Combi or Conventional Boiler

[WSB]

As detailed in other posts, I'm replacing my back boiler with either a combi or Conventional Boiler.

Am in the proceedings of getting quotes and trying to work out which to go for.

We have a medium sized 3 bedroom mid terrace house with single bathroom with shower over bath and total of 7 radiators.

I like the simplicity of a combi but as there's 5, sometimes 6 people living here, I'm concerned whether it'll have enough ooomph.

E. G tap, washing machine or dishwasher on when someone in shower etc.

Also, lose the backup of an immersion heater in cylinder. That said, I'd be getting insurance, so if any problems the fix hopefully won't be too slow.

When I phoned Boxt, they recommended the Worcester Bosch greenstar 4000 at 30kw.

Any thoughts most welcome and other stuff I need to be aware of.

Many thanks again

 

[oldbuffer]

1. Insurance won't necessarily get you a rapid fix to problems. Also tends to be overly expensive in the long run.
2. If anyone wants a bath, rather than a shower, be aware that the time to fill a bath can be excessive.
3. Before making any decision you need to know that the incoming mains pressure and flow rate will support a combi. Look for minimum 1.5 bar pressure and dynamic (measure one with another mains powered outlet open) flow rate of 15 litres per minute, preferably more.
4. My personal preference is an open vented (not system or combi) boiler installed as a sealed system (no feed and expansion tank) and either@
4.1 An unvented hot water cylinder (needs 2.0 bar pressure and 20 lpm dynamic flow) OR
4.2 A vented hot water cylinder and a Stuart Turner Monsoon shower pump.
5. I prefer open vented because all accessories (pump, motorised valves, controls) can be sourced independently of the boiler manufacturer and not subject to their pricing regime.

 

[WSB]

1. Insurance won't necessarily get you a rapid fix to problems. Also tends to be overly expensive in the long run.
2. If anyone wants a bath, rather than a shower, be aware that the time to fill a bath can be excessive.
3. Before making any decision you need to know that the incoming mains pressure and flow rate will support a combi. Look for minimum 1.5 bar pressure and dynamic (measure one with another mains powered outlet open) flow rate of 15 litres per minute, preferably more.
4. My personal preference is an open vented (not system or combi) boiler installed as a sealed system (no feed and expansion tank) and either@
4.1 An unvented hot water cylinder (needs 2.0 bar pressure and 20 lpm dynamic flow) OR
4.2 A vented hot water cylinder and a Stuart Turner Monsoon shower pump.
5. I prefer open vented because all accessories (pump, motorised valves, controls) can be sourced independently of the boiler manufacturer and not subject to their pricing regime.

Thanks. Water pressure from mains is good to check. Will ask british gas bloke to check on Monday visit

 

[Madrab]

All depends on what you are expecting from the system but one key point you touch on is

but as there's 5, sometimes 6 people living here, I'm concerned whether it'll have enough ooomph.

E. G tap, washing machine or dishwasher on when someone in shower

Simply put, a combi will usually struggle to supply more than one hot outlet at a time without impacting on the other outlet(s), without using fancy water saving, flow limiting or bespoke lower flow outlets. The alternative is to manage it tightly where more than one outlet wouldn't be used at the same time and that's not considering the mains dynamic water pressure and flow and whether it is adequate.

The 30Kw 4000 will deliver ~11L/Min @ 40 deg rise (this boiler is flow limited to achieve @ 40deg rise), so using more then 1 HW outlet at the same time will be noticeable

 

[Elsa TLC]

If you have room then conventional.

If space is tight and your not likely to create another Bath/shower room then Comvi may be best.

Lots more to ho wrong with a combi, but they have a purpose.

 

[WSB]

If you have room then conventional.

If space is tight and your not likely to create another Bath/shower room then Comvi may be best.

Lots more to ho wrong with a combi, but they have a purpose.

Thanks. That's a good point to consider.
Do combis go wrong more?
On average, do they have a shorter life span?

 

[ericmark]

I had two boilers, one for DHW and one for CH, next to each other, before combi's were popular, the CH no real problem, the problem is the DHW.

1) Takes an age to fill a bath.
2) Will only support on shower.
3) Unless there is a reservoir the taps need to be on nearly full bore.
4) Hot days the shower would not use enough water, had to turn sink tap on at a dribble.
5) Can't connect to any solar array.

And as a combi, often the boiler is larger than required for the central heating, so will not modulate down enough for economic heating.

With a storage tank, I use the spare solar power, and have option of either CH boiler or the immersion heater, but that does highlight one advantage of the combi, there is no pipe run to loose heat when using it for DHW in the summer.

I was surprised, since my boiler does not modulate (oil) energy used is easy to work out, and I found in summer to heat the DHW, I had to run the boiler 4 times a week at 20 minutes each time it ran, so a 19 kW boiler, so around 25 kWh per week. Most of this to heat the pipes.

The immersion heater is connected to an iboost+ and solar panels, so easy to work out how much used, it displays it on the read out, and around 4 kWh per week, if that, so electric needs to be 5 times more expensive to oil to break even. And of course free as comes from solar.

