To cut out summertime natural gas use my daughter wants to have DHW heated by electricity.
Her combi boiler is in the loft and there is room for a 100ltr cylinder in an airing cupboard or the loft.
Is it possible to have them work together or must the water be switched between winter and summer running?
Extraordinary. End up paying more and emitting more CO2.
What's to like?
If the combi is actually heating a hot water TANK, then no reason not to simply switch on the leccy full time.
Combi will kick in when its trimer is on and water is cold,
No additional CO2 emitted
ok.. might not be everyone's first choice :-)
It can work, but you will need an external valve, a cylinder stat, a dual channel timer, and some wiring changes.
Probably easiest / most efficient configuration would be a W plan system. This will give hot water priority operation, and allow the full output of the boiler to be directed to the cylinder. Unvented cylinders tend to have large indirect heating coils, ans so can be very fast recovery.
(this assumes that the combi does not have weather compensation - since that would complicate it a bit)
The DHW maximum temp would be limited by the flow temp set on the boilers CH temperature setting, *not* its DHW temp setting. The set temp would be controlled by the cylinder stat.
Additional to what?
Do you think that electricity is clean and doesn't emit any CO2?
Day time electricity is extortionate, plus the heat losses from the tank. I suggest you do your sums, which may only add up if you use night time rates and then of course pay more during the day!
At the moment gas is king.
Right now, the carbon intensity is 41 gCO2/kWh here (east of England). The current spread goes from 0 g (north Scotland) to 94 g (south west England).
Natural gas including distribution[1] has 202 gCO2/kWh when burnt.
So yes right now the electricity is cleaner than burning gas, and so switching to electricity is carbon negative.
Obviously the gCO2 figure varies with time, but in a lot of the country it's less than 202 most of the time. Northern Scotland is pretty much zero carbon all the time nowadays.
Theo
[1]
Could make sense if you have solar, so you can divert generation into the cylinder immersion for 'free' hot water.
By 'large' do you mean 'long' or 'wide'?
Thinking that if you're going to install a cylinder you might want it to be heat pump ready, which means having a long coil to extract maximum heat from a low-temp flow input. Not sure how compatible that would be with a low-flow high-temp output from a combi, or would the end result be similar?
I'd likely want more than a 100 litre tank though: one tankful would be just enough for a bath, give or take. If you had a tank of hot water and emptied it, wouldn't you then have a tank of cold water with the combi frantically trying to reheat it - not ideal for that to happen mid bath, although it would probably be hot again after 10 mins or whatever.
How would the boiler know to fire up when the tank got cold, would it need a thermostat in the tank?
Wouldn't that mean condensing takes a hit, if your flow temp now has to be
60C or whatever, rather than a lower more efficient flow?Theo
when it comes froma wind farm or solar panels -it doesn't - at the time of use.
The embedded energy that was used to get the PV panels on her roof, inverter etc..
Yes this is the case, nearly every day from now until October there will be surplus over her household use and probably her car charging.
John's helpful solution presupposes she will want to heat the tank with gas at some stage. This is unlikely and the energy is better stored as gas rather than hot water. In winter the combi boiler will provide DHW and CH without the need for the hot water cylinder.
It would need to be a very well insulated tank in case it needed topping up with cheap rate electricity to coincide with car charging, which is sometimes needed after a long journey. Apart from heat loss and over capacity the gas at 6p/kWh for instant hot water or heating a cylinder at 7.5p/kWh should be about the same.I think that would be a maximum of
8kWh/day if the tank of 120ltr was used up each day.I wonder what the heat loss would be over 24 hours.
So the system would be electrically heated cylinder OR the combi.
The extra cost of a heat pump/cylinder combination doesn't seem worth it compared with an immersion element, though a clever one that heated from the top down like I have would be better.
Three person household, it looks like 40ltr per person is about right, so maybe a bigger for resilience is worthwhile.
So would you want a setup where you can draw water from the cylinder or from the combi, but not use both together. ie some kind of switch to change from one to the other? Maybe you could do that with a 3 port valve and a tank stat: when the cylinder is hot, use that. When it's not, use the combi. Solar puts energy into the cylinder but will carry on heating it without drawing while it's lukewarm.
The downside of that is aren't using any of that solar heat until the water is 'hot enough', ie you're not using it as a preheat for the boiler on days the solar is weak. Maybe the amount of energy wasted there is small enough to not matter.
I wouldn't suggest a heat pump at this point, just that when the boiler needs replacing and a heat pump makes sense, it would be sad if you had to replace the cylinder as well. Spend a little now on a better cylinder and leave you options open in future.
Where's your 40 litre figure coming from?
This:
The problem with averages like 40 litres per person per day, but you might have a whole bath every few days, not a third of a bath per day. So you need a cylinder enough for peak demand.
