Hi,

I planning on descaling the DHW heat exchanger on my combi. It's an
Ariston Microgenus 27 and the rate of flow has gradually declined since
it's been installed (over three years ago). The descaling looks like an
easy enough job, and removing the heat exchanger itself is dead easy,
but before I make a complete fool of myself I've never really
understood exactly how the combi does the DHW. Do I need to drain the
boiler completely (and all the rads) or is just turning off the cold
water feed to the combi enough?

On Wed, 19 Apr 2006 11:49:33 -0700, Martin Pentreath wrote:

Hi,

I planning on descaling the DHW heat exchanger on my combi. It's an
Ariston Microgenus 27 and the rate of flow has gradually declined since
it's been installed (over three years ago). The descaling looks like an
easy enough job, and removing the heat exchanger itself is dead easy,
but before I make a complete fool of myself I've never really
understood exactly how the combi does the DHW. Do I need to drain the
boiler completely (and all the rads) or is just turning off the cold
water feed to the combi enough?

Have you got the service manual?
It will almost certainly involve draining the water out of the boiler but
not the rads.




--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html

On Wed, 19 Apr 2006 11:49:33 -0700, Martin Pentreath wrote:

Hi,

I planning on descaling the DHW heat exchanger on my combi. It's an
Ariston Microgenus 27 and the rate of flow has gradually declined since
it's been installed (over three years ago). The descaling looks like an
easy enough job, and removing the heat exchanger itself is dead easy,
but before I make a complete fool of myself I've never really
understood exactly how the combi does the DHW. Do I need to drain the
boiler completely (and all the rads) or is just turning off the cold
water feed to the combi enough?

You'll need at least to isolate the primary side as well, but most boilers
have valves on the flow & return so you shouldn't have to drain the rads.

In message .com,
Martin Pentreath writes
The service manual does indeed say "the boiler must be drained of all
water", but that's all it says, it doesn't say how to do that.

.... Because not every boiler is installed in an identical fashion

There should be valves to isolate the boiler, then you have to use a bit
of grey matter to find where best to break the circuit


I wasn't
sure whether this means draining the rad circuit as well, but from what
you're saying it doesn't. There's no obvious draining point underneath
the boiler. Any advice on the usual procedure for draining just the
boiler would be very welcome.


--
geoff

In article ,
raden writes:
In message .com,
Martin Pentreath writes
The service manual does indeed say "the boiler must be drained of all
water", but that's all it says, it doesn't say how to do that.

... Because not every boiler is installed in an identical fashion

There should be valves to isolate the boiler, then you have to use a bit
of grey matter to find where best to break the circuit

....bearing in mind the pressure vessel (which is presumably in
the boiler) is going to squirt the water out either straight
in your face or over the electrics inside the boiler, if you
are not careful. There may be a drain point inside the boiler
to release the pressure and water in a more controller manner.

--
Andrew Gabriel

Thanks for all advice. I have now found and studied the water circuit
digram and am a little wiser about how combis heat the domestic hot
water. However, this boiler seems very annoying - I can isolate the
primary circuit easily at the point where the CH flow and return enter
the boiler, but I can't then find any way of draining the water from
within the boiler. I'm just going to have to drain the whole system
down and replace the inhibitor.

On Thu, 20 Apr 2006 02:28:32 -0700, Martin Pentreath wrote:

Thanks for all advice. I have now found and studied the water circuit
digram and am a little wiser about how combis heat the domestic hot
water. However, this boiler seems very annoying - I can isolate the
primary circuit easily at the point where the CH flow and return enter
the boiler, but I can't then find any way of draining the water from
within the boiler. I'm just going to have to drain the whole system
down and replace the inhibitor.

I've looked through the manual and it seems, sadly, you are right.
However you may find that you can release the pressure with a drain cock.
Then isolate and then remove the water from the boiler by opening a a
union (you won't get sprayed if you have released the pressure).

If the boiler is above the rads you have a much better than even chance of
keeping the water in the rads.

IMHO it is exactly this sort of feature (or lack of it) that is the
difference between the differing quality of boiler models.


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html

Thanks for the confirmation Ed.

Unfortunately the boiler is on the ground floor with two storeys above
it. I think I'll just take the opportunity to flush the system through
anyway while it's drained. Also while everything is empty I'm intending
to put isolators on the flow and return pipework into the boiler and a
drain cock above one of the isolators to make draining the boiler alone
possible in future.