Cheaper to leave CH on 24 hrs?

[reg091]

As we have TRVs on most rads, and the other rads are turned off permanently I am wondering about the most efficient (cheapest!) way of running the system. (There is no room stat).

My assumption on how it works is that the gas heats up the CH water to the temp set on the boiler dial, the pump sends it around the system, and the temp of the returning water is sensed. Once the temp of the return is hot enough the flame is extinguished. Is that correct?

Does the pump run continuously so that can keep being checked?

So does that mean that with most of the rads either off or TRVs set to low the water pretty much by-passes most of them and returns more quickly to the boiler, and therefore is more likely to return hotter, quicker?

So if I leave the CH on 24/7 then the cost involved is the continuous running of the pump, and the bursts of flame to keep the water at the temp set. I am wondering if that is cheaper than to to have it come on, say, every two hours and have to heat the water up from cold again every time?

Any thoughts appreciated.

 

[Agile]

So if I leave the CH on 24/7 then the cost involved is the continuous running of the pump, and the bursts of flame to keep the water at the temp set. I am wondering if that is cheaper than to to have it come on, say, every two hours and have to heat the water up from cold again every time?

Any thoughts appreciated.

Why are so many people unable to understand the costs of running heating?

When heating a house you are supplying the heat loss of the house, that's often about 10 kW.

So for every hour you use 10 kWh.

For each hour the heating is off you save 10 kWh

Tony

 

[JohnD]

I am wondering about the most efficient (cheapest!) way of running the system. (There is no room stat). .

Get a room stat then.

So if I leave the CH on 24/7 then the cost involved is the continuous running of the pump, and the bursts of flame to keep the water at the temp set. I am wondering if that is cheaper than to to have it come on, say, every two hours and have to heat the water up from cold again every time?

No.

 

[D_Hailsham]

There is no room stat.

Get one installed.

My assumption on how it works is that the gas heats up the CH water to the temp set on the boiler dial, the pump sends it around the system, and the temp of the returning water is sensed. Once the temp of the return is hot enough the flame is extinguished. Is that correct?

When the boiler lights the pump starts, water circulates and temperature rises. When the water temperature leaving the boiler has reached the set temperature, which is not immediately as heat is being removed (rads) and added (boiler) at the same time, the boiler will stop until the temperature has dropped a few degrees, then it starts again. The temperature of the return will depend on how much heat has been extracted from the water by the radiators.

Does the pump run continuously so that can keep being checked?

The pump only runs while the boiler is alight and (on most boilers) for a few minutes after the boiler stops.

So does that mean that with most of the rads either off or TRVs set to low the water pretty much by-passes most of them and returns more quickly to the boiler, and therefore is more likely to return hotter, quicker?

The rate at which the water circulates is determined by the pump. Unless you have one of the latest variable speed pumps, the flow rate actually slows down when TRVs close down as the pressure loss round the circuit increases. If there is less heat required by the rads the boiler will either modulate down or turn off.

 

[D_Hailsham]

When heating a house you are supplying the heat loss of the house, that's often about 10 kW.

So for every hour you use 10 kWh.

For each hour the heating is off you save 10 kWh

The house may need 10 kW to raise the temperature from -1C to 21C, but it doesn't need that all the time. If it did, the boiler would run continually at full output all the time the heating was on.

Say the house has reached the required temperature and the room stat turns the boiler off. The temperature drops, say 2C due to hysteresis, and the boiler lights. It will only require 2/22 x 10kW to bring the house back up to temperature again.

 

[Agile]

When the boiler lights the pump starts, water circulates and temperature rises. When the water temperature leaving the boiler has reached the set temperature, which is not immediately as heat is being removed (rads) and added (boiler) at the same time, the boiler will stop until the temperature has dropped a few degrees, then it starts again. The temperature of the return will depend on how much heat has been extracted from the water by the radiators.

Does the pump run continuously so that can keep being checked?

The pump only runs while the boiler is alight and (on most boilers) for a few minutes after the boiler stops.

If the room stat is still demanding heat and the boiler has turned off because the set flow temperature has been reached then the pump will continue to run until the boiler fires up again!

