Gas combi v electric combi = 3.75kw solar being instaled

neilmoonan

Hi, I have a 10 year old gas combi boiler and in march i will have 3.75kw of solar, i know ive missed the best FIT but thats the way it is.
So should i get a new electric combi boiler or should i get a hot water storage tank to work along side my gas combi for hot water and how would it work?
What are my options for heating house with solar power heat central heating efficiently?
I would like to maximise the use of solar in the winter by heating house all day and having water ready to use.
If anyway one has any advise or ideas which way to go please post.
thanks in advance Neil

penrhyn

I would have thought that any output you get from your solar PV during winter months will be minimal, certainly not enough to run any form of electric heating.
The 3.75 kw you quote is peak output, the sun would have to be shining at the optimum angle on your panels to generate that output, and remain for an hour to get 3.75Kwh.

Andy_WSM

neilmoonan wrote: »

I would like to maximise the use of solar in the winter by heating house all day and having water ready to use.

I live in an all electric property and have Solar PV (4kWh) & heat pumps for both hot water and space heating (now backed up with oil filled radiators in each room).

Even this time of year there are very few days that the solar produces enough power to run the heat pumps (approx 2kWh each) without taking some power from the grid. It is highly unlikely that solar will help very much through the winter months at all - more likely that it will just absorb the background load throughout the day.

However, the train of thought I use is that the FITs earn me enough money to pay my electricity bills and then some, so having paid out to buy the system I no longer need to worry about my energy consumption month to month.

Pincher

You could pretend it's a solar thermal system, and use the solar electricity to heat the hot water using the immersion element.
You need to connect the inverter directly to the immersion element, whick is 3kW, because if the inverter is only producing 1kW, you will be using 2kW from the mains if that's connected. You can use the cylinder stat to control a relay (which can switch 3.75kW), so that the inverter is connected to the mains when the water is hot.

It's not worth doing for winter, but you will get lots of hot water in summer for 3.75kW.

If you have a 180litre direct cylinder, with no gas boiler heating coil,
there is an interesting variation. Get a gas water heater, and feed the water from the cylinder into it. The cylinder pre-heats the water, the gas water heater just tops it up. You have to get the Rinnai Infinity 16i type of water heater, which only adds enough heat to maintain output temperature. In the summer, the water is already hot, the so the Rinnai barely burns any gas. In winter, the cylinder is tepid, but at least not totally cold, so the water heater has less work to do.

You can go wild with thermal store, underfloor heating, etc. but the amount of solar energy you collect in winter is just not worth it.

energysavingexp

Pincher wrote: »

You could pretend it's a solar thermal system, and use the solar electricity to heat the hot water using the immersion element.
You need to connect the inverter directly to the immersion element, whick is 3kW, because if the inverter is only producing 1kW, you will be using 2kW from the mains if that's connected. You can use the cylinder stat to control a relay (which can switch 3.75kW), so that the inverter is connected to the mains when the water is hot.

It's not worth doing for winter, but you will get lots of hot water in summer for 3.75kW.

If you have a 180litre direct cylinder, with no gas boiler heating coil,
there is an interesting variation. Get a gas water heater, and feed the water from the cylinder into it. The cylinder pre-heats the water, the gas water heater just tops it up. You have to get the Rinnai Infinity 16i type of water heater, which only adds enough heat to maintain output temperature. In the summer, the water is already hot, the so the Rinnai barely burns any gas. In winter, the cylinder is tepid, but at least not totally cold, so the water heater has less work to do.

You can go wild with thermal store, underfloor heating, etc. but the amount of solar energy you collect in winter is just not worth it.

this idea will not work as the inverter needs a mains voltage connected to it for it to work.
it would work if you had an off grid inverter but that is no good if you want / need to export your leccy to maximise your production meter
ie if its off grid then when your tank is at temps your system will not show any production.
if it is a grid connected inverter then if you are producing 1kwh with an 3kw heater then you will be using 2kwh from the grid.
it cannot be grid connected and off grid at the same time

Pincher

Good to know the is a difference.

I assumed there would be a need to sync to the 50Hz, but you seem to be saying a grid connected inverter will shutdown in the event of a power cut. This is rather worrying. I was thinking of using the solar PV in combination with some batteries, which feed the inverter. In the event of a power-cut, cut the main breaker to isolate the house, and run the inverter from the batteries. But the inverter now doesn't work, presumably needs a 50Hz to lock into. Bum.

This is like the Micro-CHP, which can generate 1kW, which is just enough to keep the central heating going, but it still wants mains electricity.

