Heating and Hot water. Same time or separate?

abssorb

Hi,

I've always had combi boilers in the past, but now we've moved to a house with an oil fired system boiler with a hot water tank.

In general, when is it most efficient (in terms of fuel use) to time the hot water? At the same time as the heating, so it's sharing the same energy, or separately?

macman

It doesn't 'share the energy'. If you want to space heat and hot water heat at the same time, then it has to burn more oil to satisfy the increased demand.
It's true that any lost heat from the hot tank serves to warm the house too, but this is a marginal efficiency.
Most people set it up to ensure some hot water in the morning and evening, but if the house is occupied during the day, then you may want something different.
With a combi, you cannot heat rads and hot water simultaneously.

AndyPK

you can put the hot water on 20 - 30 minutes before the heating, this gives a good compromise. (allows time for the water tank to warm up quickly)


But if there aren't many people in the house, and want to save pennies then try putting them on together in winter

molerat

My only rule, as above, is don't have them both start from cold at the same time as it takes a bit longer to get the benefit of either if you do. Outside of that rule anything that suits your particular requirements.

tonycottee

I used a smart meter to measure how much it cost me to leave the hot water tank on all day compared to just turning it on when needed. It only cost around £2 extra.

Alter_ego

[QUOTE=tonycottee;74291745_It_only_cost_around_£2_extra.[/QUOTE]

Is that daily?

tonycottee

Alter_ego wrote: »

Is that daily?

Sorry, that was per month.

There's some more about leaving hot water on constant.

System

tonycottee wrote: »

I used a smart meter to measure how much it cost me to leave the hot water tank on all day compared to just turning it on when needed. It only cost around £2 extra.

There is no one size fits all here. I have a 250l Oso unvented cylinder that is on from 5.30 to 9am as our system is configured for hot water priority. The cylinder temperature is managed in the range 60 to 55C during the 4 hour on time. With CH off, my cylinder usually takes about 40 minutes to re-heat and uses an average of 6kWhs of gas per day.

The problem with constant re-heating is that most modern boilers are profiled to go to max flow temperature for hot water re-heating. If they didn't then on a warm day like today, the cylinder would be re-heating the HW boiler flow. Constant boiler on/off each time there is a HW demand is inefficient according to my 15 minute optical spot gas meter reader. We find that it is perfectly possible with a large cylinder to bath and shower in the evening without a water re-heat. That said, no one size suits all.

Cardew

tonycottee wrote: »

I used a smart meter to measure how much it cost me to leave the hot water tank on all day compared to just turning it on when needed. It only cost around £2 extra(per month).

All modern tanks have their loss of heat, measured to a British standard, stamped somewhere on the body(normally where you can't read it!!) Typical losses are 1kWh to 1.5kWh a day with water in the tank held at 65C. So in practice losses will be smaller than that figure.

My large unvented tank loses heat at the rate 1.3kWh a day. Which would cost around 4p a day for gas if I kept the water at 65C but in practice I believe my losses are less than £1 a month.


Also as said before the 'lost' heat isn't wasted as it warms the fabric of the house.

matelodave

My heating unit will heat either the tank or the rads, but not both at the same time. many other systems are similar depending on how the timeswitch/programmer is configured and whether the system has a tank stat, room stat and motorised valves.

We heat the tank for about an hour in the mornings before we get up - as it's a heat pump we use about 2kwh of leccy in the winter, a bit less in the summer. The hour a day gives us enough hot water for all our requirements (including a couple of showers in the mornings) and the water is still hot enough for two more showers if we want them. The secret being to minimise the time you stand in the shower and make sure that you control the flow rate.

I'd suggest that you try heating the water for an hour or so before you get up to see if there's enough hot water for your morning ablutions and see how long it lasts. If you need a bit more for the evening then put in on for half an hour or so. Just adjust your timings to suit your lifestyle

In the end there's no benefit in keeping the tank hot all day or even all night if you aren't using it but as Cardew says, a decently insulated modern tank doesn't lose all that much heat anyway so once it's hot is stays hot unless you use it all. You could always put another insulating jacket over it.

If oil costs you 4p/kwh and you lose 1.5kwh a day = 6p/day = £22 a year.

abssorb

Sorry for not returning to this thread sooner, I'm only an occasional user of this forum. Thanks for the replies, lots to think about

macman wrote: »

It doesn't 'share the energy'.

True - I worded it badly, I mean share the "same energy source" which in this case tops out at 30KW.

The problem with oil of course is that being unmetered, there is no reliable way to measure actual consumption accurately enough to do my own tests.
Also because insulation is yet to be improved (old house + inaccessible roof spaces), external temp and windchill add another variable.

Can I assume that the system boiler operates the same as a combi in that any demand for heat causes the boiler to come on flat out (to dial setting) until demand satisfied? I.e. it consumes oil at a fixed flow rate, and doesn't vary according to type of demand; space / water / both.
That would make my understanding simpler.




Then it's just about which will burn the least oil. This is probably going to be really hard to answer, but I'm interested in anyone's views
Our system is configured such that it can do heat and water at the same time. Even given the losses through insulation, both the below scenarios result in a warm house and plenty of hot water:

Scenario A
Heating comes on at 7:00am to 8:30am
Hot water comes on 7:00am to 8:00am

Scenario B
Heating comes on at 7:00am to 8:30am
Hot water comes on 6:00am to 7:00am

I've tried both over several weeks, but lack of accuracy in measuring consumption and a fluctuating windchill of up to -15degC make my results pretty meaningless.

With your systems can you notice a distinct difference in fuel consumption between A and B? Or is it pretty much the same and down to comfort preference?