Loft conversion - unvented cylinder

14 Jan 2017
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United Kingdom
Hi all.

I’ve got a three bedroom, one bathroom house and I’m looking at converting the loft into an additional bedroom and a bathroom. I currently have a vented hot water cylinder, cold water storage tank, and 2.0 bar hot and cold pump in the loft supplying the bathroom shower and taps. I have a 18kW system boiler in our kitchen.

As part of the loft conversion work, I’m looking to remove the cold water storage tank to free up space in the loft and also remove the pump because I think it would be too noisy if the loft is to become a bedroom, so I was thinking an unvented cylinder could be a good solution.

I’ve had two plumbers visit and to be honest it has left me with more questions than answers!

The first plumber did not bother checking the mains flow rate or pressures, but has just quoted for installing a new 32mm mains supply pipe from the street up into a new unvented cylinder in the loft.

The second plumber took measurements of static pressure to be 1 bar and dynamic pressure to be 0.5 bar which led him to believe that if we wanted an unvented cylinder we would need to install a break tank and pump in the loft before the inlet to the unvented cylinder, as well as upgrading the incoming mains supply. As mentioned earlier we are not keen in installing a pump in the loft room because of noise.

Since the second plumber has visited I have checked the static pressure myself at our outside tap a number of times at different times of the day and I have found so far that it is consistently around 2.0 bar and the tap has a flow rate of around 14 litres per minute when fully open. The dynamic pressure reading on the outside tap is about 1.6 bar when I turn on the kitchen tap with a flow rate of about 11 litres per minute at the kitchen tap. So this seems quite a bit higher than the measurements the second plumber obtained.

If I understand correctly the existing mains definitely has an insufficient flow rate for an unvented cylinder which I understand could be increased to a suitable level by upgrading incoming mains pipe diameter to 32mm.

My question is if my mains supply is at 2.0 bar static and 1.6 bar dynamic @ 11 litres per minute then even if it is upgraded to 32mm would it suitable for an unvented cylinder located in the loft without a pump and break tank before it? Am I right in thinking that that the 1.6 bar dynamic pressure would actually be more like 1.1 bar when it reaches the unvented cylinder at loft level around 5m above ground because of gravity? Unvented cylinders such as Megaflo seem to require a minimum of 1.5 bar dynamic pressure to operate, so I guess this wouldn't work?
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What diameter / material is your existing incoming main? Without that information the rest is just educated guesswork, but I'd not normally expect to see a significant improvement with a mains upgrade against what you have now with the figures you've given. Do you have space for a Flomate iBoost anywhere? If you do, an unvented can be made to work. If you don't, you may as well put a high output combi boiler in and accept the reduced hot water performance if both bathrooms are in use at the same time, or if someone flushes the loo while you're in the shower
Thanks for your reply muggles.

Inside the house the incoming main is 15mm. It's a black metallic pipe. I have a 1930s house so original pipework I assume. I'm not sure whether I have the same pipe from the my water meter to our house, but I'll have a look at the water meter in the pavement later to see what is connected to it.

We might currently have space in our garage to install the iboost but we were looking to convert it to a habitable (office/laundry) room in the future, so I'm not sure whether it work well with our future plans.

How noisy are these units? I've seen the Grundfos Home Boost mentioned on a few other threads? How do the two units compare?
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The iBoost is quieter than the Grundfos (iBoost advertised at 50dB, tech specs on the Grundfos say 65dB), more powerful, and cheaper. It's about the size and shape of a fridge-freezer, shouldn't be too hard to find a corner for it for the sake of much improved performance should it? Put it in an insulated cupboard and I bet you'll never hear it run.

Upgrading from your existing main might help a bit, but I wouldn't hope for anything spectacular

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