Ideal Default DHW Temperature

MicktheR3d · Wednesday at 3:54 PM

Hi everyone,

I have an Ideal Max Heat H18 system boiler with weather compensation installed. My understanding is that while weather compensation controls the heating flow temperature based on the outdoor temperature and the setpoint at the boiler, when there’s a call for hot water, the boiler overrides weather compensation and runs at a fixed 80°C until the cylinder thermostat is satisfied.

I’d like to know if there’s any way to reduce this DHW override temperature, either:
1. Through an installer or advanced settings menu on the boiler.
2. Via the weather compensation controller, if it allows DHW modulation.
3. Using OpenTherm or an alternative controller to gain more control over DHW temperatures.

The system includes a hot water cylinder with a thermostat set to 60°C, but that only dictates when the boiler stops heating – it doesn’t change the fact that the boiler always runs at 80°C for DHW demand.

I’m looking to see if I can lower the DHW override temperature, ideally to match the heating curve more closely or to improve efficiency. Any insights or advice from those familiar with Ideal boilers and weather compensation would be much appreciated!

Thanks in advance.

Chuck · Wednesday at 10:51 PM

I don't think you want to reduce the DHW temperature. It's set to 80°C to ensure that the cylinder is heated to 60°C rapidly and without the boiler cycling. The return temperature from the cylinder, which governs the efficiency of the boiler, should gradually rise to not more than 60°C as the cold water at the bottom of the cylinder is heated by its coil.

MicktheR3d · Thursday at 8:29 AM

Chuck said:
I don't think you want to reduce the DHW temperature. It's set to 80°C to ensure that the cylinder is heated to 60°C rapidly and without the boiler cycling. The return temperature from the cylinder, which governs the efficiency of the boiler, should gradually rise to not more than 60°C as the cold water at the bottom of the cylinder is heated by its coil.

Hi Chuck, so would you suggest i don't have the CH and DHW selected to be on at the same time, currently they are?

MicktheR3d · Thursday at 8:30 AM

MicktheR3d said:
Hi Chuck, so would you suggest i don't have the CH and DHW selected to be on at the same time, currently they are?

Chuck · Thursday at 12:07 PM

I'm not sure which boiler+controls combination you have but broadly speaking...

If your installation has 'PDHW' (Priority DHW) then when the cylinder calls for heat the flow to the radiators will be stopped automatically while the flow temperature is raised so you don't need to worry.

If, however, you have 'S-plan' plumbing, i.e. CH and DHW each have their own normally-closed zone valve you need to have the tank set to heat only when the CH is off. A common programme is to have the hot water on for an hour during the second half of the night when the CH is off. With an 80°C flow and a decent cylinder it will probably take less than an hour to heat the tank. (It's possible to convert an S-plan system to an 'X-plan' PDHW system by changing the CH to a normally open ZV that closes when then DHW is running.)

Whoever installed the boiler should have taken you through the controls and made sure you knew how to set them to achieve best comfort and fuel efficiency.

MicktheR3d · Thursday at 1:44 PM

Chuck said:
I'm not sure which boiler+controls combination you have but broadly speaking...

If your installation has 'PDHW' (Priority DHW) then when the cylinder calls for heat the flow to the radiators will be stopped automatically while the flow temperature is raised so you don't need to worry.

If, however, you have 'S-plan' plumbing, i.e. CH and DHW each have their own normally-closed zone valve you need to have the tank set to heat only when the CH is off. A common programme is to have the hot water on for an hour during the second half of the night when the CH is off. With an 80°C flow and a decent cylinder it will probably take less than an hour to heat the tank. (It's possible to convert an S-plan system to an 'X-plan' PDHW system by changing the CH to a normally open ZV that closes when then DHW is running.)

Whoever installed the boiler should have taken you through the controls and made sure you knew how to set them to achieve best comfort and fuel efficiency.

Hi Chuck, i have now set the DHW & CH to not be on at the same time, i'll monitor how it performs. The tank and boiler are both in the garage and what always concerned me with having the DHW on by itself was that because they are both so close together the return temp from the tank was almost the same as the flow should i be concerned about that?

Chuck · Thursday at 5:33 PM

MicktheR3d said:
The tank and boiler are both in the garage and what always concerned me with having the DHW on by itself was that because they are both so close together the return temp from the tank was almost the same as the flow should i be concerned about that?

The return temperature is (virtually) unrelated to the distance from the boiler to the cylinder as long as the pipes are properly insulated. It is the rate at which heat is transferred from the flow to the water inside the cylinder that governs the change in temperature from flow to return. Since the DHW demand is controlled by the 60°C thermostat on the cylinder, the return temperature will rise from a low value (perhaps 30°C) to 60°C as the cylinder warms up at which point the boiler will be switched off by the cylinder thermostat. This means that the boiler should be in 'condensing mode' for a majority of the time it is heating the cylinder.

Ideal Default DHW Temperature

View the thread, titled "Ideal Default DHW Temperature" which is posted in Boiler Advice Forum on UK Plumbers Forums.

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