Heating hot water cylinder -gas or economy7?

sheenas

The efficiency of the boiler depends on age. A modern boiler will be 95% efficient.

michaels

matelodave said:

FreeBear said:

QrizB said: Also, if your gas tariff is Octopus Tracker, you're currently paying around 5p/kWh which means there's even less chance of saving money with electric hot water heating.

But... You also need to factor in the energy that the boiler uses to heat up the water circulating around the system before it gets to the water tank. If the boiler is just firing up to top up the temperature rather than heating a full tank from cold, electric could well work out cheaper.

I'm guessing that just heating a hot water tank from a gas boiler is probably only around 50-60% efficient by the time that overall system losses are taken in account (boiler start-up, pump running, plumbing heat losses etc) v 100% efficiency for an immersion. Boilers are very inefficient when they start up - most of the heat goes out the flue until they have warmed up and settled down, thats why combi boilers aren't everso good for short draw offs of hot water.

I'd suggest that without doing complex sums, the break even point is probably around 10-11p/kwh for leccy compared to using a gas boiler at around 7p/kwh+50% = 10.5p/kwh.

Hmm - if the OP is reheating the tank at night that suggests a longer burn, perhaps 30 minutes rather than an on/off top up burn so the start up efficiency is less relevant. Against that the water temp in the heating coil probably needs to be >70 to achieve a tank temp of 60 so the boiler is probably not operating not condensing mode.  Not sure if 80% might not be a more realistic figure.

Higher gas efficiency could be achieved by storing hot water in the cylinder at 50C with a flow temp from the gas boiler of less than 60 to achieve condensing operation levels of boiler efficiency (95%?) and then using a weekly burst of the immersion to heat to 60C for a legionella cycle as happens with a heat pump.

[As an aside our electricity night rate is 6.7p per unit and we heat the cylinder to 65-70C on electric to maximise heat stored energy stored]

FreeBear

sheenas said:

The efficiency of the boiler depends on age. A modern boiler will be 95% efficient.

Whilst a modern condensing boiler might hit 95% when running the central heating, efficiency will be much lower for hot water. The flow temperature needs to be quite a bit higher (depending on how hot you like the water). So ~72% efficiency would be fairly typical.

Veteransaver

Another complication is that depending on your hot water tank it probably won't heat the full tank anyway. Likely just the top half will get heated.
I am on Tomato energy at 5p overnight so I do use the immersion to heat the tank. But it generally only takes about 3kWh to heat. I think I am saving a little bit (I'm on oil although oil is generally cheaper than gas now). Benefit is it gives enough hot water for daytime use then the boiler kicks in to heat the full tank in the evening.
Also saves a bit of wear and tear on the boiler I suppose.

michaels

Veteransaver said:

Another complication is that depending on your hot water tank it probably won't heat the full tank anyway. Likely just the top half will get heated.
I am on Tomato energy at 5p overnight so I do use the immersion to heat the tank. But it generally only takes about 3kWh to heat. I think I am saving a little bit (I'm on oil although oil is generally cheaper than gas now). Benefit is it gives enough hot water for daytime use then the boiler kicks in to heat the full tank in the evening.
Also saves a bit of wear and tear on the boiler I suppose.

What size is your tank and what is the immersion set at (and where is it in the tank) - we have twin immersions, use the lower one and can get up to 12kwh into our 180l tank

Veteransaver

michaels said:

Veteransaver said:

Another complication is that depending on your hot water tank it probably won't heat the full tank anyway. Likely just the top half will get heated.
I am on Tomato energy at 5p overnight so I do use the immersion to heat the tank. But it generally only takes about 3kWh to heat. I think I am saving a little bit (I'm on oil although oil is generally cheaper than gas now). Benefit is it gives enough hot water for daytime use then the boiler kicks in to heat the full tank in the evening.
Also saves a bit of wear and tear on the boiler I suppose.

What size is your tank and what is the immersion set at (and where is it in the tank) - we have twin immersions, use the lower one and can get up to 12kwh into our 180l tank

Ours is 150l, immersion at the top. OP stated that theirs was a "traditional" low pressure tank, these tend to be single immersion. You can get dual immersion but don't think they are particularly common especially if you have a boiler and aren't all electric.
To be honest I don't even know if ours is a long or a short immersion! I think it's a long one though the fact I can only get around 3-4kWh in suggests not!
It's the original one from the 70s I think, was gravity fed with a small coil (now pumped) so has quite slow reheat times. Probably takes an hour to reach temp from the boiler if it's stone cold too. 
Though it's generally ok for our use.

sheenas

Normally the immersion heat rods are quite short and so it would not heat very much of water in the tank. You can get longer rods as replacements or even tanks with two heater elements one top and the other for the bottom. These could be used with two timers if you wanted to get fancy.  This is typical whats happen in the tank during the day,



In terms of saving money, keeping the water temperature at 55 in the tank saves money and avoids scolding partially by kid. This way the gas boiler will be around 95% efficient verses 100% with electric. So ultimately your electric would need to be noticeably cheaper to make any real difference.

