Combi or not?

[WhoMe?]

Hi

Can people please offer some advice on the following choice?

We are in the process of fixing our old central heating system which is open vented and has almost no hot water pressure upstairs (cylinder and boiler downstairs). Our plumber suggests sealing the system and installing a mega flow to run off our existing system (i think it called) boiler. Its a four bed house with wet underfloor heating (soon to be installed) in the kitchen / diner, an up and downstairs shower with a new electric shower upstairs. Our mains pressure is 6 bar.

Our alternative is installing a combi boiler (donated from family and less than 6 months old). Both plumber and builder think this is not the right choice but I do like the idea of instant hot water.

Are there any recommendations?

 

[Mottie]

Unvented cylinder, not necessarily a mega flow though. You can still leave the heating system vented I believe.

 

[WhoMe?]

Upstairs pressure problem with the vented. Installed a pump but that did not help due to an old shortcut someone made in the plumbing (water pumped out the vent). 4 plumbers couldnt come up with a simple solution

 

[Mottie]

If you go unvented, there will be no connection with the old vented system and your cold water storage tank can be removed. You should have a decent pressure at all hot taps. I think!

 

[fixitflav]

but I do like the idea of instant hot water

You'll have that with a conventional HW cylinder, or a mains pressure one. With a combi the hot water still has to get from the boiler to the taps.

6 bar should be plenty of pressure, but it's worth checking it under flowing conditions.
A sketch of the existing system would be useful.

 

[WhoMe?]

I tried a sketch of the HW. The mains water is more of a puzzle, however we just installed a new mains feed from the stopcock and it'll be re-done: mains to water softener (and then to appliances and bathrooms etc.) and also mains to kitchen sink and outside tap. The flow is very good out if the new mains if that's what you meant .

Sounds like the cylinder is the way to go.

 

[flameport]

a new electric shower upstairs

Don't bother with that.
Use the new unvented cylinder for all of the showers.

 

[gas112]

Unvented cylinder, not necessarily a mega flow though. You can still leave the heating system vented I believe.

recommending an unvented what basic are you forgetting the OP to check ?

 

[Mottie]

recommending an unvented what basic are you forgetting the OP to check ?

Apart from that sentence making no grammatical sense, I have no ****ing idea. I'm not a plumber and neither is the OP. Why don't you make a constructive post for a change and help the OP instead of trying to score points by questioning me?

As an unqualified person I would assume that whoever is competent and qualified to install an unvented system will carry out whatever checks are needed before installing one.

So, you tell me and the OP, what can anyone who has zero knowledge of such things check for themselves?

 

[fixitflav]

The flow is very good out if the new mains if that's what you meant .

The pressure will reduce as flow increases, so worth checking at a few flow rates (measure it with a bucket and stopwatch) to be sure it will meet your needs, like maybe 2 showers at the same time.

 

[WhoMe?]

Also, I think I have a conventional boiler, not a system boiler. Would some conventional boilers do the job?

What's so special about mega flows, more pressure?

Thanks

 

[fixitflav]

Also, I think I have a conventional boiler, not a system boiler. Would some conventional boilers do the job?

Thanks

Only difference between a conventional (heat-only) boiler and a system boiler is the latter has built-in pump and expansion vessel. Heat-only has separate pump and can be fed from a feed/expansion tank or have a remote expansion vessel. I prefer heat-only as the bits are separate, easier to get at if something goes wrong. Either type can supply a HW cylinder, vented or mains pressure.

What's so special about mega flows, more pressure?

A megaflow or other unvented (mains pressure) cylinder can give a powerful shower with no need for a pump, as long as the mains supply is adequate. The primary circuit can be via a system or heat-only boiler, open vented or pressurised. I believe it's common, especially on new-builds, to have all the cold taps direct from the mains, with no cold water storage tank, but as you have a CWST there's no need to go down that route.

 

[Mottie]

Also, I think I have a conventional boiler, not a system boiler. Would some conventional boilers do the job?

What's so special about mega flows, more pressure?

Thanks

Yeah, I have a conventional boiler with my unvented system. My heating system is sealed too so I have no expansion tank either.

A megaflow is a brand of unvented hot water storage tank. I think the difference between those and other unvented tanks are that they have the expansion vessel built into the tank instead of being separate. As far as I know, most people have separate expansion tanks as they are easier/cheaper to maintain/replace.

 

[MeldrewsMate]

...I have a conventional boiler with my unvented system. My heating system is sealed too so I have no expansion tank either.

Mottie, with a sealed heating system some form of expansion facility is absolutely necessary, so I doubt that you meant it like it read.

A megaflow is a brand of unvented hot water storage tank. I think the difference between those and other unvented tanks are that they have the expansion vessel built into the tank instead of being separate...

No, it is not. If you (or anyone else) reads all the letters in the right order you will see that the brand is Megaflo. The remainder is correct. For the OP's purposes, the expansion vessel (EV) referred to here is NOT the heating system EV.
Not trying to score cheap points Mottie, you have been upfront about your qualifications and competancies...just correcting this often heard error.

Back to the OP, I support what others have said, that the dynamic water pressure is what matters. If your 6bar incoming pressure remains above 3bar when 12 litres/minute are being drawn then you will have good service from an unvented cylinder, Megaflo or otherwise. It would be wise to upgrade the cold water service pipework to 22mm all the way to the UV cylinder, and to feed all the previously tank fed cold taps and showers from the regulator (pressure reducing set) for the UVC. This means the hot and cold pressures will be balanced (better for showers). For this reason it may be advantageous to site the UVC either in the loft or in the airing cupboard.
Give careful consideration to locating the cylinder as close as possible to the main hot water point of use, to reduce the wait until hot water flows, and to save heat energy (reduce the cost of use).

A combi can be used for your heat source. Just about all modern combis are of the sealed system type, so have a pump and expansion vessel built-in, though you will very likely need to fit a supplementary external expansion vessel. The only change when using a combi as a system boiler is that a single hot water outlet (basin, utility room sink, etc.) should be fed from the combi's DHW outlet. In your case feed its cold inlet from the pressure regulator as above, because 6bar is too high for most combis.

 

[Mottie]

Mottie, with a sealed heating system some form of expansion facility is absolutely necessary, so I doubt that you meant it like it read.

Yeah. I meant header tank. Whoops!

Not trying to score cheap points Mottie, you have been upfront about your qualifications and competancies...just correcting this often heard error.

No problem lol. I'm happy to be corrected on stuff I don't know about.