I want to change from combi to unvented

[FoxtrotOscar]

Hi all

4 bed detached, two bathrooms and cloakroom, 16 rads, and 4 people including two young children.

Out hot water demands are growing by the day and if it wasn't already awful for just the two of us, our 10 year old Ravenheat combi boiler is proving entirely insufficient for our water demands with two children approaching school age.

We currently have a shower off the combi in our main bathroom (not bad, as long as you forbid all other water consumption when someone is showering), and a 10Kw electric shower in the ensuite (might as well be using a watering can).

My desire is to install a large unvented cylinder (300L) and replace the combi with a system boiler, power to be determined by a professional, but probably in the region of 30Kw such as Worcester Bosch Greenstar 30CDi. Might seem a lot but we need good DHW recovery...my wife's showers alone are about 20 mins! That's one good thing about a combi I suppose...

Our flow from the garden tap is 17L/m, static pressure is 4 bar, dynamic pressure is 2.2 bar, pointing obviously to some hefty pressure losses at peak flow.

What I want to do is:

- Upgrade 15mm copper main supply to 32mm MDPE and pipe to (concrete floor) double garage, with required trench digging (approx 4m from exterior garage wall)
- Move water metre, stop cock, and boiler from utility room to attached garage
- Install boiler (probably Worcester Bosch Greenstar) and unvented cylinder (probably Worcester Bosch Greenstore) in garage
- Upgrade all other pipework to outlets to suitable pipe diameter to ensure best possible flow to all outlets simultaneously, and create two heating zones from the current single zone
- Supply and fit controls to suit. A Tado smart thermostat is already operating the heating.
- Install secondary return, as kitchen is approximately 30m from the proposed new location of the boiler and cylinder
- Run suitably sized gas pipe from current boiler location to proposed in garage

My questions are:

* Does my basic plan sound reasonable / sensible?
* Is my water main likely to be sufficient for unvented if I upgrade the water supply?
* I've budgeted £5k for this job. Am I in the right ballpark?

Thanks very much.

 

[Agile]

At least unlike many you have made a reasonable budget.

It sounds generally fairly reasonable.

Tony

 

[FoxtrotOscar]

Thanks for the reply Tony. I'm investing in this house so I want a proper job, and am happy to pay for this if it means no shortcuts will be taken in the name of profitability. I far prefer it when a tradesman prices for a proper job rather than a cheap / quick one.

What sort of performance could I expect from 4 bar static and 32mm worth of flow? I'm looking to install proper drenching power showers in our two bathrooms when we renovate them.

Thanks again.

 

[muggles]

It's hard to give a true figure of performance without you having actually upgraded your main - chicken-and-egg situation I'm afraid, you won't know what you're going to get until you've got it. Having said that, it should be pretty decent. Another option, if you have space, would be to install an accumulator to store water pressure at 4 bar and supply your house from that.

Have a look at the Intergas HRE 30SB - cheaper than the Worcester, arguably a better boiler, seven year warranty, and capable of providing you with two flow temperatures without having to use a lot of additional expensive gadgetry, which is something the Worcester can't do. The advantage of two flow temperatures is that you then get the best of both worlds - high temperature water flowing to the cylinder for ultra-fast heat recovery, lower temperature flowing to the radiators to significantly reduce your gas consumption. It will also accept an OpenTherm connection, which will further reduce your gas bills by modulating the flow temperature to the heating to match the heat in the room, thereby providing a more stable temperature, and weather compensation is available for about £25 too. Have a look at OSO Super Coil cylinders - they look much nicer when installed as all the pipework is concealed, so you just see the cylinder. They're also very good cylinders.

Sounds like you have a decent plan though. Your budget is probably in the right area

 

[Agile]

Can you measure the dynamic flow rate which leaves 1.0 bar in the pipework?

What size is the current water supply pipe?

Be aware that an open pipe flow rate from a garden tap is no indication of the performance of a shower which needs a residual pressure of about 1.0 bar otherwise the water will not be ejected through the shower head holes fast enough to give an invigorating shower. This aspect is often poorly understood even by many plumbers.

By comparing the cross section of the existing pipe with the 32 mm then you can get a pretty good idea of the likely improvement in dynamic flow rate.

It is nice to see someone that wants a well specified system rather than something to a low budget. Even so with a good boiler it may well cost closer to £6k. You don't say how old your Ravenheat is, but although it could be used to heat a cylinder they are rather seen as budget boilers.

Tony Glazier

 

[muggles]

Ravenheat

This is the point at which I decided NOT to suggest using the existing boiler!

 

[FoxtrotOscar]

It's hard to give a true figure of performance without you having actually upgraded your main - chicken-and-egg situation I'm afraid, you won't know what you're going to get until you've got it. Having said that, it should be pretty decent. Another option, if you have space, would be to install an accumulator to store water pressure at 4 bar and supply your house from that.

