We have a Heatrae Sadia Megaflo hot water cylinder. I need to replace
the cartridge in the "hot tap" of one of the shower mixers, but there's
no stopcock on the output side of the Megaflo to turn off the hot
water.

Is it safe to drain the cylinder by shutting off the cold input, and
just leaving the hot taps open until they run dry ?

Thanks.

On 2006-12-06 21:25:00 +0000, said:


We have a Heatrae Sadia Megaflo hot water cylinder. I need to replace
the cartridge in the "hot tap" of one of the shower mixers, but there's
no stopcock on the output side of the Megaflo to turn off the hot
water.

Is it safe to drain the cylinder by shutting off the cold input, and
just leaving the hot taps open until they run dry ?

Thanks.

Yes.

It makes sense to turn off the heat source so that the expanding water
doesn't tend to run and drip out while you are working.

wrote in message
oups.com...

We have a Heatrae Sadia Megaflo hot water cylinder. I need to replace
the cartridge in the "hot tap" of one of the shower mixers, but there's
no stopcock on the output side of the Megaflo to turn off the hot
water.

Is it safe to drain the cylinder by shutting off the cold input, and
just leaving the hot taps open until they run dry ?

Thanks.


Yes, that will work fine.

It will take a surprisingly long time for the pressure in the cylinder to
bleed off. This is due to the expansion air volume inside the cylinder.
On my 250l megaflo, opening the hot bath tap with the cold supply to the
megaflo off pretty much runs a decent bath before spluttering to a stop.

--
Ron

Andy Hall wrote:
On 2006-12-06 21:25:00 +0000, said:


We have a Heatrae Sadia Megaflo hot water cylinder. I need to replace
the cartridge in the "hot tap" of one of the shower mixers, but there's
no stopcock on the output side of the Megaflo to turn off the hot
water.

Is it safe to drain the cylinder by shutting off the cold input, and
just leaving the hot taps open until they run dry ?

Thanks.

Yes.

Agreed.

(By the way you won't and don't need to actually drain the cylinder; if
there's no mains pressure (ie stopcock off) then no HW will flow.)

It makes sense to turn off the heat source so that the expanding water
doesn't tend to run and drip out while you are working.

And if you open all the other hot taps, especially downstairs, that will
empty the pipework and further dissuade water from coming out where
you're working.

David

--


"Lobster" wrote in message
...
Andy Hall wrote:
On 2006-12-06 21:25:00 +0000, said:


We have a Heatrae Sadia Megaflo hot water cylinder. I need to replace
the cartridge in the "hot tap" of one of the shower mixers, but there's
no stopcock on the output side of the Megaflo to turn off the hot
water.

Is it safe to drain the cylinder by shutting off the cold input, and
just leaving the hot taps open until they run dry ?

Thanks.

Yes.

Agreed.

(By the way you won't and don't need to actually drain the cylinder; if
there's no mains pressure (ie stopcock off) then no HW will flow.)


No, that's not right.
Well, at least it's not on my system.

On an unvented system, there is an air-filled accumulator ( either external,
or in the case of the megaflo, internal ). This is to take up expansion of
the water as it heats. Upon filling, it will initially equalise up to the
3 bar cold mains pressure. It will pressure up even more as the water
heats. The result is that even after the cold mains inlet is closed,
there's one *heck* of a volume of water under pressure to discharge before
the accumulator is back to the state it was before either filling to mains
pressure or heating.

You are right in that the cylinder will not fully drain: In the case of the
megaflo, it will only partially empty as the air expnsion volume expands
downwards and displaces water out untill it bottoms out and everything
equalises.

So a 250l megaflo will not pour 250l out the tap under this situation - just
enough volume to discharge the overpressure in the air expansion volume.
But this is a surprisingly large amount.

--
Ron

wrote:
We have a Heatrae Sadia Megaflo hot water cylinder. I need to replace
the cartridge in the "hot tap" of one of the shower mixers, but there's
no stopcock on the output side of the Megaflo to turn off the hot
water.

Is it safe to drain the cylinder by shutting off the cold input, and
just leaving the hot taps open until they run dry ?

Thanks.

Yes and no...

I would recommend only turning on such taps as are needful..or all off them.

I was doing similar, and accidentally a loo was flushed downstairs..the
ballcock opened and let air in..and how water came out of the pipe i was
working on ;-) A LOT of it.

Lobster wrote:
Andy Hall wrote:
On 2006-12-06 21:25:00 +0000, said:


We have a Heatrae Sadia Megaflo hot water cylinder. I need to replace
the cartridge in the "hot tap" of one of the shower mixers, but there's
no stopcock on the output side of the Megaflo to turn off the hot
water.

Is it safe to drain the cylinder by shutting off the cold input, and
just leaving the hot taps open until they run dry ?

Thanks.

Yes.

Agreed.

(By the way you won't and don't need to actually drain the cylinder; if
there's no mains pressure (ie stopcock off) then no HW will flow.)


Don't count on it if you have a tank in the loft like me..

and someone uses a loo downstairs..

It makes sense to turn off the heat source so that the expanding water
doesn't tend to run and drip out while you are working.

And if you open all the other hot taps, especially downstairs, that will
empty the pipework and further dissuade water from coming out where
you're working.

The trick is to empty teh tank as well, if its above the level you are
working at.

This is done by e.g. accidentally flushing a loo..air rushes INTO the
cold feed to the tank ad allows all the hot water to drain out..


David