Efficiency of heating DHW by gas

michaels

Sterlingtimes wrote: »

I had the iBoost installed supposedly "free of charge" although the reality is that it is built into the price. The iBoost saving in gas i seven months has been 437 kWh >>> 437/0.77 * 0.0303 = £17. That's almost inconsequential.

But with the installation of my Smart Meter my export is measured accurately and I move to actual export payments. The iBoost is probably now of dubious value.

Once export is measured, there is no financial case for buying an iBoost.

Ouch, ours is 'free' but no doubt a 250 discount could be negotiated without it that I was hoping would pay for itself in about 6-7 years but if there is a good chance of smart-metering before then then it would be a complete waste of money - I wonder what they go for on eBay.....

Sterlingtimes

michaels wrote: »

Ouch, ours is 'free' but no doubt a 250 discount could be negotiated without it that I was hoping would pay for itself in about 6-7 years but if there is a good chance of smart-metering before then then it would be a complete waste of money - I wonder what they go for on eBay.....

Having had the experience, I would today request a discount in lieu of the diverter.

tunnel

I wouldn't worry about selling the diverters just yet. It's not mandatory to have a smart meter, no one can force it on you...even after the national roll out has started. Think you'll see anyone with PV and a diverter will be refusing the upgrades.


Sterling, couldn't you just change supplier to one that doesn't do smart metering or one that Ovo's meter will become dumb, hence keeping your deemed 50% export?

michaels

Sterlingtimes wrote: »

I had the iBoost installed supposedly "free of charge" although the reality is that it is built into the price. The iBoost saving in gas i seven months has been 437 kWh >>> 437/0.77 * 0.0303 = £17. That's almost inconsequential.

But with the installation of my Smart Meter my export is measured accurately and I move to actual export payments. The iBoost is probably now of dubious value.

Once export is measured, there is no financial case for buying an iBoost.

7 months to date = 437kwh - do you think this equates to half the likely annual savings. I was hoping to use about 7kwh per day in water heating and achieving this on most days where over 10kwh is generated which I was hoping would be about 180 days a year so 1260kwh /gas efficiency (which I thought might be more like 90%) gas units per annum saved (1260/0.9*0.0303 = £42) but your experience suggests this is wildly optimistic?

Sterlingtimes

michaels wrote: »

7 months to date = 437kwh - do you think this equates to half the likely annual savings. I was hoping to use about 7kwh per day in water heating and achieving this on most days where over 10kwh is generated w ...

Yes, I made over optimistic assumptions from the outset. With the immersion mounted at the top of the cylinder, I may dump 3.0 kWh on a good day. This is increased if my son has a shower midday in which case I may get 5 kWh (a second shot at heating). But not all days are good days.

Those with a bottom mounted immersion are more successful, and some have even fitted mixer pumps to mix cold water at the bottom with hot water from the top.

For me, I think the annual saving would be £30 on the deemed export model.

michaels

Thanks. WE have top and bottom immersions so will put the bottom one on the iBoost. We run at lest one bath and 4 showers morning and evening so I was hoping if I put the immersion temperature at 70 and the gas at 50 we should never have no hot water but could store as much pv heated hot water as possible, our tank is new, highly insulated I think 210 litres.

Andy_WSM

This won't work for everyone, but I fitted a powerstream 12kW instantaneous water heater that feeds the whole house. I live alone though and am always showered before the economy 7 has switched off in the morning, so my hot water bill is pennies, probably about 15p per day. For the small amounts of hot water I need in the day for hand washing or washing dishes in the kitchen I might pay another 10p or so minus whatever the PV is generating.

I reckon my hot water bills are less than most peoples standing losses from their tanks.

pinnks

I don't think the maths is quite as simple as portrayed here. In my experience I achieve hot water over the summer using no gas compared to about £80 for heating water alone before I could turn the gas off as a result of buying the diverter. Over the winter it is less easy to determine but the diverter probably contributes about £20. So £100 per year for an investment of £ whatever you spent.


And don't ignore losses in the pipes to heat water using gas in the summer. Again, based on my experience, I need about 2Kwh to heat my water using diverted electricity but about 6kWh using gas - something to do with 20m or so of unlaggable pipes between boiler and tank perhaps...


I would certainly invest in a diverter, even with measured export, though the maths would then be quite complex. I know I have a large system but I am exporting more than 50% in summer despite diverter. In winter I export virtually nothing and get the water heated using the diverter...

phsci

Sterlingtimes wrote: »

The Ideal Classic FF270 is an 'E' rated boiler with an efficiency of 77.2%.
So what is my real efficiency likely to be?

Sterling, the quoted 77.2% is the SAP annual efficiency for your boiler; the figures for winter seasonal efficiency and summer seasonal efficiency are 78.1% and 68.0% respectively (the boiler is more efficient when it’s working hard). The efficiency for hot water production only is a mere 49.7%, and then there will be transmission losses between your boiler and the hot water storage cylinder. I believe this makes a good case for continuing to use your immersion diverter and sacrificing your export payments (if and when they are measured) in favour of an almost 100% efficient way of producing domestic hot water.
You can see these figures at the Building Energy Performance Assessment Support Website:
http://www.ncm-pcdb.org.uk/sap/pcdbdetails.jsp?pid=26&id=005931&type=105&mid=000008

For other boilers go to http://www.ncm-pcdb.org.uk/sap/index.jsp and look at “Search Database”, “Product Characteristics Database (PCDB)”, “Boilers” and then enter as much info as you can.

HappyMJ

Be careful of that hot water figure of 49.7% it is measured in test conditions and takes into account losses in the pipes between the boiler and cylinder (they use 1.5m of pipework in the test), losses from the cylinder (they use a 120 litre insulated cylinder) and losses from the tap when you run it for a period of time waiting for useful hot water to come through.