Heat only boiler timings

RavingMad

During the week I set the water to start heating up at 5.30am, and daughter is in the shower at 6am. It usually only takes 20 mins to get to temperature in the warmer months.
Now that the heating will start to kick in too before we get up, this will take priority over heating I'm guessing or do they work together? Only got to thinking about this as I thought the water seemed a bit cooler than normal the other week (heating has only kicked in once so far)
I'll see if it happens again but was wondering if anyone knows how my boiler works (WB Greenstar if that makes any difference) and whether I need to adjust my timings (I think I up the 30 mins to 45 or even 60 over winter months)
Thanks 

MattMattMattUK

Is your water in a hot water tank or is it pre-heat on a combi boiler? If it is the former you can set it to heat the water at any time, there is no need to set it to heat the water just before someone showers 

RavingMad

Thanks Matt, it is a tank that we heat up. I guess it's the thought of heating the water up well in advance and the waste/loss of temperature (my brain is wired that way) but I guess it's only a few degrees lost if that

MattMattMattUK

RavingMad said:

Thanks Matt, it is a tank that we heat up. I guess it's the thought of heating the water up well in advance and the waste/loss of temperature (my brain is wired that way) but I guess it's only a few degrees lost if that

On the old tanks it did matter, the losses on a modern tank are very small, doing it an hour earlier will make such a tiny difference it is not worth worrying about. Also in general that heat escapes into your home warming it slightly. Set it an hour or more before and it will all be fine. 

QrizB

RavingMad said:

Now that the heating will start to kick in too before we get up, this will take priority over heating I'm guessing or do they work together? Only got to thinking about this as I thought the water seemed a bit cooler than normal the other week (heating has only kicked in once so far)
I'll see if it happens again but was wondering if anyone knows how my boiler works (WB Greenstar if that makes any difference) and whether I need to adjust my timings (I think I up the 30 mins to 45 or even 60 over winter months)

I've got a heat-only boiler (a 15kW WB Greenstar until it was replaced last week, now an 18 kW Vaillant EcoFit) and a HW tank. My central heating system is Y-plan with a 3-port valve,which means that if both the heating and the hot water call for heat simultaneously then the boiler flow is split between both.

However, my "little" 15kW WB boiler* was still big enough to feed both at once; my radiators totalled ~10kW, so even if the house was stone-cold there would still be 5kW of excess boiler cpaapcity to heat water.

I don't know how well matched your boiler is to your house, or whether you've got a cleverer control system than me that would let you set a priority, but if the designer did their job properly you should be in a similar situation and have enough boiler output that you can heat the rads and your HW tank at the same time.

* I practically never needed the full output of my boiler, ad I think I could have got away with a smaller 9kW one. I ony chose the 18kW Vaillant to replace it as my installer would've charged more for a smaller one!

Lorian

If the tank only has it's foam jacket add a red fibreglass one. If it's y plan and the water comes on at the same time as the heating it will share the heat output. Will mean your return temps will be generally lower and boiler more efficient.

I'm ops case I would start the heating at 5am to take the chill off the house before the water is heated and shower taken.

Scot_39

Boiler to hot water tank loops slowdown heat exchange to tank dramatically as the tank water approaches the boiler HW loop flow temperature.

Simple pipe theory loss / exchange is linear with delta T iirc but tank circulation makes a big difference as the graphs here show its a common mistake to think of the hot water in tge tank as being at just one temperature.

Came across this thread on another forum when so eone else asked your sort ofvquestion.

There's some detailed examples for one guys tests here on iirc two different systems - both suggest 1/2 hour a little on the short side - albeit in one case just -  to get whole tank to temp.

And tge boiler dynamics matter in terms of potential cycling.

But for a quick shower, you should only need the top of the tank heated to temp anyway.  Which would be much quicker and half hour fine for showering - but might leave you struggling for full hot later in the day.

https://forum.ovoenergy.com/home-and-heating-138/optimise-your-hot-water-set-up-17178

vacheron

It also depends if your tank is a traditional "hot water tank" or a "thermal store". Our system uses a thermal store where the main body of the tank is filled with very hot water, and hot water is obtained by by passing fresh cold water through a coil inside the tank which is heated by the hot water surrounding it, (the opposite of a hot water tank where a hot coil inside (heated by the boiler) heats the surroinding water which is then drawn from the hot water taps)

In our thermal store system the design means that hot water automatically gets "first priority" as hot water from the boiler heats the thermal store first, any water pushed out of the bottom of the thermal store then goes to the radiators (if the central heating is on).  

Some pics of the tank would help.

QrizB

RavingMad said:

Thanks Matt, it is a tank that we heat up. I guess it's the thought of heating the water up well in advance and the waste/loss of temperature (my brain is wired that way) but I guess it's only a few degrees lost if that

Heat loss from a modern hot water tank is typically 100 watts or less.

With gas around 6p/kWh, the energy loss will cost 0.6 pence per hour.

(If your HW tank is more than a decade or so old, it might have a higher heat loss than this, but it's unlikely to be more than 1p/hr.)

Scot_39

Yes many modern tank spec sheets quote size (volume) dependent losses in the 1-2 kWh per day range assuming 60C target.