Worth getting solar pv?

Verdigris

I though DHW was supposed to be limited to 60 degrees to prevent scalding. Needs to be over 50 degrees to prevent Legionnaires Disease. What a dangerous world we live in!

EricMears

Verdigris said:

I though DHW was supposed to be limited to 60 degrees to prevent scalding. Needs to be over 50 degrees to prevent Legionnaires Disease. What a dangerous world we live in!

That's 60 deg C at taps.   If you're going to store hot water it's probably more efficient to store it at much higher temperature then draw it off via a blending valve.

Reed_Richards

EricMears said:

Verdigris said:

I though DHW was supposed to be limited to 60 degrees to prevent scalding. Needs to be over 50 degrees to prevent Legionnaires Disease. What a dangerous world we live in!

That's 60 deg C at taps.   If you're going to store hot water it's probably more efficient to store it at much higher temperature then draw it off via a bending valve.

Exactly.  There is a thermostatic blending valve on the output from the cylinder.  So no danger at all, @Verdigris, but thanks for your concern. 

Hexane

DaisyH8 said:

We use a little under 2000kWh a year and we’re at home most of the day so we’d use even a measly amount generated in winter. 

If you really do go for an ASHP as replacement for gas boiler, you will use a lot more than this, and so of course solar PV then becomes more advantageous (because more of the power generated, instead of being exported for only around 5.5p per kWh, will be used to avoid buying more expensive imported electricity that you otherwise wouldn't be able to get out of buying because you need to heat your house and your hot water.) Although of course being on the correct tariff (time of use) is perhaps more important, in my way of thinking, being dependent on ASHP for heating makes it illogical *not* to have solar PV.

Hexane

Reed_Richards said:

DaisyH8 said:

Verdigris said:

I think your best bet, if you wish to pursue ASHP is to go for a TOU (time of use) tariff. You would need a smart meter to be able to take advantage of TOU. Home storage batteries would also assist in being able to time-shift cheap electricity price/surplus solar generation to more useful times of the day.

Good luck!

Thank you. I hadn’t heard of TOU, I’ll look into that. I’ve been thinking about batteries too, definitely lots to think about. 

Time of Use tariffs tend to charge you premium rates between about 16:00 and 19:00 and you would want to minimise your use of electricity then.  So I am going to stick my neck out and say that this is not compatible with an ASHP (or an electric boiler) unless you are prepared to wear a coat indoors for three hours. 

Not sure this is really true. With modern heating controls, the ASHP could quite easily have been configured to bring the house up to a good temperature between 2pm and 4pm (making use of any solar PV generation to buffer the cost as well), and it's not going to drop too ridiculously by 7pm except on the coldest of days. (In a well insulated house...)

Reed_Richards said:

DaisyH8 said:

Verdigris said:

I think your best bet, if you wish to pursue ASHP is to go for a TOU (time of use) tariff. You would need a smart meter to be able to take advantage of TOU. Home storage batteries would also assist in being able to time-shift cheap electricity price/surplus solar generation to more useful times of the day.

Good luck!

Thank you. I hadn’t heard of TOU, I’ll look into that. I’ve been thinking about batteries too, definitely lots to think about. 

If I am wrong  I'm sure others will be along to tell you this but I bet they don't actually have electrical heating (other than Night Storage Heaters which should work very well with that sort of tariff).

I am right now at this moment putting just over 3kW, all solar generated, into electrical underfloor heating mesh in three different parts of the house. With immersion heater standing by to use any surplus automatically once the hot water cylinder temperature drops far enough. Although, I don't have a time of use tariff and I do also have a gas boiler as well (switched off for both space heating and hot water right now).  It's still cold enough outside that I need all of this heat, especially as most of it is going into the tiled floor in the centre of the house, my own sort-of storage heater equivalent

Coastalwatch

Reed_Richards said:

Time of Use tariffs tend to charge you premium rates between about 16:00 and 19:00 and you would want to minimise your use of electricity then.  So I am going to stick my neck out and say that this is not compatible with an ASHP (or an electric boiler) unless you are prepared to wear a coat indoors for three hours.  If I am wrong  I'm sure others will be along to tell you this but I bet they don't actually have electrical heating (other than Night Storage Heaters which should work very well with that sort of tariff).

I can only speak from our own experience whereby we obtain our Energy from Octopus on their Go Faster tariff. We have 5 hours of a cheaper overnight energy at 5.5p/kWh while the remaining daytime hours are at 14.12p/kWh. During the winter months when PV generation is limited we make best use of the overnight tariff to heat our hot water, charge the EV and to run our two Air to Air Heat Pumps so the house is pleasantly warm when we awake. Once daylight hours arrive there is usually sufficient PV generation to keep them ticking over all day. But we do have an 8.85 kW array.

Sorry if this causes further complication for you, just thought it worth mentioning.

Verdigris

Reed_Richards said:

EricMears said:

Verdigris said:

I though DHW was supposed to be limited to 60 degrees to prevent scalding. Needs to be over 50 degrees to prevent Legionnaires Disease. What a dangerous world we live in!

That's 60 deg C at taps.   If you're going to store hot water it's probably more efficient to store it at much higher temperature then draw it off via a bending valve.

Exactly.  There is a thermostatic blending valve on the output from the cylinder.  So no danger at all, @Verdigris, but thanks for your concern. 

Thanks both. I'd forgotten the at the tap bit. Having looked at my boiler, which has a small cylinder within the boiler casing, it does indeed have a mixer valve on the outlet.

I would argue about it being more efficient to store water at a higher temperature, though. With a higher water temperature the temperature gradient with the surrounding air is higher so the rate of heat loss, per unit area will be greater. With a larger cylinder, at a lower temperature, the temperature gradient is lessened and the surface area to volume stored is smaller.

Given sufficient space I'd tend to go for as large a cylinder as practical for storing solar heated water. The hottest is always at the draw off point in the crown of the cylinder.

EricMears

Verdigris said:

Thanks both. I'd forgotten the at the tap bit. Having looked at my boiler, which has a small cylinder within the boiler casing, it does indeed have a mixer valve on the outlet.

I would argue about it being more efficient to store water at a higher temperature, though. With a higher water temperature the temperature gradient with the surrounding air is higher so the rate of heat loss, per unit area will be greater. 

I was of course thinking of a well insulated storage tank.  However,  if your source of heating is effectively 'free', thermal losses are costing nothing and my definition of 'max efficiency' would be to get as much hot water as possible (at tap temperature) from a stored tankful .  Incidentally,  heat 'lost' from a poorly insulated tank isn't usually a real loss.  If tank is somewhere indoors,  escaping heat contributes towards warming the house.

Verdigris

Well yes, nobody in their right mind would use anything less than a foam lagged tank.

Which reminds me, the losses from my cylinder have got my sourdough starter up to peak fizz. I'd better go and make some bread.

Reed_Richards

EricMears said:
...if your source of heating is effectively 'free', thermal losses are costing nothing and my definition of 'max efficiency' would be to get as much hot water as possible (at tap temperature) from a stored tankful . 

Mine too, which is why I allow the tank to get to 75 C or thereabouts.  However my tank is well-insulated.