Combi Boiler gas consumption confusion 4 May 2021 at 7:39PM in LPG, heating oil, solid & other fuels 12 replies 222 views

60BOWENS

Hi

Our Combiboiler apart from providing central heating is also set up to provide hot water by the 300L indirect unvented cylinder.

At present boiler powers on twice a day to provide hot water and again twice a day to provide central heating-but at different times.

Hence our gas bill is higher.

I was wondering if I could get boiler timings synchronised to provide central heating AND hot water for the cylinder at the same time.

We have Worcester 38D classic combi.

Many thanks

jefaz07

A combi on an unvented y plan isn’t the norm! You would normally have a heat only or system boiler. 

With this comes a 3 port or a few zone valves. Normally these can be controlled by a programmer at the same time. 

Im not sure how your system is installed here. 

matelodave

Are you confusing combi with condensing. Combi boilers dont usually heat hot water tanks as they deliver hot water directly to the taps.
Regarding your query, heating the hot water and trying to heat the house at the same wont save you any money, it will just take longer to heat the house and the tank but you'll probably use the same amount of energy.

60BOWENS

jefaz07 said:

A combi on an unvented y plan isn’t the norm! You would normally have a heat only or system boiler. 

With this comes a 3 port or a few zone valves. Normally these can be controlled by a programmer at the same time. 

Im not sure how your system is installed here. 

Well,we do have a megaflow installed in our garage which is fed by our combiboiler.It works fine and gives us good pressure even while using showers simultaneously which was not possible with combi alone.System works.

but can you help me with my original query?

60BOWENS

matelodave said:

Are you confusing combi with condensing. Combi boilers dont usually heat hot water tanks as they deliver hot water directly to the taps.
Regarding your query, heating the hot water and trying to heat the house at the same wont save you any money, it will just take longer to heat the house and the tank but you'll probably use the same amount of energy.

At present combiboiler comes to provide central heating by the honeywell timer.Cylinder has it on timer and it fires up boiler as per how it is programmed or on demand if need be.

My question is can i have same time set up in Combi timer and Cylinder timer which should theoretically make boiler run for a set period of time only thus saving on gas consumption.

Dolor

Condensing boilers get their efficiency by running at the lowest possible flow temperature. When there is a hot water demand, the boiler flow is adjusted to its max flow temperature. If you have a S Plan system, you can run heating and hot water heating at the same time but it will not be as efficient as you might think. Most combi boilers are though Y Plan and it is either heating or hot water, and hot water has priority. 

Reed_Richards

Combi boilers are intended to heat hot water directly and so you don't need a cylinder, although there are reasons why you might prefer to have one.  I briefly had a combi boiler with a hot water cylinder to eliminate long pipe runs causing long waits for a hot tap to run hot.  I was advised that a combi boiler should retain at least one hot tap fed directly from the boiler even if all the other hot taps are fed from the cylinder.  I don't know if this was really necessary or just plumbing superstition.

Given that your central heating and hot water are controlled independently it is very likely that each has its own on/off valve so both could be open at the same time.  Some systems for heating and hot water use a single valve with two positions (i.e. 3 ports) but I think that would require a single controller that does both central heating and hot water.

Therefore I think it is highly likely that you could run both heating and hot water at the same time but your gas bill depends on the amount of heat drawn by your cylinder and by your radiators and I don't think running them simultaneously or at different times will make any difference.  [Edit:  I have assumed your central heating has some sort of thermostatic control, either a room thermostat or radiator thermostats or both.  If not then those are what you need to save on gas]

Your combi boiler will never know if it is heating the central heating or the radiators or both so some comments about boiler flow and temperature are based on a misunderstanding and wrong.       

matelodave

The amount of energy (gas) required to heat the tank will be the same whether you heat it separately or at the same time as the heatling (for a 200 litre tank , thats around 12kwh). The same with the heating, it wont use less energy to get the house up to the same temperature.(lets assume another 12kw).

I'm guessing it takes four hours or so to fully heat your 200l hot water tank from cold, as most tanks only have a 3kw coil unless it's fitted with a larger high efficiency heating coil.

Doing the sums you'll still need 24kw to heat everything to the same temperature as it was before, so sharing the output from the boiler to the heating and hot water wont save you any energy but it could take twice as long to heat them (or only get them half as hot in the same time)

The way to save energy, is to reduce your hot water temperature, dont use so much hot water (take fewer showers and dont stand in them for so long), turn your heating down and improve your insulation.

Dolor

This thread is confusing as there are now a number of combi boilers such as Intergas that can be configured to run as a system boiler. 

coffeehound

matelodave said:

I'm guessing it takes four hours or so to fully heat your 200l hot water tank from cold, as most tanks only have a 3kw coil unless it's fitted with a larger high efficiency heating coil.

A bit more than that, matelodave: no doubt varies by model, but current Megaflo 300i has nominal 24.5 kW coil.  I don't know what flow temperature that assumes.

Reed_Richards

Dolor said:

This thread is confusing as there are now a number of combi boilers such as Intergas that can be configured to run as a system boiler. 

But according to the OP

60BOWENS said:

We have Worcester 38D classic combi.

In fact any combi boiler can be configured as a "system" boiler by treating the cylinder as a second zone on the central heating.