Somebody should re-design these things for power-cut scenarios.
A standalone micro-CHP just need a battery to get itself started, presumably, and top up the battery using the 1kW it generates.
I suppose it's lookng for a 50Hz to lock onto, as well.

Can't they just flip over to auto-mode when there is no grid 50Hz?

energysavingexp

Pincher wrote: »

Good to know the is a difference.

I assumed there would be a need to sync to the 50Hz, but you seem to be saying a grid connected inverter will shutdown in the event of a power cut. This is rather worrying. I was thinking of using the solar PV in combination with some batteries, which feed the inverter. In the event of a power-cut, cut the main breaker to isolate the house, and run the inverter from the batteries. But the inverter now doesn't work, presumably needs a 50Hz to lock into. Bum.

This is like the Micro-CHP, which can generate 1kW, which is just enough to keep the central heating going, but it still wants mains electricity.

Somebody should re-design these things for power-cut scenarios.
A standalone micro-CHP just need a battery to get itself started, presumably, and top up the battery using the 1kW it generates.
I suppose it's lookng for a 50Hz to lock onto, as well.

Can't they just flip over to auto-mode when there is no grid 50Hz?

yes the inverter will shut down if there is a power cut this is to stop power going back down the grid for safty reasons

i know what you are saying that you would like it to still work during a power cut but you would have to have a automatic cutoff main breaker to the house.
also if some one (not clued up) wanted to say change a socket or light fitting in the house and turned off the main switch thinking that the power was now off, but it would not be, as the inverter would still be on if they did not shut down in a power cut

Pincher

That's useful, before I go down the wrong path.

So a 100amp "switch" that sits between the electricity meter and the consumer unit would detect loss of mains power and take action. This still doesn't provide a 50Hz for the inverter to detect, so the switch needs to be battery backed up, so it can send a signal to a diesel generator, which starts up (battery plus starter motor). On restoration of mains power, switch back to mains supply, signal the diesel generator to stop. A 1 or 2 kW generator will be good enough to keep the central heating going, and some energy saving light bulbs.

The solar PV inverter is pretty useless in winter, even with battery backup, but there is still hope for the micro-CHP. All my central heating related components that need power are wired into the central heating wiring centre: CH pump, UFH pump, 4 zone valves, boiler, smart wiring centre and low voltage programmable thermostat, extra two channel programmer. There is a switched fuse faceplate (labelled BOILER) That can isolate the whole central heating subsystem. If the micro-CHP can operate in standalone mode, then in the event of a power cut, the micro-CHP will shutdown as it should for safety. I come along, manually isolate the central heating subsystem using the BOILER switch, put the micro-CHP into standalone mode, so it will ignore the absence of 50Hz, and the whole thing will startup, probably on battery. In case I forget it's in standalone mode, the standalone switch should be a momentary rocker switch, so that when mains power is restored, it detects the 50Hz, goes back to standard mode, and will switch off on the next power-cut. Now that can't be hard to do, can it? Baxi, start working on it. I just changed my boiler a year ago, so I'll need it ready in nine years time, so no hurry.

energysavingexp

if i have a power cut (only had one in last 5 years) i turn off the mains switch and run my house on a 4kw generator
this will run the gas boiler,fridge,frezzer,lights,tv,and kettle etc.

if you are looking to run your heating and some lights you could use some leasure batterys and a 1kw inverter and just plug them in

Pincher

Generator is the obvious option, I was just trying to come up with a combination where you make use of something that already serves a purpose, as opposed to a generator that you never use, but have to keep testing, so it doesn't rust up when you do want to use it.

I suppose the car's alternator might give just enough power to run the central heating. Car has be in open air to run, but can be locked using spare key. Jumper lead goes under locked garage door to inverter, which plugs into the socket in the garage. Now I just need the right inverter and the power-cut scenario is covered.

energysavingexp

Pincher wrote: »

Generator is the obvious option, I was just trying to come up with a combination where you make use of something that already serves a purpose, as opposed to a generator that you never use, but have to keep testing, so it doesn't rust up when you do want to use it.

I suppose the car's alternator might give just enough power to run the central heating. Car has be in open air to run, but can be locked using spare key. Jumper lead goes under locked garage door to inverter, which plugs into the socket in the garage. Now I just need the right inverter and the power-cut scenario is covered.

sounds an expensive way of doing it
why not charge batterys via solar then if you have a power cut
use them with a 1kw inverter to run your heating

how many power cuts are you expecting.