Combi v System for 4 bedroom house

[old duffer]

you could adopt an approach of considering that hot water supply and heating have different considerations and operations and therefore should be separate.
Hot water supply on demand and high volume then use a dedicated continuous water heater
For heating, use a dedicated combi boiler
Or you could remain in the 20th Century and use the outdated methods of a single solution for both

 

[vulcancontinental]

Simple solution keep cylinder (do you have an immersion heater for backup?). New boiler, keep pump for hot water in shower then everything else except kitchen cold is at same pressure and gravity is your friend (if that goes you'll have more pressing issues to deal with) Minimum pipework alterations; maintenance, one boiler each year, every 10 years a ballvalve on the CWS cistern.

Change to unvented: new boiler, pipework alterations, pressurise system plus pressure relief from new UV cylinder. Boiler straight swap. Immersion heater for backup. No cisterns in loft but two services a year (boiler and unvented cylinder).

Change to combi, more complex appliance. New gas supply, replace larger bore domestic hot pipework, get rid of power shower in en-suite replace power shower with electric shower as backup (needs new cabling), pressurise system, reliant on one appliance. No CWS cistern in loft. One service each year.

10l/m 25kW combi, 12l/m 30kW combi, 14l/m 35kW combi, 16l/m 40kW combi. Set cold tap at kitchen sink to whatever combi power you choose and show wife. If you have 3/4" taps on bath they'll probably need changing for appearance of flow.

No idea of property but combi will probably be cheapest after straight boiler swap.

 

[RobGra]

Simple solution keep cylinder (do you have an immersion heater for backup?). New boiler, keep pump for hot water in shower then everything else except kitchen cold is at same pressure and gravity is your friend (if that goes you'll have more pressing issues to deal with) Minimum pipework alterations; maintenance, one boiler each year, every 10 years a ballvalve on the CWS cistern.

Change to unvented: new boiler, pipework alterations, pressurise system plus pressure relief from new UV cylinder. Boiler straight swap. Immersion heater for backup. No cisterns in loft but two services a year (boiler and unvented cylinder).

Change to combi, more complex appliance. New gas supply, replace larger bore domestic hot pipework, get rid of power shower in en-suite replace power shower with electric shower as backup (needs new cabling), pressurise system, reliant on one appliance. No CWS cistern in loft. One service each year.

10l/m 25kW combi, 12l/m 30kW combi, 14l/m 35kW combi, 16l/m 40kW combi. Set cold tap at kitchen sink to whatever combi power you choose and show wife. If you have 3/4" taps on bath they'll probably need changing for appearance of flow.

No idea of property but combi will probably be cheapest after straight boiler swap.

Thanks, sums it up nicely.
I have an immerser in the HW tank, useful if the boiler is down.
Whilst I'd like mains FW in upstairs bathrooms instead of tanked water it might not be worth the hassle, and cost.

Daft question, I assume the radiators and the unvented HW cylinder are pressurised in a system boiler?

Think I'll cost up the options & see what price difference is.

 

[Mister Banks]

Thanks, sums it up nicely.
I have an immerser in the HW tank, useful if the boiler is down.
Whilst I'd like mains FW in upstairs bathrooms instead of tanked water it might not be worth the hassle, and cost.

Daft question, I assume the radiators and the unvented HW cylinder are pressurised in a system boiler?

Think I'll cost up the options & see what price difference is.

I had a vented system replaced with a pressurised system boiler and kept the vented HW cylinder.

 

[fixitflav]

Daft question, I assume the radiators and the unvented HW cylinder are pressurised in a system boiler?

The radiators and the primary coil of the HW cylinder can be either gravity, with a feed/expansion tank, or pressurised, with a expansion vessel. The latter can be a system boiler with built-in expansion vessel, or a heat-only boiler with separate expansion vessel.
The HW cylinder can be either unvented (mains pressure) or gravity, fed from the cold water storage tank.
The above in any combination.

