Starting again with heating

Pincher

prunus wrote: »

We have seven storage radiators which draw a total of 91 amps. Add in the DHW and the other electrical stuff turning on at night, including us getting up for cups of tea, and we have, in the past, blown the cheap rate supply fuse. We now cope very well by not using things like the washing machine at night.

13amp x 7 = 91amps.

A 3kW kettle is rated at 13amps, I believe.

This means you are using 3kW x 7 = 21kW

Seven hours heating up the storage heaters would imply 21 x 7 = 147kWh, at 5p per kWh that would be £7.35 per night. Are you sure you are "reasonably happy" with that?

On warmer days, this would be wasted during the day, but you cannot predict what you will need, so you are forced to heat up the storage heaters regardless.

Ideally, you want on demand heating, so you only heat up the house on days you actually need it. Since you do not have mains gas locally, I would add ASHP to the living room, and one for the master bedroom.

Assuming a COP OF 3, and day time peak rate of 15p per kWh, you will be effectively paying 5p per kWh, same as E7 night rate.

If the weather report says cold spell coming, then heat up all seven storage heaters overnight, otherwise switch off a few, and use the wall mounted heat pumps for day time top up if needed. This frees up capacity for the kettle and washing machine, as well as save some money.

bobmedley

Why not trial one of these?

http://www.airconwarehouse.com/cgi-bin/sh000002.pl?REFPAGE=http%3a%2f%2fwww%2eairconwarehouse%2ecom%2facatalog%2fcatalog%2ehtml&WD=windy&PN=DIY_Air_Conditioning_Units%2ehtml%23a465#a465

No need for professional installation, COP of 3, low noise level, gives over 2.5 kw of heat (and cools in the summer if you wish) and they only cost £299 each.

grahamc2003

Pincher wrote: »

13amp x 7 = 91amps.

A 3kW kettle is rated at 13amps, I believe.

This means you are using 3kW x 7 = 21kW

Seven hours heating up the storage heaters would imply 21 x 7 = 147kWh, at 5p per kWh that would be £7.35 per night. Are you sure you are "reasonably happy" with that?

On warmer days, this would be wasted during the day, but you cannot predict what you will need, so you are forced to heat up the storage heaters regardless.

Ideally, you want on demand heating, so you only heat up the house on days you actually need it. Since you do not have mains gas locally, I would add ASHP to the living room, and one for

I'm afraid your storgae heater calcs are a lity the master bedroom.

Assuming a COP OF 3, and day time peak rate of 15p per kWh, you will be effectively paying 5p per kWh, same as E7 night rate.

If the weather report says cold spell coming, then heat up all seven storage heaters overnight, otherwise switch off a few, and use the wall mounted heat pumps for day time top up if needed. This frees up capacity for the kettle and washing machine, as well as save some money.

Agree about the ashps - I would hope to get a higher overall cop than 3 by limiting their use in really cold weather, when their efficiency drops for a couple of reasons. At 7 degrees and above, new systems claim cops of over 5, which would ensure very cheap heating (if the unit is anstalled and used correctly).

I'm afraid your storage heater calcs ar a little off. To blow the 100A fuse, you need about 23kW, which is only 6 or 7 of the largest domestic storage heaters. They don't all have to be on full - even if they were all set to their minimum input, they'd still blow the fuse, because they'd be trying to draw more than 100A when the e7 rate time starts. I have 8 storage heaters, but not all the biggest ( 3x1.2kw, 2x2.4kw and 3x3.6kw), which together are getting very close to 100A. When they are all on, I don' use the kettle! Some are on low, so soon turn themselves off via the thermostat, so I'm not pulling over 20kw for long.

Also, you can't get a full charge into any storage heater in one night, so you don't get 7x3.6kwh stored overnight. Even when on full input and zero output, the thermostat will still cycle on and off as the core temp rises before having tme to dissipate into the storage material. So the max cost, even when on full, is less than 7x3.6x5p per large heater. I'd estimate it's about 80% of that, say £1 per night.

While I'm rambling, any subsidy which encourages people to shift from nightime use to daytime use is really counterproductive as far as the grid is concerned. It is excasperating very well known problems instead if alleviating them. Much better, in all respects (including environmental) to use 3 times the electricity at night rather than 1/3rd at the peak in the day, but that is what the rhi is encouraging me to do.

Pincher

grahamc2003 wrote: »

I'm afraid your storage heater calcs ar a little off. To blow the 100A fuse, you need about 23kW, which is only 6 or 7 of the largest domestic storage heaters. They don't all have to be on full - even if they were all set to their minimum input, they'd still blow the fuse, because they'd be trying to draw more than 100A when the e7 rate time starts. I have 8 storage heaters, but not all the biggest ( 3x1.2kw, 2x2.4kw and 3x3.6kw), which together are getting very close to 100A. When they are all on, I don' use the kettle! Some are on low, so soon turn themselves off via the thermostat, so I'm not pulling over 20kw for long.