Thanks, I've looked into accumulators and am not sure I fancy another huge tank taking up space so I'm going to do the mains and see if that means I don't need one. 4 bar static should forgo the need for an accumulator with a nice fat water pipe, I hope...

Can you measure the dynamic flow rate which leaves 1.0 bar in the pipework?
What size is the current water supply pipe?
Be aware that an open pipe flow rate from a garden tap is no indication of the performance of a shower which needs a residual pressure of about 1.0 bar otherwise the water will not be ejected through the shower head holes fast enough to give an invigorating shower. This aspect is often poorly understood even by many plumbers.
By comparing the cross section of the existing pipe with the 32 mm then you can get a pretty good idea of the likely improvement in dynamic flow rate.
It is nice to see someone that wants a well specified system rather than something to a low budget. Even so with a good boiler it may well cost closer to £6k. You don't say how old your Ravenheat is, but although it could be used to heat a cylinder they are rather seen as budget boilers.
Tony Glazier

I've had two plumbers out to quote now, the first turned on every outlet downstairs and couldn't get dynamic pressure below 2 bar. Not sure what it would have taken to drop to 1 bar.

The second plumber I had to convince to measure the pressure and flow, because he said if my combi was working OK then an unvented would be fine too! Eventually he did and got 22L/m at 4 bar static. Not sure how to explain the fact that the first guy got 17L/m and the second got 22! He didn't bother measuring dynamic flow...not sure if he was lazy or in a rush or what, but I don't want to dismiss him out of hand because he's WaterMark certified so he can do all the Severn Trent stuff for us too.

The current supply is 15mm copper, from what I can tell. I can only see where it comes through the concrete in our utility, can't tell from the external stopcock as the hatch is full of murky water.

The Ravenheat is ten years old as stated in my opening post .

Thanks again all. Can't wait to get it sorted now, let's see what the quotes are like.

 

[bernardgreen]

I've had two plumbers out to quote now, the first turned on every outlet downstairs and couldn't get dynamic pressure below 2 bar. Not sure what it would have taken to drop to 1 bar.

Neighbours turning their taps on would reduce the pressure in the mains and thus a pressure drop in your house. How much depends on how good the local water supply network is at supplying peak demand.

 

[FoxtrotOscar]

Thanks bernardgreen. At the beck and call of the water company then I guess.

I wonder what people do when they spend £ks on a shiny new unvented installation, then suffer a large drop in mains pressure (but still at or above the legal / guaranteed minimum). That can't be fun.

 

[bernardgreen]

then suffer a large drop in mains pressure~~~. That can't be fun.

It isn't, a tank of water may not give much pressure to the shower but it does supply toilets when the water supply fails due to a burst water main.

 

[FoxtrotOscar]

I meant a permanent reduction in main pressure not as a result of maintenance, but yeah, that's not great either!

On a separate note, someone has told me 'you can't over size a regular boiler for a property, your system wouldn't comply with building regulations'...is this true? Surely a home owner has the right to choose whatever boiler they flipping well wish, as long as it's not dangerous?!

 

[bernardgreen]

If you oversize the boiler then it may not be efficient when it has to supply a low amount of heat. If the minimum heat output is more than the radiators can disipated then the temeprature of the water returning to the boiler is too hot for the boiler to condense. It may even have to cycle on and off.

Hence a combi that is able to heat enough water for a shower ( at maximum heat output ) may not be able to throttle down ( modulate ) far enough to match the much lower amount of heat the radiators need to keep the house warm. Most boiler manufactures quote the highest efficiency the boiler can achieve, in almost all boilers this high efficency is only obtained when the boiler is working at it's optimum heat output level. The efficiency at other heat output levels can be considerable lower.

 

[FoxtrotOscar]

If you oversize the boiler then it may not be efficient when it has to supply a low amount of heat. If the minimum heat output is more than the radiators can disipated then the temeprature of the water returning to the boiler is too hot for the boiler to condense. It may even have to cycle on and off.

Hence a combi that is able to heat enough water for a shower ( at maximum heat output ) may not be able to throttle down ( modulate ) far enough to match the much lower amount of heat the radiators need to keep the house warm. Most boiler manufactures quote the highest efficiency the boiler can achieve, in almost all boilers this high efficency is only obtained when the boiler is working at it's optimum heat output level. The efficiency at other heat output levels can be considerable lower.

Great info. I was wondering whether or not building regs actually required this though?

 

[bernardgreen]

I believe ( but don't quote me ) that Building Control accept the quoted efficiency if there is any reason to doubt whether the overall energy efficiency of the house meets the requirements of the Building Regulations.

 

[FoxtrotOscar]

Hi guys

Update: work is underway!

Specs:

- Worcester Bosch 30CDI system boiler
- Worcester Bosch 300L unvented cylinder with immersion, twin coil, and secondary return connections
- 32mm MDPE pipe to upgrade the water supply pipe
- Secondary return circuit
- Worcester Wave controls

They spent most of the afternoon trying to get through the concrete garage floor for the supply pipe! Can't wait for a decent system now.