 

[RobGra]

The radiators and the primary coil of the HW cylinder can be either gravity, with a feed/expansion tank, or pressurised, with a expansion vessel. The latter can be a system boiler with built-in expansion vessel, or a heat-only boiler with separate expansion vessel.
The HW cylinder can be either unvented (mains pressure) or gravity, fed from the cold water storage tank.
The above in any combination.

So it's feasible to use a heat only boiler, with a vented system, and it could be changed later with a pressurised / unvented HW cylinder?

 

[oldbutnotdead]

So it's feasible to use a heat only boiler, with a vented system, and it could be changed later with a pressurised / unvented HW cylinder?

Absolutely. The primary heat circuit (the water that flows through the boiler, radiators and the coil in the cylinder) is completely separated from any secondary circuit (the water in the cylinder that eventually comes out of the taps). So you can have open vent primary and secondary or open vent primary, pressurised secondary or pressurised primary and secondary or pressurised primary, open vent secondary.
Pros and cons for all setups pretty much explained by @vulcancontinental above.
EDIT A boxshifter (BG or any of the large online we'll do the job in a day merchants are unlikely to offer exactly what you want- they tend to offer limited options probably because they get huge scale discounts. You'll be better finding a good local company- might cost you a bit more initially but you're more likely to end up with what you want rather than what they can sell you)

 

[fixitflav]

So you can have open vent primary, pressurised secondary or pressurised primary and secondary or pressurised primary, open vent secondary.

Or open vented primary, open vented secondary. That used to be the norm, and it's what I have.

 

[fixitflav]

So it's feasible to use a heat only boiler, with a vented system, and it could be changed later with a pressurised / unvented HW cylinder?

Not quite sure what you mean, but if you mean keep the vented heat-only boiler, and later change the vented HW cylinder to a pressurised / unvented one, yes, no problem. It's one of the combinations I gave in #20.

 

[oldbutnotdead]

Or open vented primary, open vented secondary. That used tp be the norm, and it's what I have.

Oops, forgot that one and yes of course you can, simple, cheap, protected from external factors (water pressure reduction the main one), more options available.

 

[RobGra]

Thanks all, much appreciated! I'm going to go for a regular, heat only boiler, and perhaps later, convert to a pressurised HW cylinder, keeping the "primary circuit" vented.
I now need to choose a bloody boiler! (Leaning towards a WB 30KW boiler)

 

[fixitflav]

Thanks all, much appreciated! I'm going to go for a regular, heat only boiler, and perhaps later, convert to a pressurised HW cylinder, keeping the "primary circuit" vented.
I now need to choose a bloody boiler! (Leaning towards a WB 30KW boiler)

Unless the house is somewhat unusual, IMO 30 kW is likely to be oversized. Worth doing a heat loss calculation.

 

[Mister Banks]

Thanks all, much appreciated! I'm going to go for a regular, heat only boiler, and perhaps later, convert to a pressurised HW cylinder, keeping the "primary circuit" vented.
I now need to choose a bloody boiler! (Leaning towards a WB 30KW boiler)

If I were looking to replace my boiler today I would look for one that supports Opentherm, I dont think WB do, although they do have their own version using their proprietary heating controls.

 

[Agile]

I would agree that a cylinder system is far better.

One not mentioned yet is that if the gas boiler or supply fails you can still heat it with the immersion heater!

If you must get a combi you need an adequate dynamic flow rate from the mains water! That is not even understood by many fitting boilers!

Most current boilers expect a pressurised heating system.

On my Energy Efficiency course we were told that an open vented heated system was about 4% to 5% less efficient. As far as I know they are still permissible. But I would always go for a pressurised one. Very few systems give many leaks when pressurised.

 

[DP]

Look at the Intergas offering.
Fit a 24kw combi as open vented and hook it up fo the cylinder.
Boiler can deliver water to kitchen and utility area, cylinder to washing drawoffs