Upgrading combi vs. moving to unvented

[Participant]

Hi all

I just want to run this thinking past you knowledgeables to check if it is sound or not.

We currently have a Ravenheat 120csi combi boiler in our house, which was there when we moved in. More than one outlet in use at any one time (e.g. someone uses a tap when the shower is running) causes poor performance from both outlets.

We've managed so far as there were only really two of us. However, our eldest is now three and we have a three month old, so our hot water demands are going to increase, with regular baths and showers becoming more of a feature.

House is a four bed with two bathrooms and a cloakroom.

Our choices seem to be: upgrade the combi to a beast with more power that can deliver a greater flow of hot water, or go unvented and swap the combi to a system boiler.

I'm fairly sure that the combi will be the cheaper option, but I love my super duper power showers, and both showers need to be usable with a strong flow at once.

Assuming that my flow and dynamic pressure is sufficient for both options, which would you go for? I have space for a cylinder if needed, but unless it'll blow the socks off a top combi, I'm not sure I want the added complication and expense.

Many thanks.

 

[Derek W]

I'd go for the switch to an unvented system. Purely from a specification point of view, one of our developers only uses combi boilers for smaller 2 / 3 bedroom houses <100m2 floor area. Anything larger they install an unvented system so with you having a 4 bedroom house its going to be a decent size.

 

[Dangerous Dave]

If you want more than one hot water outlet running at the same time you need to go unvented (or vented with a pump), no domestic combi boiler can supply two showers simultaneously.

I converted to unvented from a old Worcester Highflow 3.5 storage combi and it is superb, my system can supply three showers simultaneously (I do have a very good mains supply topping 30l/m and 4 bar)

My installer said to supply a shower you need 11l/m and 1 bar at each shower, the biggest domestic combi boilers that can come close have water stores and can only mange 20l/m for a short periods once the water store is depleted you are back to sub 15l/m (Vaillant 937 and Worcester Bosch CDI 440/550). A combi boiler is also limited to heating the water by 30-35c so in the winter this could be sub 40c hot water temperatures when the unvented system can still supply hot water at the required temperature.

Cost wise my install was around £1000 more over a large combi boiler.

 

[Participant]

Cheers guys. Sound like I need to measure my cold water flow rate and get someone out to assess for an unvented solution.

Will an unvented support those body jet / rainfall showers? Or would I need a pumped solution for that?

 

[Dangerous Dave]

Will an unvented support those body jet / rainfall showers? Or would I need a pumped solution for that?

You would need to know the flow rate /pressure required for the shower, an unvented cylinder will supply hot water matching the flow rate of your mains supply limited to a max pressure of 3-3.5bar. A quick google shows a jet /rain shower will use 16-30l/min so should not be a problem for an unvented system as long as your mains can keep up. A combi boiler wouldn't be able to supply a shower over around 15l/min

 

[Participant]

You would need to know the flow rate /pressure required for the shower, an unvented cylinder will supply hot water matching the flow rate of your mains supply limited to a max pressure of 3-3.5bar. A quick google shows a jet /rain shower will use 16-30l/min so should not be a problem for an unvented system as long as your mains can keep up. A combi boiler wouldn't be able to supply a shower over around 15l/min

Cheers for that.

I've just measured flow rate at the garden tap and it's 21 l/min. According to the internet, this only just fits the minimum requirement for unvented.

What I'm not clear on is what level of performance I could expect from 21 l/min? Since it's minimum, does that mean performance would only just be adequate?

Your comment earlier about 11 l/min for each shower seems to suggest this would be OK for running multiple outlets simultaneously at reasonable flow rates.

 

[b0rn2sk8]

Cheers for that.

I've just measured flow rate at the garden tap and it's 21 l/min. According to the internet, this only just fits the minimum requirement for unvented.

What I'm not clear on is what level of performance I could expect from 21 l/min? Since it's minimum, does that mean performance would only just be adequate?

Your comment earlier about 11 l/min for each shower seems to suggest this would be OK for running multiple outlets simultaneously at reasonable flow rates.

Was your garden tap fed off 10mm pipe? The un-vented will be fed off a 22mm pipe as close as possible to where the water comes in for the maximum performance.

EDIT: 16-30L/hour :O your wallet wont like one of those either.

 

[Participant]

Was your garden tap fed off 10mm pipe? The un-vented will be fed off a 22mm pipe as close as possible to where the water comes in for the maximum performance.

Garden tap is fed by 15mm sir. My water main is 15mm too, as far as I can see it anyway, which is basically just before the stop **** before it goes under concrete.

EDIT: 16-30L/hour :O your wallet wont like one of those either.

True, but I want the option, and the cost of water doesn't overly concern me cause it's pretty cheap .

 

[b0rn2sk8]

Garden tap is fed by 15mm sir. My water main is 15mm too, as far as I can see it anyway, which is basically just before the stop **** before it goes under concrete.



True, but I want the option, and the cost of water doesn't overly concern me cause it's pretty cheap .

Sorry I meant 15mm in my post.

Though I dont pretend to be an expert for an un-vented its normally recommended that you feed it with 22mm to get the performance from it.

Its also not the cost of the water that is I was referring to, heating it is where the £££ lies.

 

[Firestar_3x]

You would need to know the flow rate /pressure required for the shower, an unvented cylinder will supply hot water matching the flow rate of your mains supply limited to a max pressure of 3-3.5bar. A quick google shows a jet /rain shower will use 16-30l/min so should not be a problem for an unvented system as long as your mains can keep up. A combi boiler wouldn't be able to supply a shower over around 15l/min

That would be 15L/min of just hot water though? And you won't be showering with just hot water, it will be a combination of hot and cold water.

My parents have a house which is over 300m2, it has multiple bathrooms, washrooms etc, granted only two of them live in it, but this boiler (recently installed), does a fantastic job, even at christmas etc.

http://www.vaillant.co.uk/products/domestic-boilers/combi-boilers/ecotec-plus-937-2/

 

[Dangerous Dave]

That would be 15L/min of just hot water though? And you won't be showering with just hot water, it will be a combination of hot and cold water.

My parents have a house which is over 300m2, it has multiple bathrooms, washrooms etc, granted only two of them live in it, but this boiler (recently installed), does a fantastic job, even at christmas etc.

http://www.vaillant.co.uk/products/domestic-boilers/combi-boilers/ecotec-plus-937-2/

It depends on the temperature you run the shower at I like a hot shower past the 38c overide around 40-42c, most combis are pretty much maxed out depending on incoming water temperature as most have a 30-35c temp rise, in the winter our old combi couldn't heat the water quick enough to require any cold mixed in and I ran the shower at full heat.

The 937 is a water store combi that I looked at, if installed correctly it also needs a blending valve on the hot water outlet to achieve the quoted flow rates this cost plus the cost of the boiler took the parts price over the cost of a system boiler/unvented cylinder and it still can't supply two showers simultaneously.