Best & most efficient is "most are the same".
There are 2 key types of storage heater. a) Domestic
- 13-14cm deep
- 9-18-24kWhr capacity (35-70-105cm wide)
- heat is released by convection via flap & "all over"
- may have additional integral convective/fan heater
b) Industrial
- 27-30cm deep
- 24-32-40-48kWhr capacity (80-125cm wide)
- fan outlet at bottom, lots more insulation
- heat is released fan, little convection
- remote thermostat controls fan operation
Type a) tend to have the same insulation.
- Rockwool at the sides
- Silica wrapped in glasscloth front
- Silica slab top & bottom
Type b) tend to have similar, just more of it.
- The idea is they lose less by convection
- Heat is thus available on demand by thermostat
- Their aim is office-hours plus boost for late workers
The benefit of storage heaters for commercial premises is they can sometimes get better rates on electricity AND they can eliminate all the maintenance & risk of water based systems. Typically good for shops, although air heat pumps are taking over (cooling in summer can be desireable) and will probably take over fully once reasonably priced CO2 based air heat pumps are available.
The bigger industrial units are Dimplex VFMi & Creda TSR. They are pricey - a bit too pricey because they are a single point of failure - although there is precious little in the things to ever fail. That is their attraction to many pensioners, they just work for 25yrs+. They are also branded AEG, Miele and so on in Europe. Forget importing now though re exchange rate, the Germans have quite a range though - about
6-7 industrial types of various functions.
The domestic units are Creda, Dimplex, Sunhouse, a few others. You have automatic & manual versions - automatic adjust their charge based on room temperature, so can save about 10% since you do not fully have to guess the next days weather (the real problem with pure storage heaters!). The manual versions require you to guess the weather - and then if it suddenly goes hot you are going to get a bit cooked.
There is a hybrid, the most popular is Duoheat. Duoheat come in 3 sizes - the largest is 2.5kW not the more usual
3.3kW, instead it uses a panel heater to provide boost heating after about 12-midday through to midnight-or-2am (there is a jumper inside re "disable 2hrs extra during charge"). The panel heater is small, about 0.39kW on the large 2.5kW (Duo500 unit and 0.29kW or so on the smallest unit (Duo300). The controller is electronic, you set the background (E7 overnight) charge, and you manually set the day boost (peak). This avoids the "cold in evening" or "sized to keep heat in evening but fry you overnight" - ie, it aims to fix the problem of insufficient insulation (basically) since their temperature is limited to about 920-980oC).
Duoheat should be very good. I did look at them some time ago for a relative, and will probably go for them "someday". Water based would not be ideal, she is a retired pensioner, and they provide a foolproof solution. I did enquire about parts prices, they are sensible re control-panel on top cost (about =A312), internal PCB (=A329) and internal thin-film element panel heater (about =A350). One issue is that they are electronic and thus might not be as reliable as the "capillary tube automatic type" such as the Creda TSR24AW (A =3D Automatic, 24 =3D 24kWhr charge or 7hrs * 3.3kW). So if I did go for them I may also buy a few select spares (do not want to bin a heater because a part failed.
Creda's Credanet is similar, but appears more ugly - Duoheat seems more popular.
Now, the fundamental problem is *insulation*. Storage Heaters work fine for pensioners with lots of insulation - roof insulation to 200-250mm, cavity wall insulation. Without those you are losing the bulk of the charge overnight and going to end up buying lots of heaters to compensate and paying a lot more. With cavity wall insulation, they should work *very well*. The reason is the insulation by virtue of being in the cavity uses the inner block/ brick as a thermal store, so the storage heater's night-charge limitations are then masked by the *thermal mass* of your wall. That evens out the up/down UK winter weather (9oC overnight one night, next night may be -3oC with a howling wind which for most houses strips heat out of them).
Storage heaters are thus a good "part of house or background heat" for a wood-burner / gas fires (not decorative which put more heat up the chimney than they give out and so suck the heat out of a house in doing so). They are good for pensioners, they are not good for people coming home at 7pm expecting the house to be warm :-) That would require careful sizing AND really good levels of insulation (1995+ if not 2007+ noddy house :-)
Very old storage heaters suffered 2 problems:
- They had VERY poor insulation so lost a lot of heat overnight, running cold quickly the next day
- Potentially asbestos content, which can be an issue
I think the old ones were limited to 480-520oC core temperature which is pretty hopeless. Modern units from 920-980oC which is a huge difference re temperature delta over ambient air through them as the thing depletes. They were a dead duck from the get-go, relying on gargantuan size and miserable insulation to basically warm your bum.
Alternatives?
- Now, staywarm or warmup (can't remember the name) do grant assisted GCH installs. If your house is solid floors, complex joist layout, complex rooms re alcoves, doors, chimneys, multiple floors, unusual construction the result may not be pretty - and the misery is the grant is not "choose a local installer" which in my view is a key problem. They can install storage heaters in place of GCH, which can save a fair big lump of money - the biggest domestic storage heaters are
3.3kW and =A3350-400 each, if you need 2 big, 1 mid, 1 small, that is a lot of money. The electricity supply needs to be able to handle it - generally a 60A supply is limited to 2 big and 1 small , although it may be possible to upgrade it. Other alternatives are gas wall heaters, about =A3200-450 to buy and "whatever" to install. The UK products are a bit crap, a good one is Rinai 308 which is a 3kW output, microprocessor controller intelligent timer-controlled unit. It is Japanese and should be pretty reliable. They are not well known, but are a lot better than the dumb UK systems (Baxi, Drugasar, etc). They could easily heat a hall, bedrooms, bathroom, toilet from a hall position - 75kW output on maximum. I know a few people who installed them as backups to GCH and one who installed it in place of GCH and it really does heat the house (it has a fan in it, so does get the heat circulating around effectively). Fuzzy logic control.
- In 10yrs I expect CO2 air-heatpumps to be as common as convectors, pretty much, and slowly supplementing then replacing gas central heating. At present we are stuck on older refridgerant technology which whilst cheap (=A3200) does not work so well at -5oC, CO2 gives 1 CoP down to -25oC and 3:1 CoP at a UK 0oC which is 3kW out for 1kW electricity in.
If they ARE great big things dating pre 1984, you WOULD get a benefit from fitting modern units.
If a 3.3kW input I would look on Ebay for the Sunhouse 24kWhr model in Automatic form, do not buy Manual. There are generics too which in Automatic form will probably be the same - although a brand name may make spares easier. Once fitted they are disliked by installers because they are called Fit-n-F***off because no maintenance or repairs are needed, you can see the hatred amongst the Wet GCH systems which I've never liked (underfloor GCH is a different matter).
Insulation first, fix that - at least loft-insulation (payback 1yr), then CWI (payback 2yr). Then do a few sizing calculations, you may want to check wiring size and check uk.d-i-y.
Alternatively she may qualify for GCH - just have someone there when they install to avoid the idiot-screwups (and there have been a few, can't use the stairlift, atrocious installs, leaks, cables drilled through, the usual trade-professional making a d-i-yer look like einstein, that sort of thing - avoidable by someone Advising them). Moben & Dolphin are little better, so take heart all such installers :-)