Vented to unvented - general queries

[Martin-W]

Having an extension built, getting underfloor heating through extension and adjoining existing room, and our boiler has definitely seen better days.

...so getting the boiler replaced at the same time seems like a no-brainer. So far, so straightforward. But a friend recently recommended I get a 'Megaflo' (other brands do exist) at the same time, to future proof my growing children wanting multiple showers a day, and to improve our shower water pressure. It sounds on the surface like a good idea.

What I have rapidly discovered is that this area is a minefield for the uninitiated, and the builder's recommended plumber, whilst apparently very good (all the other tradesmen have been so far), is flaky when it comes to answering questions...taken 2+ weeks to get a quote out of him, and that's a one-liner.

...so can I ask you good people for advice, please?

Current system is very old-school heat-only boiler in an upstairs cupboard with vented tank underneath and a water tank in the loft. With 10mm microbore for the c/heating. House is a 1990 build 4-bed / 2.5 bathroom with 13 radiators + 2 large rooms for underfloor heating.

Questions:-
1) Boiler - Do I HAVE to get a system boiler if I'm going for an unvented tank? Or does an old school heat-only with a pressurise kit work as well? (Don't want a Combi, before that's mentioned)
2) Boiler output - my maths suggests 25kW would be a sensible figure - have I got that right?
3) Unvented tank - plumber has suggested a 200 litre tank. 2 adults / 2 kids, only very occasional visitors - does this sound OK or a little parsimonious?
4) Unvented tank - risk to old pipework? I can't find a lot out about this - is it a serious consideration, or are they typically OK? (Not asking for guarantees, just an assessment of the risk that we're going to spend £4k here and then have to spend the same again or worse chasing leaks all round the old pipework)
5) Unvented tank - are there any brands to avoid, and is there a recommended or a minimum spec? I figure I only need a single coil indirect system, but do they automatically come with a backup immersion heater as a rule, or is that not a thing?
6) No-one has measured current mains flow-rate and pressure...searching and reading on here I assume that's a critical first step...but what minimum values should I be looking for?
7) Maintenance - is an unvented system any/much more onerous for an amateur like me to look after (radiator bleeding and generally keeping an eye on) than our current old-school system?

I know the recommended advice is to get competitive quotes / consider getting our own plumber in, however our trusted plumber retired pre-Covid, and we've since gone through 2 others who've come out, serviced the system once, then gone quiet (presumably because finding good ones round here is impossible). And the only other recommendation we've been given has ignored 2 voicemails and 1 text...clearly doesn't need the business. So we'd just be going to google and hoping some random stranger was better than the builder's random stranger.

(Quote is £4,500ish for a Vaillant heat-only boiler with pressurise kit, a "200 unvented cylinder" (so presumably Gledhill or Vaillant), fit 4x new radiators to replace corroding / old ones, plus drain down, fill and test)

Thanks very much,
Martin.

 

[ericmark]

As far as I can make out, an un-vented cylinder needs testing every year, which to me it rather off putting. One can get mains pressure shower without an un-vented cylinder this shows basic idea, the water is heated as it passes through the heat exchanger, but this likely limits it to one shower at a time.

Main reason to maintain a cylinder is to be able to use other power sources like solar.

Even moving from header tank to sealed system for central heating can cause problems, as the old single seal lock shield valves can't take the pressure.

Under floor heating is for rooms which are heated 24/7, and have no alternative heating, and that includes the sun through windows, as the response time is so slow. I want to decrease response time no increase it, so if I decide I want to use a room, it can heat up fast, OK 2 up and 2 down, rooms always in use, but with larger properties one wants to select when and by how much each room is heated, only used my down steps rooms about 5 times this year, but when some one rings to say coming want them to heat up in around an hour, no way will under floor heating do that.

 

[gas112]

First thing to check if having an unvented fitted is that there is an adequate sized and adequate pressure cold mains supply to the house

 

[Johntheo5]

Having an extension built, getting underfloor heating through extension and adjoining existing room, and our boiler has definitely seen better days.

...so getting the boiler replaced at the same time seems like a no-brainer. So far, so straightforward. But a friend recently recommended I get a 'Megaflo' (other brands do exist) at the same time, to future proof my growing children wanting multiple showers a day, and to improve our shower water pressure. It sounds on the surface like a good idea.

