New central heating - blank canvas (well nearly) - what to go for?

fenceposttortoise

We need to decide this week on a new central heating set-up for our house - a detached rural cottage. We have just extended it and now have a nice well insulated modern half attached to a drafty old bit.... which has been heated by night storage heaters and an open fire in the past (immersion heater for hot water).
We now have a log burner installed (but no back boiler as it wasn't possible to fit one - double sided fireplace) and are about to replace all the night storage heaters with wet radiators. The extended part of the house has wet underfloor in the floor screed and radiators elsewhere to provide the heat. The question is what boiler to use to run the CH and hot water?
We are not on mains gas, so thats out.... leaving ground/air source heat pump, bottled gas, traditional oil or electric. Heat pumps are out as we don't have the land and the air source we looked at were too noisy. Bottled gas seems to be out owing to expense from the advice we have received..... leaving traditional oil or electric.

Having planned to go oil, we have watched with horror as freinds and neighbours stuggled to fill tanks in the last few weeks as the price has rocketed...... which is making us very nervous of spending the £4k-odd to install an oil tank, all the kit and a new boiler. The plumber suggested looking at wet electric boilers. This seems to make some sense, particularly with oil prices where they are now (70p-odd per litre), as it reduces install costs substantially. We were also thinking we might be able to reduce this cost in the future by investing in photo-voltaic and using feed-in-tarrifs to reduce the net bill.

We just don't know which way to turn and there must be others in similar positions...... we are looking over a period of at least five years and our plumber has calculated 17kw as our boiler requirement. Does anybody have any advice or real-world experience of oil vs wet-electric?

Cardew

fenceposttortoise wrote: »

Does anybody have any advice or real-world experience of oil vs wet-electric?

What a dilemma!

What is the 'health warning' on investments? 'Past performance is no indication of future results'(or similar)

From a running cost point of view, Oil prices are so volatile that advice from even a few months ago is worthless, as will be any advice offered now.

You have to factor in the cost of servicing and maintaining an oil boiler and the eventual replacement of that boiler; where there is little to go wrong with electricity.

You also should consider with electric the possibility of a large heat store heated by off-peak on an Economy 7 tariff. However even though you are heating water and not bricks, you still have to second guess tomorrow's weather and heating requirement.

Pincher

Cardew wrote: »

You also should consider with electric the possibility of a large heat store heated by off-peak on an Economy 7 tariff. However even though you are heating water and not bricks, you still have to second guess tomorrow's weather and heating requirement.

You can get thermal stores ready made up to 450 litres.
Plenty of manufacturers willing to make bigger, bespoke ones.
You need to have a solid concrete floor, no cellar below to support it (put it in the cellar). If you can put an Aga on it, you can put a thermal store on it. It's the size of a fridge freezer.

It takes 1 kCal to raise 1kg (~1 litre) of water by 1 degree C.

A thermal store can keep water up to 85 degrees. Let's say useful temperature range is 35 degrees to 85 degrees. So, to raise 450 litres of water by 40 degrees requires 450 x 40 = 18,000 kCal.

1,000 kCal = 1.17kWh
So 18,000 kCal = 1.17 x 18 = 21.06kWh

If your heat loss is 17kW, a 450 litre thermal store will give you 1.2 hours of heating. At 5p per kWh, £1.05. Coincidentally, a 3kW Immersion element will put in precisely 21kWh over 7 hours!

The good thing about thermal store is, it is very well lagged, so does not lose much heat during the day, unlike storage heaters.
So, assuming your storage heaters are good till 18:00, you can pipe the heat from the thermal store to the under floor heating from 18:00. The UFH heats up slow, and cools off slow, so if you size the thermal store right, e.g. 1,000 litres, it should cool off when you go to bed.

You can also get solar compatible thermal stores, so you get free heating in the summer, which you can use for showers. The idea is that you put in a booster heater on the hot water output, so if the thermal store is heated to 40 degrees by solar thermal panels, the inline booster just has to add 10 degrees, as opposed to 40 degrees from cold, to give you piping hot water.
During winter, the thermal store would supply the hot water, without triggering the booster. Solar thermal should attract RHI subsidy.

The typical domestic electricity supply has a current limit of 100amps, so if you use two 3kW immersion elements, they will be drawing 26amps. If you have five 3kW storage heaters each drawing 13amps, that will be 91amps. Running the washing machine and tumble dryer can trip the circuit breakers. So you might need to use some 2kW storage heaters if you need more of them around the house. Alternatively, you can get a heavier duty electricity supply.

Assume two immersion elements at 3kW and five storage heating elements of 3kW, you are using 21kW, so the maximum electricity used per night is 21 x 7 = 147kWh, at 5p per kWh, that is £7.35 per day.

If you need to run both thermal store and storage heaters for 120 cold days a year, that's £882.

The thermal store has the virtue of low heat loss if you don't use it, so you can switch off the storage heaters when the weather report forecasts warms days, and use the heat only when you want to. Well, you will be using hot water from it, so nightly top up will be necessary.

So, no oil boiler (no tank), no gas boiler (no tank), just one or two thermal stores using Economy 7, hence no maintenance either. You don't have to have solar thermal, but you should keep the option open by getting a solar compatible one.

An interesting configuration is two 450 litre thermal stores, one solar, one not. The hot water is supplied from the solar thermal store, but the UFH uses the non-solar by priority.
When the non-solar is depleted, a selector valve switches to the solar store for UFH. In the summer, the non-solar store is not used.

