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PigletJohn and other clever types - new CH system help needed!

(18 Posts)
TremoloGreen Sun 17-Jan-16 10:13:57

Hi, hope someone can help, we are currently getting quotes to replace our ancient oil boiler and central heating with a whole new gas system. The house is 110m2 with two bathrooms and a loo. We are a family of four with high water demand in the mornings.

Two of the engineers recommended a heat only system with a megaflo cylinder and one thought we should have a combi system. We had a combi in our old house (similar size) and although it could take two showers at the same time, I was never very impressed with the drop in pressure/temp and the time it took to fill the bath. I don't want an electric shower.

Having read previous threads, I cone armed with the following information: the flow rate from the kitchen cold tap is 12L/ min and the supply pipe to the stopcock is copper (looks about 20mm??).

I want a reliable system that will last a very long time. I don't mind regular servicing. I am quite taken with the idea of moving everything to the loft to give more space (it is well insulated).

Finally, should I be worried that none of the engineers has looked at the flow rate?? Thanks in advance!

PigletJohn Sun 17-Jan-16 15:56:28

12lpm is about half what you need.

Copper pipe is usually 15mm or 22mm o.d.

22mm is bigger than usual, about the same i.d. as 25mm o.d. plastic, which is quite good. 32mm plastic is better for flow. Plastic is less constricting because it is somewhat flexible so less likely to have joints and elbows.

It is possible that the kitchen tap is restricting the flow, especially if it is a fashionable ceramic mixer and the tap tails (look under the worktop) are the size of pencils. It will also be restricted if you have braided flexible tap connectors, or ball-o-fix type service valves, or where it has been reduced from 22mm to 15mm.

If you have a utility room with pillar taps, or a garden tap, measure flow there.

PigletJohn Sun 17-Jan-16 23:37:49


The speed that the water gushes out of your taps will be limited by your incoming flow, but unlike a combi, where the temperature will reduce as the flow increases, an unvented cylinder such as a Megaflo will deliver the hot water as hot as you have heated it, and the temperature will not reduce. Most people get a big one, say 250litres, which would be enough for about three baths or a number of showers. The boiler will be reheating it which you splash about, starting shortly after you turn the hot tap on, so it would be difficult to use it all up.

I am not keen on putting things in the loft, where they will be less easy to access and exposed to freezing, for example if the boiler breaks down in winter or you have a power cut or the house is left empty. Some people will not maintain equipment in lofts unless there is fixed access, floor, safety rails round the hatch, and lighting.

TremoloGreen Mon 18-Jan-16 10:30:12

Thanks for your reply PJ. Now I 've got up close and personal with a steel ruler, the supply pipe, I would say it is 15mm. The kitchen tap tails are the flexible braided type so I've been to the outdoor tap and measured the flow rate there. The flow rate there is 30L / min. It was much more powerful than I was expecting. There was not aLaurel and Hardy esque scene of a woman jumping about trying not to get her slippers muddy with a newborn baby attached to her in a sling

So if I get the megaflow system, do you think it will meet our needs/priorities? My main concern is that there is more stuff to go wrong and more expense to fix, but I want a system for modern living. I would use a Worcester-Bosch approved engineer to get a 7 year warranty but I understand that if there is sludge etc in my existing system this will fall outside the warranty. The quote I have includes a MagnaClean(?) filter. Is there anything else I should ask about?

TremoloGreen Mon 18-Jan-16 10:35:14

They quoted for a 180L cylinder as well, does it sound like that would be enough for say 4 showers in the morning and 1 bath in the evening (plus handwashing and washing pots etc - we do have a dishwasher).

PigletJohn Mon 18-Jan-16 13:15:59

if you get 30lpm at the outdoor tap, that sounds really good, better than I would expect from 15mm. I wonder if the house is close to the main, or is modern enough to have a plastic pipe underground. Measure it again to make sure. 30lpm is about six and a half gallons (three buckets) per minute.

There may be a slight advantage in changing some of the piping, and any stopcocks or service valves, to full-bore ones, especially between the stopcock and the cylinder, and the cylinder and the bathtaps, to get the best out of your flow. I would prefer 22mm to the cylinder, but the benefit might be small if the main supply pipe is only 22mm. Elbows in the pipes reduce flow more than bends.

Unless you have long showers under a drencher head, 180litres would do four, especially if you time the boiler to start heating the cylinder half an hour beforehand, and to continue running until at least twenty minutes after the last shower has finished, that will leave it pretty well full for the rest of your day's use.

Bear in mind that running all your hot taps at full power, if they deliver the full 30 litres, they would use up all the hot water in 6 minutes. A bath uses about 100 litres. Unless you are tight on space in your airing cupboard, I would go for a 250 litre cylinder.

