Oil boiler questions

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  • Oil boiler questions
  • andyl

    Got the keys to our new (rental) house this week so moving stuff in.

    It has a new WB Heatslave 25/32 condensing boiler so a pretty hefty model (irritatingly the shower is electric though) but I am a bit confused by the set up.

    There are TRVs on all rads except the towel rail in the bathroom.

    There is no thermostat anywhere

    The controller is on the front of the boiler and seems to be intended for a non-combi boiler.

    The other day the boiler just fired up by itself with the heating side of the controller showing off and the hot water side being set to on.

    My first thoughts were that it has some kind of small tank inside for instant hot water and when it’s left to ‘on’ it will keep the water in this tank hot. I turned the water side to ‘off’ and the next day we still get hot water as the boiler is firing up as a combi should.

    The instruction book doesn’t mention combis at all so I am at a loss to what is going on.

    My main concern is how to make the system as efficient as i can without a thermostat? Will it shut down when it detects the flow back to the boiler is too warm – ie if all the room TRVs close off as they are up to temp leaving just the small bathroom towel rail.


    this is it: http://www.worcester-bosch.co.uk/homeowner/boilers/oil-boilers/greenstar-heatslave-1825/controls

    Seems to mention there should be a thermostat. Am wondering if the installer left it out as it doesnt seem to have any ‘link’ lights for a wireless one.


    The heatslave I had did indeed have a small HW resovoir internal to the boiler, I would guess about 10 gallons. If it’s run without a room stat the boiler limit stat should kick in once the temperature in the CH system is at set point. From memory mine had 2 adjustable limit stats on the front panel, one for HW and one for CH. It worked really well, no waiting for a combi to heat the HW up to temp as it was already hot in the storage tank. Seem to remember that you had to limit the rate you filled a bath though as the water would gradually get cooler as the boiler struggled to keep up!


    Without looking at the link, that boiler is probably one of those that have a tank of hot water inside the boiler. Take off the front cover, and it is on the left hand floor side.

    A room thermostat will certainly save you some fuel, usually £100ish a year for a typical house. As you say, the boiler reads the outgoing temperature, and return, and compares them. When they reach parity minus 10 degrees, or whatever it is set to, it will turn off, or more likely just turn off the burner, and leave the pump running.

    The non-TRV rad is a safety feature to ensure there is always somewhere for hot water to go.


    I’m familiar with old systems (and their quirks) and have just installed a new WB gas combi with the Honeywell 927 controller in my flat I rent out (I really want the same controller in this place 🙁 ) but this confused me a bit as I couldn’t work out why there is no thermostat. In my flat I have the honeywell in the hallway and the rad in the hallway doesnt have a TRV.

    I found the old model (heatslave II is now the current model as of September) and it states that:

    A room temperature controller must be fitted to control the central heating. Refer to instructions
    supplied with the thermostat for information on siting and setting.”

    The only place the thermostat could be is in the bathroom but it’s not there. Will have a look in the drawer with all the manuals tomorrow and see if there is a thermostat that’s not been wired in.


    dont call my wife an old boiler !


    Oil fired combis heat a small store of primary water so that the hot water demand can be satisfied instantly. Unlike a gas combi which has a small amout of primary water in them oil boilers have large water jackets which take a few minutes to warm up to be of any use fro domestic water.


    so it’s worth setting the water side of the timer to be on around times we will need water? ie morning and evening so we have instant hot water or just it on to keep itself topped up with heat?

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