10.5kw Shower and Storage Heaters?

Discussion in 'Electricians' Talk' started by dandare1980, Jan 11, 2020.

  1. dandare1980

    dandare1980 New Member


    I would be grateful for some advice regarding a potential electrical upgrade which I intend to get completed this summer when we have our kitchen and bathroom refurbished. I will engage the services of an electrician nearer the time, but wanted to explore some possibilities to help with planning the works.

    A bit of context first - The property is a small ex-council 3 bed semi built in 1937 and most electrics appear to be from the 80s.

    Everything is set up around economy 7 (no option for gas). Heating is delivered via 7 storage heaters, hot water comes from a cylinder with immersion element and the kitchen has a free standing electric cooker.

    EON upgraded us to a smart meter when we moved in and fitted a separate non-adjustable time switch which activates the storage heaters via their own consumer units (2 in total).

    The economy 7 night rate is active for 7 hours between 1:30 and 8:30 am in the summer, and 12:30 and 7:30 am in the winter.

    The storage heaters collectively draw as much as 13.6 kw at peak. The 3 kw immersion is on a timer and generally used between 5:30 and 7.30 am, then a short boost 5:00 to 6:00 pm.

    We don’t have a shower and would really like to fit a 10.5 kw unit when we upgrade the bathroom. This is where I have concerns – although our supply is rated at 100 amp, would I be in an overload situation if I was to shower when the storage heaters and immersion are all going together?

    This is likely as we are all up and getting ready before the storage heaters switch off. Worst case scenario would be the storage heaters, immersion, hob plate and kettle are all going while the shower is active (other than these items we have very few other power demands).

    I understand that principles of diversity may apply. In practice, when the storage heater circuits are first activated there is likely to be a big draw, but as we’re all in bed, by the time morning comes and we want to use the shower etc. most heaters will be up to temperature and only drawing intermittent power through thermostatic control. Of course, none of this is an issue in summer.

    Should I instead consider a less power hungry 8.5 kw shower or do away with the idea and opt for better stored water and a pump (not preferred)?

    Our current collection of consumer units (4) is a total mess so I will be getting quotes to have the lot upgraded. I am hoping it will be no more than 2 units, one for storage heaters the other for everything else.

    I wanted to consider RCBOs instead of a dual RCD setup, but the cost may be prohibitive. I have thought about only applying the RCBOs to the main board, leaving the storage heaters protected by a single RCD. I have often seen showers on their own consumer unit but assume this wouldn’t be necessary with an appropriately sized consumer unit.

    The electrician will need to install it all to meet regulation, so I may have no say in the matter on this point anyway.

    Any advice or recommendations will be gladly received.


  2. Bob Rathbone

    Bob Rathbone Screwfix Select

    If you don't take a shower at night, ie 1:30to 08:30 then you should be OK. A morning shower before 08:30 will probably be OK as the storage heaters will probably be charged and their charge thermostats will have switched them off.
  3. dandare1980

    dandare1980 New Member

    Thanks Bob, that's what I'm hoping.

    If I can get my smart meter's old power display working I'll see what the storage heaters are pulling down tomorrow morning.

  4. Jimbo

    Jimbo Screwfix Select

    Maybe it is possible to add a load monitoring controller to the storage heater distribution board, like the EV chargers are starting to do for the same reason.

    Another option would be to fit a pumped thermostatic shower like Aqualisa Quartz, and just used the stored hot water you already have. This will give a much better shower but with only an electric immersion heater, hot water recovery time could be a problem.

    Although a much, much bigger job... converting to a heat pump could be worth a look, as there are government grants available that will cover half to two-thirds of the cost and the electricity cost will be around half too. Obviously depending on the running cost of the existing system it may or may not be viable.
    WillyEckerslike likes this.
  5. dandare1980

    dandare1980 New Member

    Thanks Jimbo,

    I've had a suggestion from another forum's member that I could ask the electrician to consider using a contactor with a priority shower device - these are used in properties that have more than one electric shower and effectively prevent them from operating at the same time - although in this scenario it would be used to temporarily de-energise the storage heater circuit/consumer unit when the shower is active.

