Ideal Isar 35

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.
Reply to
stan
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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

Reply to
Philipj.cosson

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).

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.

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.

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.

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.

Reply to
John Rumm

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