If you know someone with a powerful shower, try filling a bucket to see how many lpm that is. I would consider 10lpm to be a good shower. Some drenchers or American showers deliver more. Most people might set the mixer to, say, 75% hot and 25% cold.

The installation of a new boiler on old radiators should include a powerflush, which is about half a days work, and done before the new boiler is fitted. The Magnaclean will capture residual circulating particles before they can settle into new sediment. Ask the engineer to show you how to open the lid and clean out the trapped sludge, it is no harder than emptying a hoover bag, and you should do it monthly until the amount removed dwindles away.

As for reliability, a conventional or heat-only boiler is less complicated than a combi, and has less to go wrong. If it stops working, you can get by using the electric immersion heater(s) fitted to the cylinder (preferably one at the top and one at the bottom). This will be less economical that gas, and take longer to heat up (an immersion heater warms about one litre of water per minute, so you need to turn in on well before your showers). The unvented cylinder needs to be serviced by a qualified person (not all heating engineers are) and you should include it with your annual boiler service. There is not much to go wrong, but they have a pressure vessel and safety devices to prevent excess heat or pressure so should be regularly inspected or they might squirt hot water out of the relief pipe.

TremoloGreen Mon 18-Jan-16 14:01:35

PigletJohn, thank you so much for all your advice! This is very much appreciated as I am clueless about plumbing!

I have measured the flow again twice to be sure (wore my wellies this time) and I filled a 10 L bucket in around 20sec both times. The house is a funny hybrid - front part 1930s, back half (where the stopcock and garden tap located) were built in '88. We are 25m and slightly downhill from the main. The bathroom taps (old part of the house) deliver nothing like this pressure, so I guess this is due to old pipework. The quote mentions removing the boiler and all redundant pipework but nothing about replacing any feeds to the taps, so I will ask him about this. I will also ask why he recommends 180L and if we should get a bigger one. I'll also ask the other lot when they're here to survey tomorrow - maybe they'll think I know what I'm talking about.

Quote also mentions System chemically cleaned and a chemical inhibitor added on completion, which I guess is a 'powerflush'?

I will indeed be taking all your other advice about the MagnaClean. This is fab, thanks grin

PigletJohn Mon 18-Jan-16 14:49:39

"System chemically cleaned" no, that sounds like adding a chemical and letting it run round for a bit. A cheap and easy (DIY) job, not as good as a powerflush. You could do it yourself for £15.

Unless he has looked inside the pipes and sent a sample of water away to be analysed, so he knows there is not much scale, corrosion and sludge, it is a second-class job. If you go into the loft and look at the small water tank (the feed and expansion) see if there is thick brown mud at the bottom. This is (loosely speaking) rust from the steel radiators that has settled out of the water.

Powerflush involves attaching a large and powerful pump to the pipework, and forcing a chemical backwards and forwards round the pipes, while turning radiators off and on to concentrate the force through one at a time, possibly while using a vibrating tool to shake loose clumps of sediment and dislodge blockages. Done properly, it is half a day's work in an average house.

TremoloGreen Tue 19-Jan-16 10:48:42

Have now chatted to the first guy - turns out I did not read your earlier comment thoroughly enough either. He is replacing all the radiators. He thought that this would get rid of most of the sludge and was concerned that the stronger chemicals used in a power flush might pose a risk to some of the pipework that is embedded in concrete floor, but he would do one if it was apparent that it was necessary.

Whatdoiknow31 Tue 19-Jan-16 22:23:43

If your having all the radiators replaced I have to agree with your first guy in only doing a chemical flush.

Reason being most of the sludge would be in the rads anyway and presuming your existing pipe work was put in in 1988 that makes it 27yrs old. From what you've said you would be going from a open vented to a sealed system which then would be pressurised to a minimum of 1bar (should be 1.4bar, rising 1/2bar when boiler hot). We would not recommend a powerflush to a customer in this scenario with pipe work in concrete floors as we would not want them to have to spend more than necessary if a joint under the floor did not hold - which would mean repulsing to bypass that part of the system.

Unfortunately there are no guarantees that you won't get leaks under the floor if the joints don't hold after just a chemical flush. Ask who ever you choose to do an Air Pressure Test after cleaning but prior to final filling. Should be left for 2-4hrs at 3bars. Any drop then at least they are there to sort it out and you haven't got water everywhere.

Yes a powerflush is the optimum and is recommended, especially on open vented where oxygen in the water from the F&E tank causes oxidisation/corrosion. The minimum the manufacturers require is a chemical clean and with a magna clean on the return to the boiler it is protected from any debris.