Pincher

http://www.plumbnation.co.uk/site/gledhill-torrent-heat-pump-solar-open-vented-mains-pressure-thermal-store-cylinder/

This one goes up to 450 litres, has solar connection, heat pump connection if you change your mind, and extra sensor pockets for thermostats.

http://www.jaspi.co.uk/products.html

Check out the ThermoStore 3000, 3000 litres!
They are in Devon, but they do design and installation for further afield.

http://www.jaspi.co.uk/thermalstorage.html

A good read if you are a nerd.

fenceposttortoise

Wow! Thanks for those replies - excellent information and it has allowed me to assess this more critically.
First off, I have checked with my electricity provider (I am on with Southern Electric) to see what tariffs they offer and whilst I am on Economy 7, they also offer Economy 10 with rates of 6.615p for off peak, 14.55p peak and 20.86p daily standing charge (all including vat) - which would seem ideal for a wet electric boiler. (Could these rates be bettered by an alternative supplier?)

Now I have done some rough back of envelope calculations to assess the running costs of oil vs electric.... bear with me here as I am stabbing in the dark a little and am not entirely sure how to use all the figures! My plumber's heat loss calcs came up at 17kw - am I right in thinking that is the amount of energy or number of kw I need to use to keep the house at 21deg with an outside temp. of -1deg per hour? Anyway, I have done some calcs based on 10 hours of heating at 17kw output per day for 182 days (i.e. 6 months of 'heating').... I am afraid I have no experience at all of what sort of heating kw one might consume in an average house in an average year and don't know if this assumption is wildly pessimistic or overly optimistic, BUT

for OIL - assuming oil price at 50p/L (rough eyeball average for 2010) and a modern boiler running at 97% efficiency with 1L of oil producing 10.2kw,
I reckoned 5.05p/kwh (50p divided by (10.2kw X 97%)).... multiplied by 17(kw) = 85.85p/hr in running costs or £8.59 per ten hours

Right now oil prices are 63p/L (source: Boilerjuice) which brings the same calcs out to 108p/hr in running costs or £10.81 per ten hours


For WET ELECTRIC - assuming a wet electric boiler running at 100% efficiency and using electricity at 6.615p/kwh,
I reckoned 6.615p/kwh.... multiplied by 17(kw) = 112.5p/hr in running costs or £11.25 per ten hours

Quite interesting results (assuming I have the calculations right!) and the point at which the two systems are equal in running costs per hour over ten hours is when oil prices only reach 65.45p/L. Now there are some big assumptions here..... chiefly that I would only ever be using 17kw on off-peak rates - as soon as I creep into using it during peak times, the cost rockets as the cost per hour rises from 112.5p/hr to a horrifying 247p/hr (based on a peak time unit of electricity costing 14.55p/kwh). This clearly breaks down when you stray in peak rates and the key would be sticking to off-peak rates..... and cranking up the log burner (9kw flat out) to fill any chilly gaps between off peak bands.

The next issue for me to consider is the install costs as I have no pre-existing hardware - the boilers are about the same in price (possibly slightly cheaper for electric), so we will ignore the costs of those but the peripherals are expensive for oil - the oil tank (I reckon at least £1000 plus fitting), possible chimney lining etc.etc. lets say £2000 total...... that goes an awfully long way making up the difference between running a marginally cheaper oil system and an electric system spread out over the days.... plus the ongoing maintenance costs (annual service etc.)..... and the fluctuations in oil prices (call me a doom-monger, but I can't see oil getting substantially cheaper, granted neither will electricity but at least there is a regulator watching pricing!).

Using the above calculations, but including installation costs and assuming oil prices of around 50p/L, the savings made running an oil boiler over electric would take over 4 years to start making a difference and that is assuming oil prices at 50p/L for all that time.... you can probably see which way I am leaning

SO.... before I start dipping my hand in my pocket, are my assumptions about right do you think? any glaring errors in my calculations? any alterations you would make to the comparisons? have I over-simplified it?

Assuming I am going to go with a wet electric boiler, my next task is to start investigating the heat banks / thermal stores as referenced above as this seems to be an even cleverer way of managing it. A big part of the assessment in this will be space to install the unit (we are very cramped with every nook & cranny utilised) and the structural support for the weight (screeded floors over insulation board over drainage membrane 'egg-crate') - but these are builder / architect questions.
Then to look at solar... either water to feed a heat bank or PV electric to take advantage of the Feed in Tariffs on offer....

I am beginning to see the mist clear with the help of you good folk at MSE and a crash education in the finer points of heating - any comments welcome :money:.

Pincher

Once the wet radiator and under floor heating is connected to the thermal store, you can choose from any heat source. The key is to have enough sensor pockets (for thermostats) in the cylinder to enable the control set up for the chosen heat source.

For a rural location, with possible access to cheap wood, a back burner is an obvious choice, but you seem to be saying that's out. Thermal stores are particularly solid fuel friendly. In the summer, the idea is to charge the thermal store in one session,
and it will give you hot water for days.

The other common and cheap heat source is the heat pump, which you have also rejected. You can forget Economy 7, just run the heat pump 24 hours a day to maintain a small (say 300 litre) thermal store at 40 degrees. This is ideal for under floor heating, and is perfectly usable for showers, but if you need it hotter, put in a boost heater. Assuming you pay 10p per kWh for a non-Economy 7 tariff, you will get heat for

at COP = 2.2, 4.54p per kWh
at COP = 3, 3.33p per kWh

So, even with a low yield heat pump setup, you will be paying less than Economy 7 (6p?) for heat. You will also pay less for electricity when you don't need heating the rest of the year, and can use the washing machine any time you like.

Without land, they can still do Ground Source by drilling a vertical bore hole, 50 to 100 meters straight down.