Should I use immersion heater or gas boiler for hot water with solar panels?

thegentleway

Now we're getting more sunshine , my panels are producing excess energy so I wonder if we should be using immersion heater (we have a pressurised cylinder) instead of gas boiler to heat up our water tank?
I'm with Octopus Flux: excess is sold back to grid at 13p kWh and gas is 5.9p kWh so presumably it's cheaper to use gas boiler?

QrizB

thegentleway said:

I'm with Octopus Flux: excess is sold back to grid at 13p kWh and gas is 5.9p kWh so presumably it's cheaper to use gas boiler?

Using gas will almost certainly cost you less; your boiler would have to be worse than 50% efficient before gas cost you more.

FreeBear

QrizB said:

thegentleway said:

I'm with Octopus Flux: excess is sold back to grid at 13p kWh and gas is 5.9p kWh so presumably it's cheaper to use gas boiler?

Using gas will almost certainly cost you less; your boiler would have to be worse than 50% efficient before gas cost you more.

Depending on water temperature, a modern gas boiler will be between 75% and 85% efficient. So yes, gas will be cheaper. But CO2 and NOX emissions will be through the roof compared to using the immersion heater.

If you are just topping up the DHW tank to counter heat losses, boiler efficiencies & emissions will be much worse as the boiler will only be firing up for short periods. On that basis, I'd probably opt for solar panels - You'd only need 1-2kWh to maintain the DHW temperature per day.

thegentleway

FreeBear said:

QrizB said:

thegentleway said:

I'm with Octopus Flux: excess is sold back to grid at 13p kWh and gas is 5.9p kWh so presumably it's cheaper to use gas boiler?

Using gas will almost certainly cost you less; your boiler would have to be worse than 50% efficient before gas cost you more.

Depending on water temperature, a modern gas boiler will be between 75% and 85% efficient. So yes, gas will be cheaper. But CO2 and NOX emissions will be through the roof compared to using the immersion heater.

If you are just topping up the DHW tank to counter heat losses, boiler efficiencies & emissions will be much worse as the boiler will only be firing up for short periods. On that basis, I'd probably opt for solar panels - You'd only need 1-2kWh to maintain the DHW temperature per day.

Good point about emissions.

My DHW isn't on all the time though, it's on a schedule a couple of times a day at the same time as heating so should be doing most of the heating at decent efficiency.  

Veteransaver

Your boiler may be claimed 85%+ efficiency, although in my experience hot water heating is generally far lower, there's lots more losses in the pipework so actually it tends to be closer to 50-60%. Also the boiler doesn't operate in condensing mode much when it is heating water due to the high return temperature.
Now obviously in winter those losses actually gomto heat the house too so it isn't totally wasted, but in summer it is.
So using the immersion, especially if you only need a small amount of hot water in the top of the cylinder for a shower for example, immersion can actually be cheaper.

thegentleway

Veteransaver said:

Your boiler may be claimed 85%+ efficiency, although in my experience hot water heating is generally far lower, there's lots more losses in the pipework so actually it tends to be closer to 50-60%. Also the boiler doesn't operate in condensing mode much when it is heating water due to the high return temperature.
Now obviously in winter those losses actually gomto heat the house too so it isn't totally wasted, but in summer it is.
So using the immersion, especially if you only need a small amount of hot water in the top of the cylinder for a shower for example, immersion can actually be cheaper.

Very interesting. 

Presumably when the heating and hot water is on then the return temperature is lower? and the boiler more efficient?

Summer, heating isn't on so immersion might be cheapest. 

michaels

FreeBear said:

QrizB said:

thegentleway said:

I'm with Octopus Flux: excess is sold back to grid at 13p kWh and gas is 5.9p kWh so presumably it's cheaper to use gas boiler?

Using gas will almost certainly cost you less; your boiler would have to be worse than 50% efficient before gas cost you more.

Depending on water temperature, a modern gas boiler will be between 75% and 85% efficient. So yes, gas will be cheaper. But CO2 and NOX emissions will be through the roof compared to using the immersion heater.