Also, you can't get a full charge into any storage heater in one night, so you don't get 7x3.6kwh stored overnight. Even when on full input and zero output, the thermostat will still cycle on and off as the core temp rises before having tme to dissipate into the storage material. So the max cost, even when on full, is less than 7x3.6x5p per large heater. I'd estimate it's about 80% of that, say £1 per night.

Since the OP stated that he was drawing 91amps and has 7 storage heaters, 13x7=91 is what the information says to me.
I agree the startup transient tips the current over the rated 100A,
and my guess is the OP put timers on some of the storage heaters to stagger the start up time.


I agree the storage heaters will be cycling early in the morning, so the amps drawn will be lower. 147kWh is the maximum for energy used for 91amp for seven hours, which is of course rather high, but it is simply the consequence of overspecifying with spare capacity. Like you said, the OP can switch off or turn down the input and use as little as he wants, even £1 per day, but that is far too low.

£1 at 5p per kWh is 20kWh. That is a heat loss at the rate of 0.83kW, hardly realistic. At that rate, you can keep the whole bungalow warm with a 2kW oil filled heater.

260 cubic meters bunglow says 9m wide x 10 meters deep x 2.9 meters high, which is one story of a four bed semi. Since it is a detached bungalow, with heat loss all round, I would have thought heat loss rate would be 5 to 10kW. Even at 5kW, you need 5 x 18 (up and about hours) = 90kWh.

Consider non-Economy 7.
If you are paying 10p per kWh, COP of 3 means 3.33p per kWh, LESS than Economy 7! AND it's on demand, so you use less kWhs overall. 12,000kWh at 3.33p is £400. Don't forget, with non-Economy 7, you are also saving on day time electricity the whole year round.

So, I would migrate to ASHP and non-Econmy 7 using the RHI subsidy. You can do it one room at a time, to see whether you like it or not.

grahamc2003

£1 per night is my estimate of the maximum cost of 1 3.6kw heater when set to maximum (as I stated) - not the cost to heat the whole house! (which I didn't state).

Pincher

"7x3.6x5p" 7 hours, 7 heaters. I must have mentally skipped a beat.

I thought you were disputing my figures, which was £7.35 per night at maximum, and £1 x 7 heaters = £7 doesn't seem very disagreeing.

prunus

Pincher and graham2003,

Your maths are about right. I figure that we are paying just over £7 per night when our storage radiators are all on full intake.

Reasonably happy? Well, as said before, you have to look at the long term. I bought them and put them in and there's has been virtually no maintenance or service costs in over twenty years.

I was forced to have a new modern outside meter installed a few years ago when I needed an overhead supply cable buried underground. Both the day rate and night rate supplies were then fused by the authorities at 80amps with the main cable staying at 100amps. It was the 80amp night rate fuse that then started blowing. Something was done about that fuse rating of course but at a draw of 91amps we still do have to be careful.

I also like the maths on the cheaper running costs of an ASHP. I'm going to dwell on that for a bit. Any idea about the 'livability' of a hot air warming system? My only experience of them was when a relative moved, briefly, into a house with a central unit blowing hot air through ducts in the flooring. It wasn't very good.

bobmedley,

The links to the brochures of the unit you posted about don't work. I also have doubts about the difficulty of getting parts from Italy should something go wrong. Nice idea but itt might be a case of buying cheap and regretting it later.

Thanks a lot for your very helpful replies.

bobmedley

At least Italy is nearer than Japan/China:D Once I had a Fujitsu air-con unit and the delivery time for a new fan was 4-6 weeks - it was literally on a slow boat from China.

I have some Hyperinverters (these are air to air and do not heat water for DHW or radiators/UFH), and as a heating medium they are absolutely fine. The indoor units are whisper quiet and they can warm a room up extremely quickly - of course they can also cool in summer if you wish. They are inverters so they modulate & don't just blast out air until the set temp is reached.

Temperatures around here got to -15 recently, but they were still pumping out hot air no problem.

MHI cost £575 and ME £875 for the same size units, but there seems to be little difference between them so I would recommend the cheaper MHI units. The COP can reach over 5.

Note these are single systems (one indoor/one outdoor) - you can get others with multiple indoor units linking to one outdoor unit which can work out slightly cheaper.

prunus

bobmedley,

What make are the MHI and ME units? Googling those alone brought up a variety of different makes.

Pincher

ME is Mitsubishi Electric,
MHI is Mitsubishi Heavy Industries