Boiler doesn't start when Heating on + HIVE

[DarrenR]

Hi

I had HIVE in my old house so thought I'd buy it again for the new house. I've only had combi boilers in my houses but the new house has an oil boiler on ground floor (grant euroflame 70/90), hot water tank (immersion) on the 1st floor and two tanks in the loft.

I've had HEATING on , in the Honeywell programmer and the thermostat set to full for an hour now. I can feel the pump working but the boiler isn't making a noise and no pipes are getting hot. When i turn the HW on , the boiler fires.

It would appear to me - from the wiring and testing - that the boiler only starts when the hot water is on. Why would anybody want to heat water whenever the heating is on? Like I say ive never had this type of heating system before. The shower is an electric shower, so we only use hot water for washing hands and washing up.

Also will HIVE ever work in this scenario without OFF wires?

Thanks

 

[flameport]

Hive can be used with that.
Hive unit replaces the programmer.
The Heating On connection must go directly to the pump brown wire. Wiring to the old thermostat disconnected and not used.
HW On connects direct to the boiler as it does now.
Off terminals not used.

Hive must be set to Gravity mode, which means you can only have hot water only or hot water and heating.
There is no heating only option with that system.

 

[ericmark]

When would one not want DHW but want CH? Much depends on pipe work, if done correctly when pump is running very little water goes through the heat exchanger, but the C Plan has been the standard for oil boilers for years.

The problem is in the summer your DHW is heated only by time, better if it uses a tank thermostat, and the non volt free contacts of Hive make that a problem.

 

[DarrenR]

Thanks flameport.

Makes total sense and what i would have guessed after creating the wiring diagram. Didn't know such a setup existed. Should there be a way to control the temp of the water?

 

[DarrenR]

I had to do some Googling then ericmark !!

"The C Plan is not used in heating systems anymore but can still be found in older systems."

I guess having the DHW being heated when I only want the CH isnt actually using anymore oil..

Thanks

 

[flameport]

Should there be a way to control the temp of the water?

Not with the arrangement as it is. The only control is the temperature of the boiler, which will also be the temperature of the water in the radiators.

A thermostat for the hot water could be added, but that will require additional components and some plumbing alterations.

 

[ericmark]

There are three C Plans each one slight improvement to the last, the first one in the summer only control of domestic hot water is the time the boiler runs, I have my boiler run 3 times a week for ½ hour each time, it does not actually run for ½ hour more like 20 minutes as the boiler cuts out on temperature before time has lapsed. But water is not fully heated in that time, but warm enough to wash hands.

With the tank thermostat it means in summer you can just leave boiler on DHW only and it fires up when required, without the tank thermostat it would mean the boiler cycling on/off many times a day.

Adding the motorised valve means you can also use the tank thermostat in the winter, so DHW can be cooler than central heating water, however my plumber advised against it, as it means no where for the hot water to go when boiler turns off.

Likely the Y Plan is best as default is still domestic hot water, and boiler can cool by heating the DHW, however over summer I use very little oil, I can't see any drop in oil level in the tank, although clearly I must use some, so not really worth the effort threading the extra cables required.

Nest Gen 3 is not the best control, but it does allow ½ hour slots, 3 days a week.