Unvented Cylinder vs. Combi

Hello all,

I’ve been pointed to this forum as a source for help in a question over heating / hot water systems. Apologies for the long winded post to follow!

My 4 bedroom, 2 bathroom house was built on 1999 and is currently heated by a Potterton Supreme heat only boiler and a gravity fed hot water cylinder.

In December, we will be having both bathrooms refitted (no real issues, other than they are tired), and wanted to try to improve the shower performance at the same time.

Herein lies the problem. We are getting conflicting advice between the various people who have quoted for the above work, and wanted some independent/ real world advice.

I cannot measure water pressure, but the property (on a housing estate) gets around 12-15 ltrs/min, depending on the time of day and tap.

While the property is fed by a large diameter water inlet (32mm), this reduces to a 15mm pipe at the stopcock.

Interestingly, the Mira Thermostatic Mixer shower in the en-suite (which is the main reason for the question), outputs around 7 ltrs/min, with the dial set to cold only and shower head removed, where as the bath tap outputs over 15ltrs / min.

We have been offered three options to improve the shower performance:

1) Unvented Cylinder.

Question : Most systems (Megaflo and RM) seem to suggest a minimum flow of 20 ltrs/ min, but some plumbers are still suggesting this. Will it work and give us a good balance of pressure / time of hot water?

2) Combi Boiler

At some point, I know the boiler needs to be replaced. One installer has suggested just installing a large (36KW) Combi, which will provide a cost effective solution and offer improved water performance. I recognise I lose the backup to a hot water tank, but I also understood that Combis are not recommended for larger (18 radiator) houses?

Question : Is this worth considering and will it have any effect on shower performance (other than having unlimited hot water)?

3) Digital Showers

Only one installer has suggested leaving the heating system alone and buying two digital shower pumps, located in the loft space (to reduce sound issue) and plumbed to the current hot water tank.

Question: I would guess our current cylinder is around 200ltrs, meaning that, even with a mix of cold water, showering time would be limited at a flow rate of 20+ltrs / min. Is this a viable option?

Apologies again for the long post, but does anyone have any thoughts / experience on the above and can offer some advice?

Thanks again!

I’m not a heating engineer but I’d be concerned that none of those have checked your water pressure and flow before making suggestions. What would happen if you followed their suggestion and it turned out there isn’t enough pressure or flow? It’s not rocket science to check those things and it takes minutes.

An unvented cylinder can deliver unsurpassed hot water, if the pipes can supply it.

Ash D said:

While the property is fed by a large diameter water inlet (32mm), this reduces to a 15mm pipe at the stopcock.

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Fit a bigger stopcock, with a bigger pipe coming off it. I don't doubt that your flow would greatly increase.

You could run a new 22mm copper pipe to your proposed unvented HW cylinder, and it could deliver much more HW.

look at the pipe from your existing cylinder to the bath tap, it is probably 22mm already. That's why

Ash D said:

the bath tap outputs over 15ltrs / min.

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I am not a plumber, but I favour the full-bore pipe coming off the stopcock, with the hot and the cold pipes (and the garden tap, if you have one) coming off it. This is likely to avoid the common annoying combi experience, where shower temperature fluctuates when somebody turns on a tap or flushes a WC, or a washing machine automatically draws water for a wash or rinse cycle, and they are all in contention for a limited amount of water.

A modern combi can commonly deliver about 12 litres a minute of hot water, but in winter, either the temperature or the flow will be reduced. This is annoying when you try to fill a bath or run a shower.

IME a combi is also noisier, and most are slower to deliver HW to the tap.

Mottie said:

I’m not a heating engineer but I’d be concerned that none of those have checked your water pressure and flow before making suggestions.

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Quite agree! Without telling you all my woes, trying to find someone remotely interested in this work has been a real headache, mixed in with Covid.

It was the suggestion of the very helpful plumber I spoke to over the weekend to measure the water flow rates, although he has not been around to see the property yet.

JohnD said:

Fit a bigger stopcock, with a bigger pipe coming off it. I don't doubt that your flow would greatly increase.

You could run a new 22mm copper pipe to your proposed unvented HW cylinder, and it could deliver much more HW.

Click to expand...

Thanks John. We did consider a new pipe, but I think it would mean quite major surgery to the house to get it from the kitchen to the upstairs bathroom, and would mean destroying part of the kitchen, and probably pulling down the ceiling!

Although, I will ask the question, as maybe there is a simpler way.

if you have a modern house, isn't there a plumbing duct in the corner of the kitchen and bathroom?

You can also use plastic pipes. I did. It was not very bendy. There are several types, though. The longer the piece, the fewer joints you need.

Sorry John, thick question, but “a plumping duct”?

