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Thermal store system efficiency

Discussion in 'Plumbing and Central Heating' started by neilht, 20 Sep 2021.

  1. neilht

    neilht

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    Hi Gents,

    currently using just over 30,000Kwh of gas a year for hot water and central heating in a detached 4 bed that is well insulated and a 1990 build. Need to reduce usage if possible as seems a bit excessive.(cooking is electric)

    The gas boiler feeds a vented thermal store tank in upstairs airing cupboard. The hot water is mains pressure so comes from a coil in the tank. The CH also is piped off the tank with another pump for circulation so both HW and CH come from the same tank.

    Problem is if someone has a long shower when heating is on the heat is depleted from the tank, you end up with cold radiators whilst the boiler tries to regenerate the thermal store and heat the house at same time. It’s only going to get worse as the three kids grow up.

    It seems a bit crap to me. Should I put a larger thermal store in for the mains hot water and have the boiler directly heating rads with a Y plan system?

    Its a IMI flow 30 cylinder 180 litres and boiler is a potterton profile looks to be original from 1990.

    Also there is a wall switch downstairs that turns on the immersion heater in the cylinder . Electric meter is economy 7 so would the switch be just for overnight heating of the tank?

    Any opinions welcome!

    cheers
     
    Last edited: 20 Sep 2021
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  3. jeff the gasman

    jeff the gasman

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    You need an unvented cylinder, new controls and maybe a new boiler.
    Get a trusted local guy to give you some options and prices.
    No cheap fix!
     
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  4. oldbuffer

    oldbuffer

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    1. I'm a little surprised the cylinder is by IMI, I thought they only made controls / valves. The most common, though not the only, make of thermal store is Gledhill.
    2. Thermal stores are inherently a little less efficient than, for example, and unvented hot water cylinder. For any given level of insulation, heat loss is proportional to the difference between the temperature of the medium (water in this case) and the ambient temperature. Most thermal stores have the water at around 80 degrees, possibly even a little higher. However, I wouldn't expect this to account for your using almost double the typical higher end gas usage.
    3. The unvented hot water system suggested by jeff the gasman is probably the best route for you, provided you have the cold mains water pressure and flow rates required. You should be looking for a minimum of 2 bar dynamic pressure and 20 litres per minute to make it viable. Be aware that installation can be quite disruptive, and can only be undertaken by someone with a current "G3" registration.
    4. The Profile is getting a bit old now, and you could consider changing it for a more efficient one, although I doubt you would recoup the cost in any sensible period through gas saving. However, it doesn't HAVE to be changed to install an unvented hot water system.
    5. The immersion heater in the thermal store is likely to be linked to the economy 7 tariff. It would be used to re-heat the full body of water at the overnight rate. The electricity on that tariff is only available for a few hours overnight, and the immersion will contain a thermostat to turn off the electricity once it has reached temperature. Provided it IS on economy 7, I'd suggest leaving the switch on permanently provided a kW of economy 7 electricity is less than a kW of gas. You could check via your latest bills.
     
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  5. oldbutnotdead

    oldbutnotdead

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    180litres is a bit small for heating and hot water. You'll be better off splitting the heating so it runs direct from the boiler but you'll still have a problem of thermal capacity on the store (3 kids, overlong showers). You could try increasing the temperature of the thermal store (as long as you have blending valve(s) on the DHW outlets- yes the heat loss over time is greater but higher store temp means more energy stored. Any idea what your boiler heat output is?- long as it's 15kw or above it'll cope, as above a new boiler would be more efficient (and give you options eg if you have more than one bathroom you could use a combi to run dhw to one bathroom and keep the thermal store for the other).
    Yes the modern way is unvented but if your store is well insulated and in good condition then keep it, you're getting the benefits of mains dhw without the annual servicing costs.
    Economy 7 immersion heater- check your bills (gas and electric), see what the price per kwH is. With the gas, divide that cost by 0.7 (assumes 70% efficiency)- the electric heater is near enough 100% efficient. See which is cheapest, use the cheapest one.
     
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  6. oldbutnotdead

    oldbutnotdead

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    PS Thermal stores work well with various alternative energy sources- eg woodburner with back boiler, wet solar panels, PV solar panels, heat pumps and (if you have a smart tarriff and smart meter) as an energy sink when there's too much wind or solar on the grid so don't be too hasty to ditch it.
     
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  7. neilht

    neilht

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    Thanks for your input Gents.