Veteransaver

sheenas said:

Normally the immersion heat rods are quite short and so it would not heat very much of water in the tank. You can get longer rods as replacements or even tanks with two heater elements one top and the other for the bottom. These could be used with two timers if you wanted to get fancy.  This is typical whats happen in the tank during the day,



In terms of saving money, keeping the water temperature at 55 in the tank saves money and avoids scolding partially by kid. This way the gas boiler will be around 95% efficient verses 100% with electric. So ultimately your electric would need to be noticeably cheaper to make any real difference.

Boiler will be nowhere near 95% efficiency though. Flow temp would need to be above 55 so it won't even be condensing for the later part of the heating cycle.
A more modern cylinder with an ultra long coil may achieve lower return temperatures to enable condensing, but an older cylinder return temperature will stay quite high.
Our oil boiler barely condenses at all during hot water heating (although oil boilers aren't quite as good for condensing as gas).
Plus quite a few losses in the pipework going to the cylinder, generally that's irrelevant in winter as the lost heat goes into the house. But makes a difference in summer.

tired_dad

These are our electricity and gas tarrifs

reading these threads I sense it’s quite a nuanced calculation.

how do you know when your boiler is condensing? Ours a a relatively new Worcester Bosch condensing boiler

Scot_39

sheenas said:

Normally the immersion heat rods are quite short and so it would not heat very much of water in the tank. You can get longer rods as replacements or even tanks with two heater elements one top and the other for the bottom. These could be used with two timers if you wanted to get fancy.  This is typical whats happen in the tank during the day,



In terms of saving money, keeping the water temperature at 55 in the tank saves money and avoids scolding partially by kid. This way the gas boiler will be around 95% efficient verses 100% with electric. So ultimately your electric would need to be noticeably cheaper to make any real difference.

I think above chart - linked below - is from this thread - I bookmarked it - as i like the actual use of tank measurements - to show how the tank actually behaves.  Even if have to admit not having studied it in detail.

https://forum.ovoenergy.com/home-and-heating-138/optimise-your-hot-water-set-up-17178#

If you read that full users post for the graph and others within it - you will see they measured for their setup a HW boiler flow temp of around 60 on average for the above graph to reach just under 50C HW temp.

By the way the top tank raise in the 6 AM area is the HW immersion test - in that mechanical set-up barely touching the two lower temp measurements - the full tank heat at around 5pm - is the boiler.

And if you go to the link source I assume for that chart - theres a nice expanded chart of the temperature rise c430 to 5 pm - showing it quickly slowing I guess as temperature differential to indirect coil boiler flow temperature decreases.

And the consequences for boiler power use etc that comes with the ability of coil to deliver less power - lower heat flow - across the pipe surface.

Re This way the gas boiler will be around 95% efficient

There is one example chart of estimated effective boiler efficiency too - with flow temp setpoint - it's quite low (in the 60s / low 70s at best - from a quick scan too).

So at 7p / kWh gas and 70% - quick fag packet suggests nearer 10p/kWh effective cost - which some EV and even some challenger aggressively balanced see saw off peak to peak pricing on E7 deals can beat.

But only for those wanting only to consider the off peak pricing in isolation.  Something IMO only those with consistently very high off peak use should be doing.  And I say that as someone with E10 all electric who does sometimes go over 90% in very cold snaps in winter - averages 75% but in summer dips below 50% occasionally - when SR would actually be cheaper than my E10 rates last time did the maths (I dont like to).

To get say that UW 5p rate quoted above - think how much would you pay at their peak rate - based on Ofgems 42% mix assumed number - and say 26p MR average pricing - I'd say maybe just over 40p - actual rate ?? - for remaining use over the 17 hours.

The other thing to note is in that chart - the top 2 zones dont even reach 50C - for was it just a half hour burn ?  - which is over 10 degrees below HSE rules (sorry - it's not guidance) for business HW storage tanks.  A limit by the way that also applies to domestic landlords - who are subject to the same hard 60C water tank storage limit.  Although unlikely to have to keep the same records as business - who actually have to demonstrate testing their set-ups.

And certainly businesses also have to comply with loss conditions - iirc needing 50C at tap (higher if medical) (or in many cases now TMV hot entry) within a minute.  Easy if your boilers 5-10m pipe run away - not so easy across a 100m+ in a larger office or especially factory setting - as all that pipework might also be heating up 20-30 degrees in summer - from the tank flow at 60C plus - more in winter.

And for 60C tank temp domestically iirc - the poster seems to suggest 75C flow temps for their boiler operation.