I hadn’t considered plastic piping, I know this is over simplifying it, but is it possible to pull a plastic through, like you would wiring?

No-one ever suggests sticking with the old fashioned vented cylinder and heat only (or system) boiler any more. They've gone right out of fashion.

But you don't need to upgrade your incoming main, you don't need the complexity(or possible unreliability that a combi could give) and there's an immersion heater in case the boiler did pack up (combi boiler only, of course).

You don't need a G3 service on the cylinder every year either.

The shower could even be decent if there's enough head (distance between shower and tank in the loft) ... or there's always the option of a shower pump if you need extra gusto ... and vented systems are (or were) bread and butter to experienced engineers.

One complication of switching to a combi boiler is the need for adequate gas supply and this can mean the existing pipe to the boiler has to be replaced with a larger diameter pipe from the gas meter to the boiler. Either disruption in the house or an ugly run of pipes on the wall of the house. Tempting for metal thieves.



Mr.B's comments about vented systems are all valid.

A shower coil in the top of a vented hot water cylinder can provide hot water at mains pressure to showers.

Thanks Mr. B. We can only lift the tank by around 0.5m, based on 1 bar = 10m of head, I’m not sure it’s worth the effort for 0.05bar increase?

A Mira Digital Shower and loft pump has been suggested by one person, but I guess the compromise is an increase of pressure bu a reduction of hot water time (as it is with a unvented cylinder).

bernardgreen said:

A shower coil in the top of a vented hot water cylinder can provide hot water at mains pressure to showers.

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Thanks. One good thing about this house is the boiler is near to the gas meter, so a larger gas pipe would not be hard to install.

Just Googled “shower coils”, not come across that before. Getting mixed results, can you point me in the right direction of what this is?

A Plumbing Duct is common in (fairly) modern houses.

The soil pipe is commonly in a corner of the bathroom, running vertically. The WC connects to it with a 100mm pipe, and the bath, basin, shower through smaller pipes.

The kitchen is commonly beneath the bathroom, with the soil pipe continuing downwards in the corner. The sink, dishwasher, and boiler condensate pipe usually connect to it.

The corner is usually boxed in to form a duct that these pipes pass through, and the water pipes are often also in it, to supply the bathroom and the kitchen, arranged to minimise and simplify the pipe runs.

In the kitchen, boxing in will be decorated and possibly tiled above worktop height. Below the worktop it may be exposed or have a removable panel so you can reach the stopcock and prod blockages. Depending on the size of the removable panel you may have about 800mm height to poke your plastic pipe up. I believe the duct is supposed to be boxed and panelled with a fire-resisting material such as plasterboard, and you can also stuff mineral wool in it to block draughts.

A shower coil takes heat from the water in the cylinder. I use this method for my shower, mains pressure shower and gravity feed pressure to the taps in bath and basins.

bernardgreen said:

One complication of switching to a combi boiler is the need for adequate gas supply and this can mean the existing pipe to the boiler has to be replaced with a larger diameter pipe from the gas meter to the boiler. Either disruption in the house or an ugly run of pipes on the wall of the house. Tempting for metal thieves.

View attachment 209195

Mr.B's comments about vented systems are all valid.

A shower coil in the top of a vented hot water cylinder can provide hot water at mains pressure to showers.

Click to expand...

Why do you always churn out that crap gas pipe.Why not churn out a badly installed unvented cylinder picture too,you being a sensible,level headed bod.

JohnD said:

A Plumbing Duct is common in (fairly) modern houses.

The soil pipe is commonly in a corner of the bathroom, running vertically. The WC connects to it with a 100mm pipe, and the bath, basin, shower through smaller pipes.

The kitchen is commonly beneath the bathroom, with the soil pipe continuing downwards in the corner. The sink, dishwasher, and boiler condensate pipe usually connect to it.

The corner is usually boxed in to form a duct that these pipes pass through, and the water pipes are often also in it, to supply the bathroom and the kitchen, arranged to minimise and simplify the pipe runs.

In the kitchen, boxing in will be decorated and possibly tiled above worktop height. Below the worktop it may be exposed or have a removable panel so you can reach the stopcock and prod blockages. Depending on the size of the removable panel you may have about 800mm height to poke your plastic pipe up. I believe the duct is supposed to be boxed and panelled with a fire-resisting material such as plasterboard, and you can also stuff mineral wool in it to block draughts.

Click to expand...

Thanks John. Neither bathroom is located above the kitchen (helpfully!) and the soil stack runs in the corner of the house, again, away from the kitchen.

I cannot see any obvious routing or duct between the kitchen and bathrooms, I assumed the pipes must run up through the kitchen walls (which are tilled) and across the ceiling.

More investigate required!