    It's actually a contactor that switches on our storage heater circuit now via the meter, so it must be possible when coupled with a load sensing device like you suggest.

    My only hesitation here is it's more kit that's got to be attached to the wall. I'm still thinking diversity will win overall and as long as the storage heaters, main circuits and shower have their own consumer units, we shouldn't be in a situation where the 100 amp DNO fuse is threatened (electrician will have final say ultimately).

    We have considered an upgraded tank (current one too small for pumped), but other than washing we don't have a huge demand for hot water.

    As we need to upgrade the consumer units and add a new circuit for the kitchen anyway, I would assume it's comparatively less expensive to opt for an electric shower Vs adding a new tank and pump or an unvented cylinder.

    Despite the lack of immediate temperature control with storage heaters, we've found them to be a very reliable solution to heating our house. They also have practically no servicing costs Vs an oil fired system which is our only mainstream alternative.

    In a typical year we use around 11,000 Kw on the night rate, which at our current 6.05 pence per Kw you're looking at £665.50 per year.

    Any savings made from an alternative system would take years to pay back and we're not likely to be here long enough to benefit from it unfortunately.

  6. dandare1980

    dandare1980 New Member

    I think for simplicity I'll press ahead with an 8.5 Kw shower, just to give me a few more amps of wriggle room.

    I've contacted Western Power and they're happy to send someone out to check the main fuse is rated at 100 amp (fingers crossed).

    I'm also contacting my supplier EDF to see if they can rid me of the smart meter - it hasn't been "smart" since 2015 when I changed provider. The setup with an external heating contactor is both messy and very noisy when it operates. The previous Landis + Gyr meter had an internal contactor and relay for the storage heaters which was silent in operation and much less cluttered.

    The shower priority devices are quite interesting, but hopefully an added cost I can avoid by choosing a lower rated shower.

    I want to stick with an instantaneous shower device Vs a stored solution, as it will require some fairly extensive tank upgrades otherwise.

    I'm getting the consumer units upgraded and as the shower is no more than 5 metres from this location the cost for installation shouldn't be too bad. If I opt for an unvented cylinder or bigger tank and pump, it's going to cost considerably more.

    Out of interest I observed the wattage draw when I got up this morning at 5.30 am via my smart meter's half-broken wireless screen. The storage heaters were active and the immersion had just come on timer. The draw ranged from around 2 Kw to 8 Kw peak, but the higher loads were only present for several minutes at a time.

    I'm hoping the electrician will be happy to fit all general circuits including the shower onto a 100 amp consumer unit with RCBOs. The storage heaters can be consolidated onto another 100 amp unit, but I might request a split dual RCD board to keep costs down - hopefully with a mix of heaters on each protected run so if a fault condition occurs in the middle of the night I won't lose all the heaters.

    Hopefully this approach won't see the Henley blocks, meter, contactor or main fuse overloaded - not for any considerable amount of time anyway.

  7. Jimbo

    Jimbo Screwfix Select

    You can request a free upgrade to 100A supply anyway.
  8. dandare1980

    dandare1980 New Member

    Thanks Jimbo, that's good to know.
  9. Heat

    Heat Screwfix Select

    A 8.5kw shower works well in normal weather. Only real drop in performance is when mains cold water temperature gets very low in severe weather. Shower has to heat the water as it flows through the shower, so flow will be less for required heat
  10. dandare1980

    dandare1980 New Member

    I think we should in general be ok with 8.5 Kw (we have no shower at present, so anything is an improvement).

    Some people have commented that they can't tell the difference with more powerful units, or that it's not significant.
  11. Heat

    Heat Screwfix Select

    I have been told same about little noticeable difference in performance.
    Personally, although a plumber, I can’t say as don’t have a electric shower, but 8.5kw showers with decent shower head seem to work fine that I have installed.
  12. dandare1980

    dandare1980 New Member

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