If you are just topping up the DHW tank to counter heat losses, boiler efficiencies & emissions will be much worse as the boiler will only be firing up for short periods. On that basis, I'd probably opt for solar panels - You'd only need 1-2kWh to maintain the DHW temperature per day.

WE use about 20-24kwh of hot water per day.  Probably would need up to 1kwh just to cover losses.

Veteransaver

thegentleway said:

Veteransaver said:

Your boiler may be claimed 85%+ efficiency, although in my experience hot water heating is generally far lower, there's lots more losses in the pipework so actually it tends to be closer to 50-60%. Also the boiler doesn't operate in condensing mode much when it is heating water due to the high return temperature.
Now obviously in winter those losses actually gomto heat the house too so it isn't totally wasted, but in summer it is.
So using the immersion, especially if you only need a small amount of hot water in the top of the cylinder for a shower for example, immersion can actually be cheaper.

Very interesting. 

Presumably when the heating and hot water is on then the return temperature is lower? and the boiler more efficient?

Summer, heating isn't on so immersion might be cheapest. 

Depends, the flow and return to the cylinder is usually 22mm and completely unrestricted so it does take most of the flow from the boiler and consequently the return temperature is very high. Many modern gas boilers also recognise that the system is calling for hot water so raise the flow temp too.
You'll find if you put the heating and hot water on at the same time the radiators will generally be much slower to heat up.

subjecttocontract

We have solar panels and GCH. My solar panels heat my DHW via the immersion heater so on sunny days we have plenty of hot water. I've set my GCH to heat the DHW between 16.30 & 18.30 every day. On sunny days it doesn't heat the water cos it's already hot and on dull days it tops up the temperature.

FreeBear

Veteransaver said:

thegentleway said:

Veteransaver said:

Your boiler may be claimed 85%+ efficiency, although in my experience hot water heating is generally far lower, there's lots more losses in the pipework so actually it tends to be closer to 50-60%. Also the boiler doesn't operate in condensing mode much when it is heating water due to the high return temperature.
Now obviously in winter those losses actually gomto heat the house too so it isn't totally wasted, but in summer it is.
So using the immersion, especially if you only need a small amount of hot water in the top of the cylinder for a shower for example, immersion can actually be cheaper.

Very interesting. 

Presumably when the heating and hot water is on then the return temperature is lower? and the boiler more efficient?

Summer, heating isn't on so immersion might be cheapest. 

Depends, the flow and return to the cylinder is usually 22mm and completely unrestricted so it does take most of the flow from the boiler and consequently the return temperature is very high. Many modern gas boilers also recognise that the system is calling for hot water so raise the flow temp too.
You'll find if you put the heating and hot water on at the same time the radiators will generally be much slower to heat up.

Most heating systems will have a three way valve to divert flow to the DHW and give it priority over heating.

Scot_39

thegentleway said:

FreeBear said:

QrizB said:

thegentleway said:

I'm with Octopus Flux: excess is sold back to grid at 13p kWh and gas is 5.9p kWh so presumably it's cheaper to use gas boiler?

Using gas will almost certainly cost you less; your boiler would have to be worse than 50% efficient before gas cost you more.

Depending on water temperature, a modern gas boiler will be between 75% and 85% efficient. So yes, gas will be cheaper. But CO2 and NOX emissions will be through the roof compared to using the immersion heater.

If you are just topping up the DHW tank to counter heat losses, boiler efficiencies & emissions will be much worse as the boiler will only be firing up for short periods. On that basis, I'd probably opt for solar panels - You'd only need 1-2kWh to maintain the DHW temperature per day.

Good point about emissions.

My DHW isn't on all the time though, it's on a schedule a couple of times a day at the same time as heating so should be doing most of the heating at decent efficiency.  

Old gas boiler plants at power stations - may be significantly less efficient of course.

And it's still not unusual for that to dominate the daily electirc mix - due to winds inherent unreliability.

And dont forget the c6-7% distribution losses on electric too.  Although that may have dropped a couple of percent over the years.  

But may perversely now by increasing again due to distances and for current and planned future HVDC links including the AC DC and DC to AC conversions.  But thats mainly for renewables gen - so not in the Co2 mix calcs