TremoloGreen Thu 28-Jan-16 10:24:10

Sorry to ressurect my own thread from a week ago, but now having had all the quotes done, I'm more confused than ever. I think we will be ruling the megaflow out on cost grounds as it is ~£3k more and if we don't really need that system, there are plenty of other things to spend it on in this house! The latest guy to come round suggested we keep our gravity-fed system, he was the only one to do so. He said a combi would be 'disappointing' whereas others have recommended that a 42kW combi with a HW flow rate of 17L/min would be sufficient for our needs. With 2 thermostatic showers we would be using at the top end of its capability, but it would be enough.

I just don't understand how a gravity-fed system, which relies on ~0.2 bar pressure can be better at providing two showers than a mains-fed system even with the fact that the flow is limited with a combi. Our hot taps upstairs are quite rubbish - the bath takes ages to fill and the 1 mixer shower we do have (just a basic hose connected to tap set up) doesn't give as good a flow as the electric (non-power) shower. confused

Whatdoiknow31 Thu 28-Jan-16 21:58:24

Defiantly not gravity, you will be very disappointed with your morning shower. Will be like standing under knats wee Haha. We love gong on holiday, but can't wait to get home to a decent power shower and actually get excited when the place were staying at has one!

There are cheaper unvented cylinders, mega flow are one of the most expensive and I believe only offer 25yrs warranty on the cylinder.
Joule cylinders are cheaper and have life time guarantee. Again Albion Ultrasteel is another unvented which is cheaper (we have this one, replaced a mega flow - absolutely no difference in performance)

Personally I would go for a system boiler (or regular if you don't want to risk the underfloor joints - system boiler the system would be pressurised) and an unvented cylinder, with balanced hot and colds - very important otherwise everytime someone turns a tap on whe your in the shower the pressure will drop.
Make sure the installer has G3 ticket as well - you will be surprised number of engineers installing when not qualified!

TremoloGreen Fri 29-Jan-16 10:00:17

Right, that's what I thought. My parents have gravity fed and their showers were so rubbish they had to get two electric power showers which sound like a jet engine taking off and probably cost a fortune to run. Every house I have lived in with this type of system has had rubbish showers where the temperature is difficult to adjust but I imagine they do make better showers these days.

The only house I have lived in that had amazing showers was my old house, a small 3-bed, 2 bath victorian terrace and that was run off a mid size combi boiler. You could have two showers at the same time, no problems. The only annoying thing was that it took longer to fill the bath in winter but I don't have many baths anyway. Also it broke down a couple of times in 7 years, but then we are looking at getting a worcester with a ten year warranty.

Really can't decide if it's worth the extra £2-3k for an unvented cylinder system (it wasn't brand name megaflow they quoted for anyway) when this will mean I have to live with filthy carpets and a downstairs loo that doesnt flush properly for another year!

peggyundercrackers Fri 29-Jan-16 10:32:32

We have a wb combi 42cdi and have two thermostatic showers which run off it, the power of the shower does drop but they are still useable. If you are in the shower and someone turns a tap on there is a very slight drop but it's not really noticeable. Our bath fills quite quickly, definitely less than 5 mins but we have a bigger than average bath

TremoloGreen Fri 29-Jan-16 13:18:03

Peggy, that is helpful, thank you. When you say the pressure drops, do you mean it goes from an excellent to an average shower, or is it disappointing? Would you still be able to rinse shampoo out of long hair - the critical question I haven't asked the plumbers grin

Have you found your central heating effective? We have a large 3 bed which is semi-detached at the front only (detached at back). About 12 radiators. It will be well insulated by the time we're finished, but stuck with old style double glazing for a while yet.

As long as the shower is useable when both are going, I can live with this. In reality, it will only be the odd morning that both overlap at the exact same time.

peggyundercrackers Fri 29-Jan-16 21:14:52

Tremolo yes our showers are excellent, we have adjustable shower heads as I think most are today but if we put it on the narrow jet the water can make you sore if you stand under it they are so powerful so we have it set so the water comes out the wide and the middle jet together. If both showers are on then yes I would still be able to wash long hair under it but might have to change the head to use one jet instead of having multiple jets open.

As for ch... We have a draughty old Victorian pile with no wall insulation. We have 12 radiators in use, 4 of them being 1600 long, all radiators are doubles. We usually have the control on the boiler set to 4 (I think it goes to 6 or 7) and the thermostats on the radiators are set anywhere between 2 and 4 depending on which room it is in and it's not cold. The main pipe that runs to all the radiators is 22mm with 15mm tails off it into the rooms (our hall runs the length of the house and all the rooms come off it), the return pipe is also 22mm, all run in copper. As an example when it's been mild here in the past couple of weeks DH has been sitting in his shorts & t-shirt at night, we are on me of Scotland.

peggyundercrackers Fri 29-Jan-16 21:16:22

Ne of Scotland not me.

TremoloGreen Sat 30-Jan-16 13:28:12

Brilliant, thanks Peggy. Laughing at the description of your husband in his shorts and t-shirt in January up in Scotland!

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