Which boiler & cylinder for new pressurised H & HW s

[Bcp]

I live in a 2 ½ bedroom house now with one bathroom, about to become a 3 ½ bedroom house with 2 bathrooms after a loft conversion (The ½ bedroom is only tiny and isn’t often used as a bedroom). Both bathrooms will have a bath with a shower in it.

I currently have a traditional system: boiler (now packed up), hot water cylinder (113 litres) and a cold water tank in loft. When the loft conversion is done I am going to move to a pressurised system, ie no cold water tank in the loft. I plan to have the boiler downstairs and the pressurised cylinder in the loft. I have the following questions:

1. Which boiler to get? Do I need a boiler specifically for pressurised systems? Should I get a “system boiler” (I believe this means it’s designed for unvented/pressurised systems), or a standard boiler with a conversion kit? (As I understand it the boiler will need an expansion vessel and this comes in the conversion kit or is included with a system boiler.)

2. Which cylinder? I assume I should get a Megaflow (145 litres, with built in expansion vessel), do people agree?

3. As the cylinder is so far away from the boiler I’ve been told that the hot water will take ages to get through to the kitchen from the loft, is this right?
So should I have a “secondary return” (also called a fully pumped system?) to resolve this?

Thanks

 

[ChrisR]

1. Which boiler to get? Do I need a boiler specifically for pressurised systems?
No. any modern one would be OK

Should I get a “system boiler” (I believe this means it’s designed for unvented/pressurised systems), or a standard boiler with a conversion kit? (As I understand it the boiler will need an expansion vessel and this comes in the conversion kit or is included with a system boiler.)
Doesn't matter. Down to space really. A bigger PV neeeds less attention.

2. Which cylinder? I assume I should get a Megaflow (145 litres, with built in expansion vessel), do people agree?
Should be fine. There are other makes, like Oso, Albion... Size depends on use. They reheat quickly if the boiler is up to it. (20kW coil typical)

3. As the cylinder is so far away from the boiler I’ve been told that the hot water will take ages to get through to the kitchen from the loft, is this right?
Yep. Use 15mm, insulated.

So should I have a “secondary return” (also called a fully pumped system?) to resolve this?
It isn't called a f.p. system, that's the heating. Secondary return is good if designed properly. How far is it?

Most important but not mentioned- how good is you mains pressure and flow, and can you change cw supply in house to say 22mm?

 

[Bcp]

1. Which boiler to get? Do I need a boiler specifically for pressurised systems?
No. any modern one would be OK

Should I get a “system boiler” (I believe this means it’s designed for unvented/pressurised systems), or a standard boiler with a conversion kit? (As I understand it the boiler will need an expansion vessel and this comes in the conversion kit or is included with a system boiler.)
Doesn't matter. Down to space really. A bigger PV neeeds less attention.
Sorry, what's a PV?

2. Which cylinder? I assume I should get a Megaflow (145 litres, with built in expansion vessel), do people agree?
Should be fine. There are other makes, like Oso, Albion... Size depends on use. They reheat quickly if the boiler is up to it. (20kW coil typical)

3. As the cylinder is so far away from the boiler I’ve been told that the hot water will take ages to get through to the kitchen from the loft, is this right?
Yep. Use 15mm, insulated.

So should I have a “secondary return” (also called a fully pumped system?) to resolve this?
It isn't called a f.p. system, that's the heating. Secondary return is good if designed properly. How far is it?
down two floors and across about 6m.

4. Most important but not mentioned- how good is you mains pressure and flow, and can you change cw supply in house to say 22mm?
Pressure is 4 Bar, flow rate about 14litres per min. Yes I've been told the pipe will be changed (from lead) to 22ml. I'm aware that flow rate is lower than people say is advisable, but I've been told by other people that it's ok. Other people in the street have pressurised systems and they say it's fine. I hope they're right....

5. I've just started reading about Unvented mains pressure systems and read one site where it said these are better than vented systems for a few reasons. Any thoughts?

 

[CopperCylinderMan]

take what I say under the advice that I am BIASED!!

I would steer away from an internal bubble-top type unvented -we used to make one, and now dont - we use an external vessel.

An internal airbubble WILL dissolve back into the water over time. This means that at some point your expansion relief valve will drip. at this point, your whole hot water system is sitting at the same pressure as the safety valve - on the megaflow this is EIGHT bar. Just before it starts dripping, it will be sitting anywhere from 5-8 bar. Most British plumbing equipment is rated at 5bar maximum (eg mira & aqualisa showers). teher have been instances of shower cartridges breaking from the pressure.

An external expansion vessel can be recharged with a bicycle pump (preferably one with a gauge).

If the mains pressure drops below 1bar and you open a hot tap, you will lose some of the internal airbubble up the taps, shortening the time between drain downs.

on to your other questions - I'm not a heating guy, but I assume PV is pressure vessel.

length of time it takes to deliver hot water depends on the pipe layout. an unvented cylinder has a higher flow rate, so the time to travel through the pipes is less. If you have a very long draw off, a 'secondary loop' can be fitted - pipe the hot water in a loop, close to your outlets and back to a secondary return connection on the cylinder. fit a timed pump on this loop to circulate the water, say between 7am-10pm, and your draw off times are negligible. if you have a dead leg of 12m(IIRC) or more, you must fit a secondary return loop.

14l/min is not great, but if it is old lead pipe, increasing the mains to 25mm alkathene from the mains pipe in the street to your property, then 22mm from the stopcock to the cylinder SHOULD improve this a lot.

Hope this helps

CCM

 

[ollski]

An internal airbubble WILL dissolve back into the water over time. This means that at some point your expansion relief valve will drip. at this point, your whole hot water system is sitting at the same pressure as the safety valve - on the megaflow this is EIGHT bar. Just before it starts dripping, it will be sitting anywhere from 5-8 bar. Most British plumbing equipment is rated at 5bar maximum (eg mira & aqualisa showers). teher have been instances of shower cartridges breaking from the pressure.
CCM

Hi ccm...I was under the impression that the pressure relief had to open at 0.5bar above the operating pressure of the cylinder at a maximum of 3 bar and the pressure relief on the combined temp & pressure relief 0.5 bar above that.
With regards to the boiler it must have a resettable overheat thermostat.

 

[CopperCylinderMan]

Nope, fraid not

MY cylinder has a Pressure reducing valve set 2.1 bar, the safety valve is at 3.5 bar, and the P&T relief at 4/90 deg C.

The expansion vessel is sized to keep the max operating pressure at approx 3 bar.

There are unvented units out there that operate a 1.5 bar cold fill, others (like megaflow) that use 3bar. They used to have a 5bar safety valve, but because the air bubble dissipates so fast, they increased it to 8 bar, (P&T @10bar!) to go longer between drain downs.

Aqualisa and megaflow are part of the same group but (I heard) there was a bit of infighting on a housebuilders site when one part of the company was blaming the other for their showers failing. snigger.

anyways, so long as a unit it tested and approved, there are no set rules for what you set the valves at - only common sense.

CCM