Which boiler system?

fezster

It's highly unlikely any domestic dwelling with insulation to current regs would require a 40KW boiler, unless it was a mansion. Having said that, it doesn't matter too much if the boiler is able to modulate down sufficiently to negate that - in reality, larger boilers (32KW+) use gas valves which mean they are unable to modulate down as well as smaller boilers, though this technology is improving. The efficiency loss during summer months can be massive (think a boiler able to modulate down to 6KW compared to one able to modulate down to 3KW).

A typical unvented cylinder loses approx 2KWh a day - the larger the tank, the more the loss. It's fairly negligible in a household which typically uses the majority of the hot water in the tank.

330d

One more question please;

If I am having a shower and someone puts the cold tap on, will the shower not be affected on both combi and an system/unvented installs?

On the combi, it will be because some the tap is taking away some of the water so the flow rate coming off the boiler will go down.

On the unvented system, the shower will be affected as it requires cold water to be mixed with the cylinder hot water and some of this cold water is being taken by the tap.

Is this correct?

Jeepers_Creepers

330d said:

One more question please;
If I am having a shower and someone puts the cold tap on, will the shower not be affected on both combi and an system/unvented installs?
On the combi, it will be because some the tap is taking away some of the water so the flow rate coming off the boiler will go down.
On the unvented system, the shower will be affected as it requires cold water to be mixed with the cylinder hot water and some of this cold water is being taken by the tap.
Is this correct?

In essence, yes. But it's essentially the same process in both situations; turning on a cold tap will remove some of that already-borderline 20lpm, so the flow is bound to be affected to some degree.

Just how badly will, I think, depend a lot on the supply pressure; a solid pressure of, say, 3bar will help to maintain that 'full' 20lpm flow, whereas a weakish pressure of 2 bar or below will likely see a more dramatic drop in flow.

Any drop in cold going to the shower mixer, whether it's from a combi or an unvented tank, will see a corresponding drop in the flow coming from the shower head, because the shower's thermostat will rebalance to maintain the output temp. Ie, it'll throttle back the hot to suit.

At the same time, any drop in the overall cold supply (caused by a cold tap/loo/W/M being used) will also slightly affect the supply to both a combi and unvented cylinder; both of these devices are 'driven' by their cold supplies. Again, just how much will come down to pressure as well as flow.

If you had 25+lpm at 3 bar, I wouldn't be concerned. Mind you, you'd still notice the 'effect' in the shower even with this (I know).

Between the two systems, I suspect the combi will be slightly more affected as it's more 'restrictive' to flow in any case, but I don't know for sure.

I have a GlowWorm 30CXi combi and a flow in excess of 20lpm and a pressure limited to 3bar by a PRV. Someone turning on a kitchen hot or cold - both the first items coming off the boiler - is very noticeable in the shower. The hot more so - in fact turning on the kitchen hot 'full' will have the greatest effect as it pinches more than half the available hot which was going to the shower, and the cold has to be reduced in proportion by the mixer. This is a full-flow kitchen tap, tho', so having an 'eco' model - or fitting a part-closed isolator to the supply - would sort this to a great degree.

It's all very complicated... :-(

Jeepers_Creepers

fezster said:

It's highly unlikely any domestic dwelling with insulation to current regs would require a 40KW boiler, unless it was a mansion. Having said that, it doesn't matter too much if the boiler is able to modulate down sufficiently to negate that - in reality, larger boilers (32KW+) use gas valves which mean they are unable to modulate down as well as smaller boilers, though this technology is improving. The efficiency loss during summer months can be massive (think a boiler able to modulate down to 6KW compared to one able to modulate down to 3KW).
A typical unvented cylinder loses approx 2KWh a day - the larger the tank, the more the loss. It's fairly negligible in a household which typically uses the majority of the hot water in the tank.

I can only go by the specs I read.

My 16-year old GlowWorm 30CXi can modulate its CH from a max of 23kW down to 5kW to suit demand.

A typical, current, 40kW combi - the W-B Greenstar 8000 - specs it can modulate from 35kW right down to 3.5kW.

That would suggest to me that it is perfectly suitable for pretty much any house or flat, should you want the extra DHW output for some reason (like a 'dooper shower).

fezster

330d said:

One more question please;

If I am having a shower and someone puts the cold tap on, will the shower not be affected on both combi and an system/unvented installs?

On the combi, it will be because some the tap is taking away some of the water so the flow rate coming off the boiler will go down.

On the unvented system, the shower will be affected as it requires cold water to be mixed with the cylinder hot water and some of this cold water is being taken by the tap.

Is this correct?

Essentially, yes. You can't get out more than you have coming in. If your incoming flow is 20 l/m, and your shower is able to utilise 20 l/m (that's a pretty hefty shower!), then *any* flow elsewhere will take away from that. A "decent" shower is 12-15 l/m, and if the internal pipework is designed correctly (22mm to shower, 15mm to taps) then the effect can be somewhat negated - you want it to the point where it is not noticeable.

