Unvented Cylinder vs. Combi

[bernardgreen]

Why do you always churn out that crap gas pipe

Because it happens that way when the alternative requires extensive work inside the house. A family in this village have a similar external gas run.

 

[durhamplumber]

Because it happens that way when the alternative requires extensive work inside the house. A family in this village have a similar external gas run.

Why not show an unvented system shoe horned into some ridiculous small place ..much more common.

 

[whall3y]

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.

I went through this decision making process a few months ago - similar setup with vented system - 4x bed, 2x ensuite, 1x bathroom, 14x rads ... It was a choice between unvented and combi and in the end I decided on a 40kw combi after the plumber checked the existing cold water flow - the unvented system was more expensive, required more servicing, did not free up space - but I am aware of the advantages. The boiler, incoming mains, and gas were all in the garage so easy uplift to 22mm gas pipe work. Bit more room in the attic and an extra storage cupboard where the hot water tank was, the shower pressure is also great and bath fills quickly. If two people have shower at the same time the pressure is reduced but that is very rare in our house. Overall I'm very pleased with it.

 

[Ash D]

Hi Whall3y, really great feedback, thanks. Exactly what I was was looking for.

Can I ask, what was the pressure like, particularly through the showers before and did you notice an improvement when moving to the Combi?

 

[Harry Bloomfield]

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.

Thanks for that Bernard, interesting - not something I have come across before.

 

[whall3y]

Can I ask, what was the pressure like, particularly through the showers before and did you notice an improvement when moving to the Combi?

Before the shower flow was poor and I was considering putting a pump in to help solve this. Afterwards the pressure was really great - though I'm spending more time in the shower now as it's much more enjoyable - just dreading my next water bill . I know that's not very scientific, but I'm really happy with my decision. I opted for a Worcester Bosch 8000 40kw.

 

[Bodd]

What Mottie says is true and the advice from JohnD is good.
Stay way clear of a combi.
I would go for an unvented if your bar pressure is good and the water flow.
If you can take mains to 22mm copper would be better than plastic.
If you have the room you could go for a booster tank..
Also option of a secondary return.(Only do entire hot water with copper)

If water pressure is bad and no room for a booster tank then vented may be best way but I would avoid if you can. Pumps are only good if you pipe them to the MI's otherwise they will not honour the warranty.

 

[Harry Bloomfield]

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.

The houses in my street, all semis, of various designs and many have had combis fitted. The only sensible place, depending on the house layout, has been to have the boiler at the front or the rear. Most have the gas meter at the side where the drive is, so the only route for the new larger gas pipe, was an ugly outside along the wall route.

My own gas route, is buried in the concrete floor, when my open vented was fitted in the mid 1980's. When I had a new boiler installed around 12 years ago, the fitter tried hard to persuade me to move to a combi, with the pipe run outside along the wall. He even tried suggesting a new pipe would be needed anyway, even for a vented system.

I happen to much prefer a vented system, so I stuck to my guns and did my own calcs on the pipe size needed.

From scratch, I probably would not have an open vented, because they are so much more expensive to install, but when the open vented is already in, it is simply daft to have it removed without a good reason.

 

[bernardgreen]

an open vented, because they are so much more expensive to install,

but open vented does not incur the costs of the annual safety checks that an unvented system has to have

 

[sxturbo]

Check your mains pressure and flow rate from the kitchen tap only, or outside tap if this is taken from the mains. In your system this will be the only tap that uses mains water.

The rest of the house will be fed from the cold water tank which shouldn't be used for drinking, and will not provide the info you require to make an informed decision.

Out of curiosity what water flow are you currently obtaining from your hot water tap?

Personally I would look at aqualisa pumped showers, I have one and it's very good, on high it's like having a massage of water jets.

A 2 bar shower pump will also be just as good, but you will also want a thermostatic shower.

A shower pump fitted in the loft will be quieter than the aqualisa.

Personally I wouldn't want an invented cylinder installed because of safety issues, only g3 registered fitters can service them, and it's hard enough trying to find a decent plumber or any tradesman.

If going for combi you can't have more than 1 tap on at any given moment, so if you having a shower someone can't flush the toilet in another part of the house

 

[whall3y]

If going for combi you can't have more than 1 tap on at any given moment, so if you having a shower someone can't flush the toilet in another part of the house

What utter rubbish. Yes if you don't have a thermostatic mixer the shower will get hotter, but it doesn't stop the toilet being flushed. And you can run more than one tap OK.

 

[bernardgreen]

If going for combi you can't have more than 1 tap on at any given moment, so if you having a shower someone can't flush the toilet in another part of the house

What utter rubbish.

Is it rubbish. ? A bit over dramatic for a well installed combi system but where the combi has been mis sold the effect of turning on multiple taps can be dramatic. The extent of the drama depends on how the boiler has been installed.

When heating DHW the combi has to control it's heat output ( modulate ) to have the DHW leave the boiler at the required temperature.

Alter the flow rate and the boiler must alter its heat output accordingly.

Turning ON a second hot tap is likely to increase the flow rate through the boiler

Turning ON a cold tap is likely to reduce the flow rate through the boiler

The boiler cannot respond instantly to a change in flow rate. There will a delay due to thermal mass in the heat exchanger and the response time of sensors and gas valve mechanism. There will be a short period of time when the temperature of the DHW is not correct while the combi is adjusting it's modulation setting and the temperature of the thermal mass of it's heat exchanger is stabilising

Provided the change in level of modulation is with the range that the boiler can operate safely then the adverse effect of a second tap being opened or closed in minimal. If opening or closing a second tap requires the boiler to change it's level of modulation to a level that is outside its safe operating range than the effects on DHW temperature are far from minimal.

Opening a cold tap will reduce the cold water pressure at the cold water inlet to the boiler. If this reduction in pressure causes the flow rate to drop below the minimum level for the boiler ( it cannot modulate low enough to match the low flow rate ) then the boiler has to shut down.

Either the boiler's control system recognises it cannot work at that low level and pro-actively shuts down or the boiler continues to work at minimum level until the DHW overheat safety devices operate and shut the boiler down.

 

[whall3y]

I understand the discussion. Was just commenting on the implication that you can only use one tap and that somebody could not flush the toilet if the shower was in use. As I said, utter rubbish.

 

[Harry Bloomfield]

I understand the discussion. Was just commenting on the implication that you can only use one tap and that somebody could not flush the toilet if the shower was in use. As I said, utter rubbish.

No it is not. A great deal depends on the cold water mains flow and pressure.

 

[Bodd]

Has Harry says its how a combi and the system is installed.

When showering I can tell if the kitchen tap goes on but the shower pressure is still good. If someone uses the shower room basin it makes very little difference to bathroom shower. Reason being is that I run from boiler to basin in 10mm Poly with minimum 90 bent elbows.
Still great pressure at tap that does not effect other outlets. and the water gets hotter quicker by running 10mm.


But in situation of the Op the best system is an Unvented system fitted correctly by a good engineer.