Heat only vs Combi boilers control- set flow temperature

[tellme_why]

Hi,

Combi boilers have two knobs to set CH and HW temperatures. This way one can keep CH temp low to say 50C and condense for high efficiency, and HW temp higher say 60C.

Is there a way I can have two temp for heat only boilers activated by an external switch or openterm? I only start the boiler for HW before shower so I can increase supply temp to 65 to heat up the water cylinder and return to 50 for the rest of the day to supply the radiators.

I could not find any supplier that does manage openterm for heat only boiler switching CH/HW temp.

Many Thanks

 

[Harry Bloomfield]

I could not find any supplier that does manage openterm for heat only boiler switching CH/HW temp.

My Vaillant (not opentherm) open vented, runs with separate temperatures for CH v HW. It does this by knowing which it is heating, and only heating one at a time. It uses Vaillant controls.

 

[Justin Passing]

What's the make and model of your boiler? Call the mfr. There may be third party ones as well, but...

 

[Mottie]

My Vaillant (not opentherm) open vented, runs with separate temperatures for CH v HW. It does this by knowing which it is heating, and only heating one at a time. It uses Vaillant controls.

I have a Vaillant without the Vaillant controls so only one temperature output for heating and hot water but with the Vaillant controls, if it only heats one at a time, does that mean that on a cold winters evening, if someone has a bath and empties the hot water cylinder, does the heating go off until the call for hot water has been satisfied? How long could that take - heating a cylinder of hot water from cold could be at least 30 minutes, couldn’t it?

 

[Harry Bloomfield]

I have a Vaillant without the Vaillant controls so only one temperature output for heating and hot water but with the Vaillant controls, if it only heats one at a time, does that mean that on a cold winters evening, if someone has a bath and empties the hot water cylinder, does the heating go off until the call for hot water has been satisfied? How long could that take - heating a cylinder of hot water from cold could be at least 30 minutes, couldn’t it?

Well, around 20 minutes-ish. If the place is up to temperature when it tries to serve the HW, the 20 minutes with no CH is hardly noticeable. It only becomes noticeable, first thing on a morning, on a really cold morning, if the CH is delayed by a demand for HW. The simple way around it, is to simply set HW to come on before, or well after a likely demand for CH.

 

[tellme_why]

My Vaillant (not opentherm) open vented, runs with separate temperatures for CH v HW. It does this by knowing which it is heating, and only heating one at a time. It uses Vaillant controls.

thanks - is it a heat only or combi boiler?

 

[Harry Bloomfield]

thanks - is it a heat only or combi boiler?

Heat only, open vented.

 

[tellme_why]

I have a Vaillant without the Vaillant controls so only one temperature output for heating and hot water but with the Vaillant controls, if it only heats one at a time, does that mean that on a cold winters evening, if someone has a bath and empties the hot water cylinder, does the heating go off until the call for hot water has been satisfied? How long could that take - heating a cylinder of hot water from cold could be at least 30 minutes, couldn’t it?

7 minutes with my potterton. I tap my app boots, after 7ish mins I check the temperature on a chippie digital meter to see 60+ - have the shower and the switch off once done.. saving a fair amount of gas..

 

[tellme_why]

Heat only, open vented.

can you share the model/type of boiler and controls?

 

[Mottie]

7 minutes with my potterton. I tap my app boots, after 7ish mins I check the temperature on a chippie digital meter to see 60+ - have the shower and the switch off once done.. saving a fair amount of gas..

Two of us in the house, gas hob for cooking, numerous hot water hand washing throughout the day, both of us have a shower every night, occasional baths. Hot water is on 24/7. When the heating is not used, we average 37p a day gas usage so personally, I CBA with switching the hot water on and off or even putting it on a timer. I might come home from work or the allotment in the middle of the day and want to jump straight in the shower so it’s nice to know I always have a cylinder full of hot water at any time of the day.

 

[Harry Bloomfield]

It's a Vaillant Ecofit Pure 418, to which I added VRC470f, weather compensated controls. The 'f' version, is the wireless version of the temperature sensor/display control unit. The complete kit includes heat only boiler, a wireless receiver which goes inside the boiler, a unit [1} which goes in the airing cupboard to control the 3-port valve, a wireless outdoor unit for outdoor temperature, and the 470f.

Rather than an on/off stat on the cylinder to control HW temperature, it uses a thermocouple to precisely measure the temperature of the cyclinder, thus when it nears the set temperature it can ramp the boilers heat down. Rather than an on off room stat, it measures and reports the precise room temperature, and can ramp the boilers output down, to avoid overshoot, that behaviour, also avoids much of the creaking of pipes, as they heat up and cool down.

The only extra wiring needed, was a pair of ebus wires, linking 1 to the boiler, and a three core between boiler and pump, when I swapped from a basic heat only boiler.

 

[Harry Bloomfield]

Two of us in the house, gas hob for cooking, numerous hot water hand washing throughout the day, both of us have a shower every night, occasional baths. Hot water is on 24/7. When the heating is not used, we average 37p a day gas usage so personally, I CBA with switching hot water on and off.

Similar here, but the other point to consider is unecessary wear and tear on the boiler, from constantly firing up, everytime you use a few pints of hot water.

 

[Mottie]

Similar here, but the other point to consider is unecessary wear and tear on the boiler, from constantly firing up, everytime you use a few pints of hot water.

Why would it do that - I don’t have a combi? I have a Vaillant Ecotec Plus 418. Sealed system with unvented hot water. It only fires up once or twice a day for hot water.

 

[tellme_why]

It's a Vaillant Ecofit Pure 418, to which I added VRC470f, weather compensated controls. The 'f' version, is the wireless version of the temperature sensor/display control unit. The complete kit includes heat only boiler, a wireless receiver which goes inside the boiler, a unit [1} which goes in the airing cupboard to control the 3-port valve, a wireless outdoor unit for outdoor temperature, and the 470f.

Rather than an on/off stat on the cylinder to control HW temperature, it uses a thermocouple to precisely measure the temperature of the cyclinder, thus when it nears the set temperature it can ramp the boilers heat down. Rather than an on off room stat, it measures and reports the precise room temperature, and can ramp the boilers output down, to avoid overshoot, that behaviour, also avoids much of the creaking of pipes, as they heat up and cool down.

The only extra wiring needed, was a pair of ebus wires, linking 1 to the boiler, and a three core between boiler and pump, when I swapped from a basic heat only boiler.

that's the setup I ned excepted for the wiring between the boiler and the S system wiring box in thee cupboard.. cannot find a wireless. What water cylinder do you have to fit the temperature sensor? Do you have an app to start/stop/boost manually with your phone?

 

[Harry Bloomfield]

Why would it do that - I don’t have a combi - I have a Vaillant Ecotec Plus 418. It only fires up once or twice a day for hot water.

Are you sure it only fires up once or twice per day? It won't fire up as frequently as a combi, but if there is significant use of HW from the cylinder, the boiler will be triggered to make up for what has been used - mine does, when HW is set to on. Run a bowl of HW for washing up, and minutes later, the boiler will fire up.