c – 55c)

I remember going on a week’s holiday with everything turned off and finding the water was still hot enough to provide a bath or two when we got back.

When I renovated my old house (1920s) I went from system to combi. Wish I hadn’t. It forever lost pressure and there was no obvious leak. Knowing my luck it would have been in the pipework encased in the cement floor of the kitchen.

Same happened at my parents’ house when they did it.

I wouldn’t include hot water loaf when sizing a heat only boiler.
Modern cylinders heat very quickly and you can offset hot water on periods from heating just by timing water 30 mins before heating requirement.
Also heat loss calculations are based on worst case scenario i.e. -3 outside and full heating demand needed inside in every room. The number of days that is required is so small that I would ignore hot water requirements. I realise you are supposed to but it can often save you a boiler size as large cylinders can have a large requirement, add to a few TRVS being shut then there is an ever better case for ignoring g the load of a cylinder.

I wouldn’t include hot water loaf when sizing a heat only boiler.

But I would, call me old fashioned if you like 😆 Then again, I’m not a domestic engineer.

heat loss calculations are based on worst case scenario i.e. -3 outside and full heating demand needed inside in every room

-3? Design for -5 here (-15 is worst case). Agree about no. Of days though (<5%). Depends on how you do your losses, if yer a 2 ac/hr man or not. Whether you actually calc the u value or guesstimate it, any quick heat up, any mains loss…. plus 210L 1/2 heat up is ~20kW…
Cost of a boiler size up as a % of the overall cost? One really bad winter would make it pay its way I’d think…
Besides, gas boilers are on their way out… 😀

I know next to nothing about boilers BUT since I had a condesing one installed my quarterly bill went down 2/3 from the time when I had a regular combi installed.

Metal – sorry soft southerner, hardly need heating where I am really!

Those [storage combi] boilers do have good flow but no back up should the boiler go wrong.
And they are very expensive too.

No back up with a combi either but a lot of people have those fitted.

They are expensive but you save the cost of an unvented cylinder (£500-1000), save space, and should save on installation costs as well (both labour and parts) so difference is not nearly as much as it first appears.

Given how little heat people are saying their pressurised tanks lose they can’t be adding any useful heat to the airing cupboards that they’re in – just taking up most of the space. Either create a heated cupboard (with a very small radiator or some uninsulated copper pipes running through it) or a drying cupboard with a dehumidifier – much more effective.

On the same principle as your electric immersion you could always fit an instantaneous hot water heater somewhere as a back up.

Theres always a lot of resistance to new stuff but these seem a genuinely smart innovation to me.

They are expensive but you save the cost of an unvented cylinder (£500-1000), save space, and should save on installation costs as well (both labour and parts) so difference is not nearly as much as it first appears.

A system boiler will have a longer life though and will undoubtedly break down less, if at all. Cheaper to replace the boiler when it does eventually go too. The motorized valves are usually the only thing that may give problems, but are easily fixed by homeowner compared to a combi breaking down.
Most unvented cylinders will have a 20+ year guarantee, but realistically they will last 30 maybe up to 40, depending on water softness.

If you have the space, System is best option. Even in a small property i would always prefer system.