Megaflow or open vented system

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blueagle78

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As some of you might know (from my other thread) I'm going to replace the boiler in the house. I'll also be replacing the hot water cylinder. Right now I just can't decide whether to go for Megaflow or open vented system (this is what I've got right now). A very well known company's sales advisor suggested not to go for megaflow because I'm only getting 16 litre/minute pressure into the house and they recommend 20+. Whereas a local plumber has suggested going for megaflow.
I've got main bathroom with shower plus 2 other showers and 19 rads.
Please suggest what should I do or what would you do if you were in my shoes?
 
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Provided you have the pressure. The flow isn't too important as
you can compensate by having a accumulator to build a store of
the water at pressure. You should have a minimum of 2bar pressure upstairs.
 
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Few people seem to understand what an accumulator can do and more importantly what it cannot do!

For most applications an O/V system will be the cheapest and using a shower pump the best solution.

A slightly more involved variant is to use a booster pump set before an unvented but fed from an open cistern.

Tony
 
Thanks everyone for your input. After long and hard discussions/decisions, I've decided to bite the bullet and get a new mains pipe in the house from the mains. I think it'll cost me a bit more right now but in the long run I'll have less failure points. For example won't have the headache of shower pumps, cold water tanks etc..
Now I've got one question to ask you experts. Will it make any difference whether I've got the hot water tank next to the boiler or upstairs in the airing cupboard?
I'm thinking of going with Vaillant ecotec 630 and Main 250l unvented cylinder.
 
blueagle78 said:
Thanks everyone for your input. After long and hard discussions/decisions, I've decided to bite the bullet and get a new mains pipe in the house from the mains.
Click to expand...

Before you commit to replacing the pipe into the house I suggest you get some proper measurements done. You need to establish the maximum flowrate that you can get down the pipe. The figure of 16l/min is very low and probably not correct. How was the 16l/min determined?
 
dumbrill said:
blueagle78 said:
Thanks everyone for your input. After long and hard discussions/decisions, I've decided to bite the bullet and get a new mains pipe in the house from the mains.
Click to expand...

Before you commit to replacing the pipe into the house I suggest you get some proper measurements done. You need to establish the maximum flowrate that you can get down the pipe. The figure of 16l/min is very low and probably not correct. How was the 16l/min determined?
Click to expand...

British gas sales advisor measured it for me. He fully opened up the back garden tap and had a gauge connected with it which gave him the flow reading. How could I properly measure the flow? If we took a 25mm pipe from the mains wouldn't that give me enough water pressure for megaflow/pressurised system?
 
blueagle78 said:
He fully opened up the back garden tap and had a gauge connected with it which gave him the flow reading. How could I properly measure the flow? If we took a 25mm pipe from the mains wouldn't that give me enough water pressure for megaflow/pressurised system?
Click to expand...

Opening one tap will not give an accurate reading because the tap itself is too much of a restriction. If you have a few mains fed taps then open them all the same time. Whilst they are ALL running measure the flow from each using a bucket and a stopwatch. Add the total flow together. That's going to give you a better idea of the total flow you can get.

Also if you get one of these pressure gauge (Screwfix code 82412) then you could plumb this into the system and see what static pressure you have. If you plumb it in close to the main stop cock then you can also see how the pressure drops when there is flow down the pipe from the street, ie. how much of a restriction the pipe is. I wouldn't spend any money until some more testing has been done.
 
Thanks for another detailed reply dumbrill. I'll measure the flow using the buckets and stop watch today and then would update you. Thanks
 
blueagle78 said:
Now I've got one question to ask you experts. Will it make any difference whether I've got the hot water tank next to the boiler or upstairs in the airing cupboard?
I'm thinking of going with Vaillant ecotec 630 and Main 250l unvented cylinder.
Click to expand...

You can put your new cylinder wherever you like, although the norm in new builds in particular is to have it near the incoming main, so that a balanced-pressure cold connection can be made to feed the rest of the house. You might consider putting it in your garage if you have one with the space to do so, then the new main can be brought in there and run straight to the cylinder. One major consideration of course is the current layout of pipework in your house, and whether it is therefore practical to run the cold in this way without a major repipe (unless you're happy to have one)

As for your choice of boiler & cylinder, boiler good, cylinder maybe not so good. There are better ones out there, anyway. Look at the OSO Super S, IMHO you can't really do much better
 
muggles said:
blueagle78 said:
Now I've got one question to ask you experts. Will it make any difference whether I've got the hot water tank next to the boiler or upstairs in the airing cupboard?
I'm thinking of going with Vaillant ecotec 630 and Main 250l unvented cylinder.
Click to expand...

You can put your new cylinder wherever you like, although the norm in new builds in particular is to have it near the incoming main, so that a balanced-pressure cold connection can be made to feed the rest of the house. You might consider putting it in your garage if you have one with the space to do so, then the new main can be brought in there and run straight to the cylinder. One major consideration of course is the current layout of pipework in your house, and whether it is therefore practical to run the cold in this way without a major repipe (unless you're happy to have one)

As for your choice of boiler & cylinder, boiler good, cylinder maybe not so good. There are better ones out there, anyway. Look at the OSO Super S, IMHO you can't really do much better
Click to expand...
Muggles thanks for your input. Its really appreciated. I think it won't be possible to have cylinder in the garage because garage is good few meters away from the utility room where boiler is going. I'm thinking of getting the new pipe from mains into the utility room. Then we can run a 22mm pipe to the cylinder upstairs in the airing cupboard. But if you think I'll lose a lot of water pressure by doing so, the cylinder could be fitted in the utility room.
My plumber speaks highly of Main products and even the local plumbing shop I visited, he told me Main was their most popular cylinders brand. But I'll definitely look into OSO Super S. Thanks once again
 
You shouldn't get any pressure/flow drop by having the installation done as you suggest. Make sure your installer uses the balanced-pressure cold connection, it ensures your mixer taps perform at their optimum.

Main are probably popular because they're cheap. They're made by Baxi (indirectly) who also make the original Megaflo and the Santon range of cylinders. The advantage of the OSO is that it gives a much neater installation, it's easier to install in many cases, and the heat-up and reheat times are among the best on the market. Certainly in the latter respect it knocks the socks off the Main, taking just 27 minutes to heat up from cold against the Main's 40 minutes, and 19 minutes to reheat against the Main's 30 minutes. It's also a bit better insulated than the Main, so it'll be cheaper to run
 
Although its well explained across this forum, you dont seem to be measuring the dynamic flow rate.

The open pipe flow rate is not what is important when considering an unvented cylinder.

Tony
 
I measured the water flow based on stopwatch and bucket method as suggested. I got just over 20l/min. While I did it I was getting the new pipe from mains anyway. So I measured the flow again last night once the new pipe was fitted and we had 22mm pipe indoors rather than 15mm. I must say I got around 24-25l/minute. So now hopefully I shouldn't have problem with the water pressure even when 2 or 3 showers are being used at the same time.
I got a question though and need your opinion on it.
The airing cupboard is upstairs and I've got a bathroom and ensuite on the same floor. Ensuite has got both cold and hot water pipes running from the airing cupboard upto the loft and then down to the ensuite. Would that cause a serious water pressure lost? I'll have shower fitted in the ensuite.
Thanks
 
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