As to cost to heat up the boiler every time a cup full of hot water is required, I can find no figures. At work it called the price of a match, we still use steam trains, and even if the steam engine never goes out of the shed, it costs to get up steam.

There must be a cost for a combi boiler to heat the heat exchanger, even if the hot tap is turned off before the hot water gets to it. And that heat then dissipates out of the flue, winter a valve changes over to give DHW so boiler does not heat up the heat exchanger, this only happens in the summer.

Since getting solar power, we have noted the oil used has dropped.

Again it depends on other factors, due to having a battery with the solar array, we have gone to an EV tariff, we don't have an EV, but the battery means at least ½ the electric used is at cheap rate, once the electric tariff drops to 8.95p per kWh it's hard to show using gas is cheaper, if you include the losses when running a gas boiler for a short time.

But a storage tank is not cheap, so now looking at installation cost, also to use a shower, likely need a shower pump, my parents had a power shower, and they are far better than direct from mains, and without a header tank, can't have a power shower. Theirs was ripped out when the combi went in.

 

[pcaouolte]

Do combis go wrong more?
On average, do they have a shorter life span?

They possibly don't go wrong more but everything is contained within the boiler so you need a Gas Safe engineer to change the circulating pump for example, which can easily be changed DIY on a conventional system.

The boiler fires up every time someone operates a hot tap so more switching on/off operations which you might expect to create more wear on the components in the boiler.

 

[Madrab]

Do combis go wrong more?

With combis, most have extra parts that regular/heat only and even system boilers don't, to allow them to perform their dual role of instantaneous HW and Central Heating (diverter valves/diverter valve motors/plate heat exchangers/ etc). So more components means more things can fail and invariably the things that tend to go wrong are the additional parts that combis require.

So yes I'd say combis typically go wrong more than the others.

Of course that's not applicable to all combi's as some do not use these additional components to facilitate their dual role

 

[WSB]

They possibly don't go wrong more but everything is contained within the boiler so you need a Gas Safe engineer to change the circulating pump for example, which can easily be changed DIY on a conventional system.

The boiler fires up every time someone operates a hot tap so more switching on/off operations which you might expect to create more wear on the components in the boiler.

Thanks. Makes sense.
Most likely go for a boiler insurance plan though, so they'll by someone to fix it if it is more complex.

With combis, most have extra parts that regular/heat only and even system boilers don't, to allow them to perform their dual role of instantaneous HW and Central Heating (diverter valves/diverter valve motors/plate heat exchangers/ etc). So more components means more things can fail and invariably the things that tend to go wrong are the additional parts that combis require.

So yes I'd say combis typically go wrong more than the others.

Of course that's not applicable to all combi's as some do not use these additional components to facilitate their dual role

Thank you. Makes sense.
Think I'm heading down the route of conventional boiler now. Might be more expensive but don't want the future hassle.
Plus the added bonus of not needing to change my nice pumped aqualisa quartz shower.

 

[fixitflav]

My personal preference is an open vented (not system or combi) boiler installed as a sealed system (no feed and expansion tank)

To misquote the Iron Duke, I don't know what that does for the OP, but it confuses me

An unvented hot water cylinder (needs 2.0 bar pressure and 20 lpm dynamic flow) OR
4.2 A vented hot water cylinder and a Stuart Turner Monsoon shower pump.

He hasn't said anything about wanting to improve his shower

I prefer open vented because all accessories (pump, motorised valves, controls) can be sourced independently of the boiler manufacturer

Independent sourcing applies to heat-only boiler, whether it's open vented with F/E tank, or sealed with expansion vessel.
Edit - I prefer open vented too

 

[Madrab]

Might be more expensive but don't want the future hassle

Not really that much more expensive in the grand scheme of things when comparing box to box and as suggested they're typically easier/more efficient to work with when it comes to modulation and range rating to configure it properly for the CH/HW demand.

 

[WSB]

Not really that much more expensive in the grand scheme of things when comparing box to box and as suggested they're typically easier/more efficient to work with when it comes to modulation and range rating to configure it properly for the CH/HW demand.

Thanks.
All extra weight in favour of conventional.
Thought conventional would be more expensive due to extra tanks. I'd be asking for cylinder, CH tank and cold water tank all to be replaced as they're all pretty old.

 

[Madrab]

Ah yes, of course, if you were planning to replace the whole end to end system.

 

[Harry Bloomfield]

Do combis go wrong more?
On average, do they have a shorter life span?

They suffer higher rates of wear and tear, because each time there is a demand for hot water, the boiler has to fire. The user than has to wait while it heats up to temperature, which wastes gas, and water. Because everything is in the room sealed case, any repair, needs a Gas Safe, whereas with an open vented, a plumber can deal with most of the points of failure, which are external to the boiler. The combi, also delivers water much slower, than an open vented system, so a bath fill, is much slower.

Combi's are great where space is limited, or where a place is irregularly used, where you might need to walk-in, then wait for the hot water cylinder to heat up.