What I have rapidly discovered is that this area is a minefield for the uninitiated, and the builder's recommended plumber, whilst apparently very good (all the other tradesmen have been so far), is flaky when it comes to answering questions...taken 2+ weeks to get a quote out of him, and that's a one-liner.

...so can I ask you good people for advice, please?

Current system is very old-school heat-only boiler in an upstairs cupboard with vented tank underneath and a water tank in the loft. With 10mm microbore for the c/heating. House is a 1990 build 4-bed / 2.5 bathroom with 13 radiators + 2 large rooms for underfloor heating.

Questions:-
1) Boiler - Do I HAVE to get a system boiler if I'm going for an unvented tank? Or does an old school heat-only with a pressurise kit work as well? (Don't want a Combi, before that's mentioned)
2) Boiler output - my maths suggests 25kW would be a sensible figure - have I got that right?
3) Unvented tank - plumber has suggested a 200 litre tank. 2 adults / 2 kids, only very occasional visitors - does this sound OK or a little parsimonious?
4) Unvented tank - risk to old pipework? I can't find a lot out about this - is it a serious consideration, or are they typically OK? (Not asking for guarantees, just an assessment of the risk that we're going to spend £4k here and then have to spend the same again or worse chasing leaks all round the old pipework)
5) Unvented tank - are there any brands to avoid, and is there a recommended or a minimum spec? I figure I only need a single coil indirect system, but do they automatically come with a backup immersion heater as a rule, or is that not a thing?
6) No-one has measured current mains flow-rate and pressure...searching and reading on here I assume that's a critical first step...but what minimum values should I be looking for?
7) Maintenance - is an unvented system any/much more onerous for an amateur like me to look after (radiator bleeding and generally keeping an eye on) than our current old-school system?

I know the recommended advice is to get competitive quotes / consider getting our own plumber in, however our trusted plumber retired pre-Covid, and we've since gone through 2 others who've come out, serviced the system once, then gone quiet (presumably because finding good ones round here is impossible). And the only other recommendation we've been given has ignored 2 voicemails and 1 text...clearly doesn't need the business. So we'd just be going to google and hoping some random stranger was better than the builder's random stranger.

(Quote is £4,500ish for a Vaillant heat-only boiler with pressurise kit, a "200 unvented cylinder" (so presumably Gledhill or Vaillant), fit 4x new radiators to replace corroding / old ones, plus drain down, fill and test)

Thanks very much,
Martin.

How is your present shower(s) supplied?, is it just gravity fed or shower pump boosted?

 

[Madrab]

1) Either a regular (HO) or a system boiler could be used
2) Boiler output would be dictated by what the whole system needs, that really needs on site information
3) Size of cylinder would normally be dictated by location specific requirements - number of baths - showers - outlets - HW recovery times - simultaneous demand etc
4) system pipework would and should be pressure tested prior to work commencing to ensure suitability to be converted to mains pressure, most setups shouldn't have an issue unless truly archaic and crumbling.
5) Quality of cylinders has improved massively over the years and TBH there aren't many that stand out as being 'really bad' these days TBH - Most indirects are supplied with an immersion element backup
6) Size of cold mains supply is critical and needs to be measured - min would be driven by dynamic demand but 20L/Min @ 3bar would be a great starting point. The cylinder will also need 22mm pipework to/from the unit.
7) An unvented system is not looked after by the customer - ideally it would be serviced at the same time as the boiler - by a qualified engineer

Please ensure the plumber that is quoting for the work is qualified to install and work on unvented cylinders, just because they may be GSR'd doesn't guarantee they have their HWSS quals.

BTW 10mm microbore, if squeaky clean, should be ok but TBH I would factor in a re-pipe to 15mm if you are considering such a significant upgrade.

 

[drives]

We had a similar set up (i.e. gravity fed) and swapped over to an unvented cylinder. Existing HW cylinder moved from airing cupboard to loft, and a few extra bits added such as magnetic filter.

200l tank has been fine for us. 2 adults and 2 kids.

We have very good mains pressure and shower pressure is like night and day- with the mixer tap we barely have the tap a quarter open.

It might be worth considering getting a HP ready cylinder which will be more efficient anyway- I think it was around £250 extra. To be honest if I was swapping a boiler out it might be worth considering getting quotes for ASHP. We're now looking at heat pumps and the quotes seem to factor in conversion costs, new cylinder etc so in hindsight it might have made more sense to do it all in one hit.

