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Will solar PV power my underfloor heating?

Posted by Paul Hutchens on 20 May 2013 at 9:29 am

Q: We are having a 4kW solar pv system fitted. We are also considering electrical underfloor heating. After reading some very interesting pages on your site I am beginning to wonder if electrical underfloor heating is the best option as this will mainly run when it is dark? Is my thinking flawed as I hoped that the generation payment and export payment would, in effect, subsidise any subsequent energy I have to buy?

We have a gas combi boiler, so would hot water underfloor heating make more sense? and would I have to install an immersion heater, run the central heating pipes through this, fit an Immersun, and also be able to somehow integrate the tank water in the existing combi powered DHW system?

A: This is an interesting and slightly involved question that identifies some of the key principles of renewable energy use in a home. I think that it would be good to cover a few basics and to answer this in stages:

  1. A 4kW solar PV system can produce enough energy to power an average home – around 3,500kWh per annum. This does not mean that the home will be self-sufficient by any means though as electricity is not used consistently and evenly – and is often needed when it is dark and no energy is being produced.
  2. There are battery based storage systems available but they are expensive.
  3. Therefore it is fair to say – as in your question – that the generation payment and export payment would, in effect, subsidise any subsequent energy you have to buy but without any direct link between supply and demand or the exact amounts involved.
  4. Electrical under floor heating is a “dry” system that cannot run off the combi-boiler; it is powered by electricity which could be boosted by the PV in sunlight.
  5. The combi-boiler could power a “wet” under floor heating system but there will be no direct contribution from the solar PV.
  6. To get a solar PV contribution to the hot water or under floor heating you would need to install a cylinder to be heated by the boiler – which would in effect now NOT be a combi-boiler having been converted to heat a cylinder rather than heating water instantaneously. The boiler and the immersion heater (which is electrically heated so can take a PV contribution) will heat the cylinder which will, in turn, provide the hot water for the “wet” under floor heating system. You could use a thermal store – a different type of cylinder but that may get too complicated for the purposes of this answer.

In summary, I think your set up would not make it convenient or cost effective to use PV to power under floor heating – unless you use electric under floor heating. This will be independent of the gas boiler.

More information about solar PV and underfloor heating on YouGen

Types of heating system

Heat pumps and underfloor heating: perfect partners?

An introduction to solar PV

How to use excess solar electricity for water heating


Photo Credit: kevinthoule via Compfight cc

About the author: Paul Hutchens is founder and director of Eco2Solar, which installs solar systems around the UK.

If you have a question about anything in the above blog, please ask it in the comments section below.

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2 comments - read them below or add one


arwooldridgeComment left on: 18 June 2013 at 10:32 pm

potentially exportable Yes you can, but what you need is a propoortional PV heating controller.

What this does is divert the otherwise exportable power to be used internally.

You can use immersion heaters, storage heaters or small underfloor heaters.

Obviously you cannot heat the whole house like that as you only have 4kwatt peak available at most, but just one room eg bathroom would work.

The heat is stored in the floor during the day and released during  the evening/night.

Such heating is essentially free as you don't get paid any more if you export it or not, the trick is to use a Sola Controla from SGWooldridge. Its proportional and will only export around 40 watts given sufficient load.

Its rated at a conservative 4kwatts and can even be overloaded. ( PV power will limit it to 4kwatts anyway)

It won't work so well in winter as the sun is low and days are short, so you need some backup then. However what small power is available even then, will go towards the heating.

Typically you would back it up with a timer set to come on when the sun goes down in the winter months.(or on night rate)

You can even arrange with slave controllers from the same source to feed the immersion and the underfloor heating, as there is typically more than enough power for the DHW ( spring, summer and autumn) and the balance can feed the underfloor heating or storage heater.

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jabolithoComment left on: 20 May 2013 at 2:32 pm

As per David MacKay ('Sustainable Energy without the Hot Air' -- available free online) heat is a relatively low value form of energy, and electricity is the highest value form of energy.  Via entropy, other forms of energy become heat.

So with electricity, it is best used for lights, electric motors, appliances etc. not for direct (induction) heating of water or house floors.

The only way a solar PV system could work with underfloor heating is via a Heat Pump (ground or air source).  There, each kwhr of electricity becomes 2-4 kwhr of usable heat for hot water or heating (COP of 2.0 to 4.0).  However there is a current surge when most electrical systems start up, and you would need grid connection to handle that.  Also the system would need to draw from the grid in bad weather or at night.  It' s unlikely that a solar PV system would generate enough current, by itself, to meet the demands of a HP system.

Given that in southern England 1 peak kw solar array generates c. 850 kwhr pa, and the average household uses around 3000-3500 kwhr pa (depending on number of people etc.) the best use of a 4 kw array is to meet annual household electricity consumption.  Generated by the grid, that 3000 kwhr would produce c. 1.8 tonnes of CO2 pa.  Earth dot org dot uk (google that and grid intensity) has real time figures.

By contrast, producing that same amount of heat energy in a gas boiler produces c. 0.6 tonnes CO2 pa.  With a good biofuel boiler perhaps 1.0 tpa (Nottingham Energy Partnership has decent figures.

If you are off the gas grid, the call then is between a solar thermal system or a solar PV system, for a given roof space.  Assuming a well insulated home, even heating with bottled gas, you are probably better off generating solar PV.

A better solution if you are off the natural gas grid is to install a heat pump (with the caution your house needs to be well insulated first) or investigate a biomass boiler.

There are some combined solar PV and thermal systems available as a 3rd alternative.

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