Unvented vs vented HW cylinder?

13 Mar 2016
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United Kingdom

Had a very productive visit from two separate heating engineers yesterday to make a plan to finally knock my system into shape. Likely to be going down the route of pressurising the system to ensure it works better with wet UFH later this year in the downstairs.

The second guy asked me whether I had considered whether to either replace the vented cylinder (probably nearly 30 years old) with either a unvented cylinder, or to just get rid of the cylinder entirely and go for a combi boiler. Im not really too keen on the latter for various reasons.

But it got me thinking, if the cylinder is going to need to be replaced at some point anyway (HW has never been that hot so maybe the coil is bunged up) - should I just put in an unvented cylinder now and then gain mains pressure cold and hot water throughout the house......

The downside of this as far as I can see it is that I cannot use a shower pump and am dependent on what the pressure from my mains supply is, as I understand it. I have no way to directly measure the mains pressure (although might buy a gauge as I cant resist toys) - but my kitchen tap, which is a 15mm connection directly to the 22mm blue mains pipe - gets a flow rate of 10 litres/ min - which seems to be at the low end of normal. I currently have a 3 bar shower pump which I use to supply both the bath and the two showers. This enables us to reliably run the bath/ have a shower or use both showers at the same time.

When I had a combi boiler on my old house which had a 15mm water main IIRC, we could run the upstairs shower and run a bath at the old time.

It seems to me that staying with an unvented DHW system means I get the advantage of being able to 'boost' my pressure to the places that need it most, where as I dont think you can do this on a combi/ unvented cylinder....

I would appreciate some expert advice so I can decide what to do!?

Many thanks
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You are correct.
For the best performance out of a UV, you need a decent flow rate as each outlet open at the same time will detract from the incoming.
ie: 10lpm will give roughly 5lpm to 2 showers both running... Not a lot if you want to be deluged!

A combi boiler further reduces the available incoming flow rate, as this has to be reduced to maintain temperature rise, as specified by each brand/model of boiler.
Some do 12lpm others can do up to 18lpm.

If you consider UV, worth getting your water supplier to check the pressure and flow rate at their stopcock, then you can decide if it would be worth upgrading your own supply from there.
If you have the space for header tanks, cylinder etc there is nothing wrong with the traditional vented dhw arrangement. Unvented is fine when you have adequate flow and pressure (3 bar dynamic @ 30l/min is good), trouble is the minimum supplier obligation is much lower than this (12l/min at 1 bar dynamic I believe), so you could easily come unstuck if a large housing estate was built nearby, a good current flow/pressure could easily drop to mso rendering your unvented install not useless but not very good either.
Thanks - youve confirmed what I thought - interestingly a new housing estate has been built near by recently.

Think what I'll do is get the central heating pressurised but stick with a vented HW system pumped to the upstairs. Might as well replace the cylinder now though I think!
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Not an expert on ufh heating so can't really comment but as far as I know the main advantages of pressurised Vs open vent heating are no f & e tank required (space saving) and fewer corrosion problems due to oxygen in water (sealed system you quickly know if there are any leaks, open vent if you have a small leak the system will just top itself up diluting out the corrosion inhibitors).
If you have a large house there is no reason you can't combine the technologies (eg if the boiler is in the kitchen and the dhw cylinder is up on the 2nd floor so kitchen tap takes ages to run hot) to have a combi boiler providing dhw to downstairs and the cylinder looking after dhw upstairs.
Without knowing what your mains pressure and flow rates are we can only give a subjective answer. The tap may only be providing 10lpm due to narrow flexis, flow regulators or filters within it.

If im quoting for an unvented i always check dynamic pressure and flow rates at an unrestricted outlet like a washing machine outlet.
I would take into consideration that the older part of your heating system pipework might not take kindly to a higher pressure. Just sayin
Cheers. We're probably going to disconnect all the downstairs rads in favour of UFH I suspect. Upstairs is alright and accessible if needs be. The pipes are only 30 years old, my last house had central heating pipes dating from the 70s and we pressurised that with no problems
"Normally" the pipes are fine going to sealed, sometimes the rad valves and fittings are best changed if there that age too. No reason you can't stick with the unvented hot water, if you did go down the pressurization route for hot and cold water then its worth bearing in mind if its older fittings then quite often toilet float valves, showers etc can need changed as some wont cope with higher pressure.

Unvented is great and my preference over any other option but its money wasted if the current works fine and you're not going to get any additional benefit out it.

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