A better incoming mains flow rate (whether a larger bore pipe or an accumulator) will overcome this problem to an extent. With a combi, you would still be limited on HW flow rate to whatever the combi is able to provide.

fezster

Jeepers_Creepers said:

fezster said:

It's highly unlikely any domestic dwelling with insulation to current regs would require a 40KW boiler, unless it was a mansion. Having said that, it doesn't matter too much if the boiler is able to modulate down sufficiently to negate that - in reality, larger boilers (32KW+) use gas valves which mean they are unable to modulate down as well as smaller boilers, though this technology is improving. The efficiency loss during summer months can be massive (think a boiler able to modulate down to 6KW compared to one able to modulate down to 3KW).
A typical unvented cylinder loses approx 2KWh a day - the larger the tank, the more the loss. It's fairly negligible in a household which typically uses the majority of the hot water in the tank.

I can only go by the specs I read.

My 16-year old GlowWorm 30CXi can modulate its CH from a max of 23kW down to 5kW to suit demand.

A typical, current, 40kW combi - the W-B Greenstar 8000 - specs it can modulate from 35kW right down to 3.5kW.

That would suggest to me that it is perfectly suitable for pretty much any house or flat, should you want the extra DHW output for some reason (like a 'dooper shower).

Good point. I changed my boiler in 2012 and the modulation ratio was not as good as it seems you can get now.

coffeehound

Jeepers_Creepers said:.

I have to hold my hands up; I do not know how the benefits of a combi in that it only heats up the DHW it actually requires compares with the benefits of a system boiler in how it can, often, be smaller compares in overall annual 'energy consumption'.

Do you?

Well a combi is working at 85 to 89% when heating DHW from the few I've looked at.  A modern system boiler can be from over 99% efficency with low return flow temperature, falling to high 80s at worst as return flow temperature goes above the condensing limit.  Have a look at boiler ErP data sheets for these sort of figures.  So for most of the time, the heat-only boiler is in condensing mode as it heats a HW cylinder.  Therefore potential for 10% efficiency saving, especially with lower tank temperatures.   Certainly will outstrip the kWh or so that will be 'lost' from the tank over the course of a day.  There's also the significant amount of wasted water from a combi while you stand at the sink like a lemon waiting for the water to run hot . .

coffeehound

Jeepers_Creepers said:

fezster said:

It's highly unlikely any domestic dwelling with insulation to current regs would require a 40KW boiler, unless it was a mansion. Having said that, it doesn't matter too much if the boiler is able to modulate down sufficiently to negate that - in reality, larger boilers (32KW+) use gas valves which mean they are unable to modulate down as well as smaller boilers, though this technology is improving. The efficiency loss during summer months can be massive (think a boiler able to modulate down to 6KW compared to one able to modulate down to 3KW).
A typical unvented cylinder loses approx 2KWh a day - the larger the tank, the more the loss. It's fairly negligible in a household which typically uses the majority of the hot water in the tank.

I can only go by the specs I read.

My 16-year old GlowWorm 30CXi can modulate its CH from a max of 23kW down to 5kW to suit demand.

A typical, current, 40kW combi - the W-B Greenstar 8000 - specs it can modulate from 35kW right down to 3.5kW.

I'm somewhat skeptical of that 3.5 kW figure for a 35 kW boiler, Jeepers.  Can you give us a link to where it is officially quoted pls?

Jeepers_Creepers

coffeehound said:

I'm somewhat skeptical of that 3.5 kW figure for a 35 kW boiler, Jeepers.  Can you give us a link to where it is officially quoted pls?

Blimey - had to struggle to find where I'd seen that!

https://www.worcester-bosch.co.uk/products/boilers/directory/greenstar-8000-life-combi - down the LH side is 'Related Docs' and the top one is "Greenstar 8000 Life One Page Guide Manufactured after 31.01.20"

Jeepers_Creepers

coffeehound said:

Well a combi is working at 85 to 89% when heating DHW from the few I've looked at.  A modern system boiler can be from over 99% efficency with low return flow temperature, falling to high 80s at worst as return flow temperature goes above the condensing limit.  Have a look at boiler ErP data sheets for these sort of figures.  So for most of the time, the heat-only boiler is in condensing mode as it heats a HW cylinder.  Therefore potential for 10% efficiency saving, especially with lower tank temperatures.   Certainly will outstrip the kWh or so that will be 'lost' from the tank over the course of a day.  There's also the significant amount of wasted water from a combi while you stand at the sink like a lemon waiting for the water to run hot . .

I'd agree there would have to be a 'good reason' for choosing a combi over unvented. For us it was due to a loft conversion which cut down the available amount of space, and we just didn't want a cylinder in there. Although it has been a bit troublesome over its existing 16 year life - due mainly to residual amounts of sludge in the old system - it has worked well for us, with little compromise required.

Other than a 'space' saving and a cheaper initial installation, I guess it's a performance & financial overall win for the unvented.