Tony

 

[JohnD]

or in this case, as he has no room stat, the pump will run continually until the timer stops it. Or for 24 hours a day if not timed.

 

[Agile]

The house may need 10 kW to raise the temperature from -1C to 21C, but it doesn't need that all the time. If it did, the boiler would run continually at full output all the time the heating was on.

The house needs a continuous heat source equalling the actual heat loss ALL the time if the design 21 C is to be maintained.

During warm up more power is required and that's provided by the approximately 10% extra included by the heat loss calculations.

The calculators don't give you the actual heat loss but a figure about 10% higher to cover warm up. Once warmed up only 90% of the calculator's figure.

Most systems should be within about 1 C after an hour from turning on from cold.

Tony

 

[reg091]

So if I leave the CH on 24/7 then the cost involved is the continuous running of the pump, and the bursts of flame to keep the water at the temp set. I am wondering if that is cheaper than to to have it come on, say, every two hours and have to heat the water up from cold again every time?

Any thoughts appreciated.

Why are so many people unable to understand the costs of running heating?

When heating a house you are supplying the heat loss of the house, that's often about 10 kW.

So for every hour you use 10 kWh.

For each hour the heating is off you save 10 kWh

Tony

Understood that for each hour the heating is off I save money! But my point is that the temp will drop considerably during that hour thus surely requiring the boiler to fire for a considerable time to bring it back to temp when it is timed on again.

If it was on 24/7 then would the regular short "bursts" of boiler firing to maintain temp add up to more or less ££ than if it had been required to heat from cold after every few hours?

 

[jackthom]

You house is losing heat through walls/roof etc all of the time. The warmer the inside of the house is kept, the more heat will be lost and the more money this will cost to replace by burning gas.

Allowing the house to remain cooler for at least part of the time will save money.

 

[Agile]

It will use more power being on 24/7.

That should be ( fairly ) obvious!

If that's difficult for you then just ponder that during normal heating the boiler is warming up the returning water from say 60 C to say 75 C all the time. Any normal rad circuit will warm up from cold within 10-20 minutes.

 

[kbdiy]

Along with all this you also need to consider how good your insulation is - loft, walls, pipework?

 

[Onetap]

Along with all this you also need to consider how good your insulation is - loft, walls, pipework?

Insulate, insulate.
Room temperature sensor (thermostats are so neolithic) in a programmable controller.
Insulate some more.
Outside temperature sensor and weather compensation controller, if one is available for your boiler. Probably an entire control package is available, so you won't need a different programmable controller.
Insulate.

The heating costs are least when the heating is off.
There is some columnist on the Sunday Telegraph who advocates leaving the heating on continously. He's a builder and knows very little about heating.


Did I mention? Insulate.

 

[D_Hailsham]

The house may need 10 kW to raise the temperature from -1C to 21C, but it doesn't need that all the time. If it did, the boiler would run continually at full output all the time the heating was on.

The house needs a continuous heat source equalling the actual heat loss ALL the time if the design 21 C is to be maintained.

You need the full output to raise the temperature from cold (-1°C) to the required temperature (21°C), but you do not need the full output to raise the temperature by 1°C (20°C to 21°C). You just need to replace the heat lost. So if you needed 10kW for a 22°C rise, that's 10/22 kW per degree, then if the stat switched on after a 1°C drop you will need 10/22 kW to bring the house back to 21°C.

I know that, in the days of on/off boilers, whenever the boiler was alight it pushed out the maximum amount of heat every time it was on, but that no longer happens with modulating boilers. It could easily be running at minimum output for the few minutes it takes to bring the temperature back for 20°C to 21°C.

During warm up more power is required and that's provided by the approximately 10% extra included by the heat loss calculations.

The 10% extra is only included to speed up the warm up, it's not essential. That's why you would not allow any extra in a house which is permanently occupied, e.g a retired couple, but 10-15% when the occupants are at work most of the day.

OK, the extra helps when the external temperature drops below the design value.

 

[Agile]

The 10% extra is only needed when its -1° C outside!

When its warmer then much more spare power is available.