I suppose if your answer to peak demand is just to run the combi then that works, as long as you always have the combi.
Theo
Long typically - many turns of pipe. A traditional cylinder had an indirect heating coil that could shift around 5kW tops. Hence why Y plan and gravity circulation was popular, you could trickle heat into the cylinder at the same time as running the rads. Doing the cylinder in isolation would have meant much short cycling.
A modern fast recovery cylinder may be able to absorb 20kW for a good part of the reheat.
You can have cylinders with multiple coils designed for things like solar thermal. So they can take advantage of lower temp heat sources.
If you set your control system to bring in the boiler as the temp starts dropping, then you should be able to get more out of the cylinder than volume alone will allow.
It can be worth playing with the numbers as described here:
Yes. See the W plan wiring and plumbing scheme that I linked to. The hot water would be controlled by both timer and cylinder stat. When both indicate a call for heat - that is send to the boiler, and the diversion valve directs the boiler output to the indirect coil in the cylinder. When that is satisfied (or the time slow expires on the timer). The boiler will revert to control by the timer and room stat (or possibly prog stat acting as both)
If you set your flow temp at say 70 deg, then allowing for a 20 degree drop across the rads, that would keep the return temp around 50. That should still get reasonable condensing efficiency on the CH. When doing a DHW reheat, the efficiency would fall as the cylinder temp gets closer to its set point temp - assuming you have it at 60 degrees.
Sorry, I misread the intent.
If electric only, then you would need a "direct" rather than indirect (unless you want the option of heat pump or solar thermal inputs)
If you only want electric heating on the cylinder, then indeed you could instead have manual valves to switch the DHW feed to the property from the output of the cylinder to the DHW output of the combi. You might be able to achieve some automation with motor actuated values (but not the CH system type)
(picking a larger cylinder might make sense since the reheat will be much slower on immersion(s))
The cylinder makers, normally quote this figure in their specs. I think the loss from my 210L cylinder is around 60W. Their insulation is normally pretty good - most loss will be from the parts that pierce the outer case.
(it might not always be considered a "loss" if you want some warmth for an airing cupboard / drying room etc)
If you want two showers in sequence, then you may want more. A couple of immersions will only be able to reheat at 6kW - the output you would expect from a very poor electric shower!
Yes but if the tank could be heated top down then the available water should all be at the high temperature even if the whole cylinder is not heated.
Can a combi be run in series with its water feed preheated by a hot water tank? What if the combi only supplies 50C water and the tank were at 70C?
An air source heat pump doesn't fit well at the moment. It would be different if the house had underfloor heating and the screed had enough thermal inertia, then it may be possible to store enough heat in the floor during a cheap rate but it would clash with car charging some winter days. Even putting 7kW into a heat pump with a COP of 3 would be limited to 4 hours and the house may be cosy at 5:00 but less so at
20:00. She did use over 30kWh of gas on a few days this last winter.looked it up on a .gov.uk site.
Good if you’re on an old FIT scheme where you get paid to generate rather than just export.
Nowadays if you’re not on a FIT scheme and if you have an EV you can often get paid more to export than import costs during the night.
I get paid 15 per kWhr to export at any time but only pay 7.5p per kWhr to heat my hot water at night so using “spare” solar to heat my water during the day doesn’t make any economic sense.
This also applies to EV charging from solar. If you can charge cheaper from the grid at night for significantly less than you get paid for exporting to the grid solar charging becomes redundant.
Tim
As it would only be changed for the summer this should not be a problem. Does a combi hot water outlet have a non return valve? Would the change over involve shutting off the feed to the combi and diverting it to the new hot water cylinder?
That's reasonable. My cylinder seems to lose most heat from the top mounted immersion, which cannot be insulated for safety reasons, so a bottom mounted immersion feeding hot water directly to the top and remaining stratified would be ideal, I think one of the built in air to water heat pump-cylinders does this. My Willis does the same but is not integrated.
This depends on the length of shower, I guess a shower is at ~40C and the water could be stored at 70C. I easily get a shower out of the top
30 litres of my tank. >
Whole set up was commissioned March 23 so no grants or subsidies.
Yes, this does cause a quandary but I do not expect the high export rate and the cheap 6+hours of cheap rate to last, currently the cheapest rates and high export are not available to her because of car/charger compatibility problems.
I know but...
It can also mean that one does not need PV panels to make the most of it, just a car with power to home capability.
She should look at Eon Next Drive. Only need a working smart meter giving half hourly data. Works with any car. Rates are currently better than Octopus’s.
Tim
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Not a suggestion, I know little of this, but would it be possible to charge a battery and then use an instant hot water heater? I realise that would have to probably be upwards of 20kW. Maybe even pre-heat the water using a battery/inverter cooling system.
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