 

[fixitflav]

Boiler - Do I HAVE to get a system boiler if I'm going for an unvented tank? Or does an old school heat-only with a pressurise kit work as well?

No, heat-only is fine.
The primary (boiler) circuit and secondary (HW to taps) circuit can be open vented or pressurised in any combination.

 

[Martin-W]

Thanks all - appreciate all the advice.

gas112 - water pressure was "observed" rather than actually tested, but the cold pressure here is pretty good so it sounds like it's not a problem.

How is your present shower(s) supplied?, is it just gravity fed or shower pump boosted?

Gravity fed. It's adequate but it's not a power shower by any means.

BTW 10mm microbore, if squeaky clean, should be ok but TBH I would factor in a re-pipe to 15mm if you are considering such a significant upgrade.

That concerns me a little...we're shelling out >£100k on extension + new kitchen + ancillary works already. Re-piping would be £££ and we'd then need to replaster / redecorate every room afterwards...can see that being a 5-figure bill easily, and we've not got that cash spare in the budget. What do you base your comment off? Personal experience, professional experience...?
Still time to cancel that bit of the job, but it's high on the "nice to have" list...as long as we don't need to spend £10-20k on it.

As for HWSS qualifications - good call, hadn't considered that.


Drives...we looked at ASHP, but we're an east-west house and the southern wall is against an external path (no other obvious place for the exchanger to go). We've already had THAT section of the community nick the lead flashing from the bottom of our chimneystack a few years back, so there's no way we're leaving a load of expensive metal easily accessible for them. Plus the indicative quotes I was getting were still 5-figures AFTER the government grant (I think installers are still taking the p around here). It'd take a long time to pay back several thou just in gas-bill savings...

 

[Madrab]

Personal experience - professional experience

Both - it's really hard to clean and flush properly.

I take it the radiator pipework are drops in the wall then rather than up out of the floor? As suggested though, if it's squeaky clean and you aren't looking to hang new much larger rads off of it then chances are it'll be ok.

As per the mains water supply, it's a must that it needs to be tested - need to know the the dynamic mains flow and pressure before deciding on whether an unvented will be suitable - it would be folly not to.

 

[Mottie]

We did away with our tanks in the loft, converted to a sealed system, had a flush, a new 150L unvented cylinder, new two port valves, ABV, PRV, some new pipework, 2 expansion vessels, a digital controller and thermostat etc. Kept our old Potterton profile boiler. Had a better pressure at the shower although we did spring leaks on two of the original 3 rads we had left in the system. All were approx 50 years old though! I changed all 3 to be on the safe side. I think we paid around £2200 about 7 years ago.

 

[Madrab]

did spring leaks on two of the original 3 rads we had left in the system. All were approx 50 years old though

Presume it was from an open vent to a sealed and they didn't pressure test? Didn't pressure test on the first conversion, needless to say every other has had been since

 

[Mottie]

Presume it was from an open vent to a sealed and they didn't pressure test? Didn't pressure test on the first conversion, needless to say every other has had been since

Yes, open vented to sealed. No, didn’t pressure test but did warn me that leaks could be a possibility. Luckily, the rads only had pinholes that showed up as blisters under the paintwork after a day or two and not big gushers.

 

[Martin-W]

All good cautions - thanks again.

Builder and plumber seem to be of the opinion that the pressure looks fine so we really don't need to check mains pressure. Not sure what to do there...trying to find an alternative supplier (who may be just as flaky) doesn't fill me with any great enthusiasm right now.

Pressure test the system on conversion - that sounds like a good plan...would rather find stuff with them here than after the event. Should that be prior to install or immediately after install?

(We are changing the 3 rads that look oldest / roughest, so hopefully that will reduce the risk)

 

[drives]

Plus the indicative quotes I was getting were still 5-figures AFTER the government grant (I think installers are still taking the p around here). It'd take a long time to pay back several thou just in gas-bill savings...

Definitely agree with you there. Some of the indicative quotes I've received are outrageous. One (from Heatgeek) included a £2000 charge to guarantee their work and a promise that they would come back if the installed equipment was not operating as expected. Call me old school but I'd expect that to be included in the 6k labour charge.

 

[Martin-W]

Ah, so for £6k they'll p around and f you over, but for another £2k they'll do a decent job? Perfect business model...