[stan]

I have had my back boiler and storage tank replaced with an Ideal Isar
35combi-boiler. Previously I had a pressure pump serving my bath and
shower via a thermostatic mixer unit. My installer told me that the
Isar would not cope with the flow rate of the pump and removed it.
The bath water now delivers about a litre of hot water, with the valve
selected to fully hot then runs very cool for half of the bath full. It
then runs hot but to get the bath water hot I have now to drain off the
cold water in the bath and start all over. Even then I cannot
guarantee that the hot flow will continue. I now run the bath until the
water gets cold then I run the hot tap in the wash basin until the
water gets hot, then continue running the bath. Sometimes I have to
repeat this procedure several times. The installer says that this is
normal for these boilers, so I called in the Ideal Heating engineer.
He confirmed that I have sufficient flow and pressure and that I have a
problem. His solution is to remove the thermostatic mixer and fit
conventional taps. Is it possible that I can get a back flow from the
cold supply at the mixer back to the boiler, which now supplies this
cool water back to the bath. The wash basin is fed by the same system
that runs the bath and generally has slightly higher temperature than
the bath when this is running cool.
I am reluctant to replace the mixer (which I like) and fit a separate
shower circuit, unless I can be assured with a logical explanation,
that this will definitely cure the problem. Has any one else had this
problem and have solved it. The central heating system is OK.

[[email protected]]

I have the same boiler - it works fine with a normal mixer tap (non
thermostatic). I would replace the mixer tap on the bath.

Philip

[John Rumm]

stan wrote:

I have had my back boiler and storage tank replaced with an Ideal Isar
35combi-boiler. Previously I had a pressure pump serving my bath and
shower via a thermostatic mixer unit. My installer told me that the
Isar would not cope with the flow rate of the pump and removed it.

More to the point you are not allowed to pump water from the cold main
which is what you would in effect be doing if you attempted to do with a
pump on the output of the combi.

(Given where you were starting from, it may have made more sense if the
installer left the hot cylinder and pump in place and configured the
boiler to heat that in the conventional way, rather than switching it to
the combis hot water output).

The bath water now delivers about a litre of hot water, with the valve
selected to fully hot then runs very cool for half of the bath full. It
then runs hot but to get the bath water hot I have now to drain off the
cold water in the bath and start all over. Even then I cannot
guarantee that the hot flow will continue. I now run the bath until the
water gets cold then I run the hot tap in the wash basin until the
water gets hot, then continue running the bath. Sometimes I have to
repeat this procedure several times. The installer says that this is
normal for these boilers, so I called in the Ideal Heating engineer.
He confirmed that I have sufficient flow and pressure and that I have a

I am not convinced by your installers explanation....

I have the same boiler, and it will run both our thermostatic mixer
shower, and the older standard mixer valve one without any difficulties.

One obvious thing to check would be that he has not reversed the hot and
cold feeds to the mixer - this would certainly cause odd behaviour from
the mixer.

Another thing to watch with combi boilers in general is that most
(including the Isar) don't have any form of flow regulation. Hence if
you attempt to draw hot water to quickly from them, then they will not
be able to maintain the temperature of the water. This is the phenomenon
that leads many people to believe that they combi's can only deliver
tepid water. The solution is to turn the tap down a bit so that the
water gets longer to heat in the boiler (or partially turn off the cold
water inlet valve at the boiler to reduce the maximum flow rate). Your
35kW boiler will be capable of heating about 15 lpm of water to a usable
temperature at this time of year. If you ask for water faster than that
it will get proportionately cooler.

problem. His solution is to remove the thermostatic mixer and fit
conventional taps.

Some mixers may have a problem with combis - it depends how they
regulate the temperature. One of the difficulties is the above mentioned
flow rate problem - if the mixer attempts to increase the temperature do
the mix by asking for more hot, it may just result in the temperature of
the available hot water *falling*. The second difficulty is that with
many installs the cold main flow rate may not be able to supply water at
a rate significantly above that which a reasonably powerful combi can
use it. Hence you can get a situation where if you attempt to cool a mix
by adding more cold, the demand for cold causes the hot flow rate to be
reduced as well - this then raises the temperature of the hot in direct
proportion, resulting in no overall change! (a solution here is to limit
the maximum temperature using the hot water temperature control on the
boiler). Thermostatic mixers will usually cope with this second problem
ok, but it can leave a large "dead zone" in the controls on a manual
mixer, with most of the temperature adjustment being squashed into a
small part of the total adjustment range of the mixer.

Is it possible that I can get a back flow from the
cold supply at the mixer back to the boiler, which now supplies this
cool water back to the bath.

Not really - bot your hot and cold supply will be at the same pressure
since they are both now derived from the incoming cold main.

The wash basin is fed by the same system
that runs the bath and generally has slightly higher temperature than
the bath when this is running cool.

This could be down to the thermostatic valve on the bath mixing the temp
down, or simply that the basin tap is unable to demand enough flow rate
from the boiler to cause a drop off in the temperature it is able to
deliver water at.



--
Cheers,

John.

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