    I’ll put a picture up of the cylinder tomorrow. It looks more like a standard cylinder with a couple more connections than a proper thermal store.

    Will look at the boiler specs and post them up too but I’m inclined to keep it until it fails and is unserviceable.

    Cylinder stat is set to 75c currently and boiler on 5. The cylinder feeds 2 mixer showers, 1 bath and 12 rads.

    One thing for certain I can do is separate the radiators from the store with a y plan 3 port valve.

    The immersion switches on in the daytime if I flick the wall switch so can’t be on a timer.

    The energy policy is up for renewal in October so may be able to factor in cheaper overnight rates and using immersion to regenerate.

    I’m tempted to fit a better cylinder and separate the radiators so boiler heats them directly. Is there a big difference between vented and unvented cylinder efficiency?

    Cheers
     
  8. oldbutnotdead

    oldbutnotdead

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    Main efficiency difference will be the insulation- factory fitted panels will beat lumps out of a loose blanket, the thicker the better (you get to diminishing returns at 150mm Iirc). The bigger the store the lower the heat loss (with same thickness of insulation and shape the surface area to volume ratio decreases as you get bigger).
    Immersion- old Economy 7 often had separate circuits that only ran on the cheap tarriff. Most nowadays don't have teleswitches etc, the meter records when you used the energy, usually peak rate is a bit higher than the average, cheap rate is a lot lower. Electricity (even off-peak) is generally more expensive than gas so use the immersion cautiously.
     
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  9. neilht

    neilht

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    Boiler is the potterton 50e 14.65kw max

    Pics below of the setup.

    I’m thinking of splitting the radiators into a y plan to separate from the cylinder and also putting thermostatic rad valves on all radiators at the same time.

    Will also put some sort of inline filter somewhere.

    I could upgrade the cylinder to something with a better heat exchanger and a few more spare coils for solar / heat pump in future maybe go for 250 litres? I’m going to keep a vented setup the mains pressure here is very good

    looked at my energy tariff and gas is still cheaper than the economy 7 rate even with 70% efficiency factored in. The immersion is just for emergency then!

    3B6CA878-A0D5-4FEE-9538-A00FBC7952F7.jpeg
    AD6FF828-C136-44A5-BF71-14FED7C0E56C.jpeg
     
    Last edited: 21 Sep 2021
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  11. neilht

    neilht

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    Just checked the pipes either side of the CH pump and they are hot, could I be getting a bit of gravity circulation? Rads are all cold.

    Wondering if I split the radiator circuit off will I need to add another vent / overflow pipe to the f&e tank? I would connect the rad circuit return to the boiler return that is coming out of cylinder and just feed the rads / cylinder with a 3 port valve so the return would still be there for any expansion?

    Thanks!
     
  12. oldbutnotdead

    oldbutnotdead

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    You won't be getting much cos whoever put the system in knew what they were about (heat rises. Feeds travelling downwards so no convection).
    It's the heat source needs feed not the rads so as long as the boiler is always connected to the f & e you'll be fine (yes via the return will work).
    Me personally I'd use a pair of 2 port valves (don't like 3 port). Advantage with 3 port is if dhw is the default position you don't need to worry about pump overrun and bypass- the cylinder will be in circuit to dump excess heat at the end of a firing so if you can do it neatly with a 3 port it'll save you some faff.
    My store is from this lot https://www.heatweb.co.uk/hot-water-storage/heatbank-thermal-stores/ , sensible prices still and helpful on the phone.
    Heat pump- pretty certain they don't like direct so yes extra coil to allow for that.
    Consider a plate hex for the DHW- means you can use all the water in the cylinder to heat dhw rather than relying on internal convection to keep the coil warm. Of course it knackers the stratification layer but meh...
     
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  13. neilht

    neilht

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    thanks, have you got your rads separated from the store out of interest?

    Ive been looking at some instructions / manuals and they recommend 170 litres for 1 bath/2 showers but don’t mention the heating of rads?

    couple of things I need to check, the mixer valve is giving hot water at 55 deg and the arrester vessel is charged to 3 bar (with mains off?)

    Looking at 3 port valve and thermostatic rad valves now at the very least.
     
  14. Gasguru

    Gasguru

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    Fit 8 litre/min flow limiters to the showers and 10 mins max for a shower....unless the kids want to contribute to the gas bill.
    Solar thermal is a total waste of time...it never pays for itself once the maintenance/repairs are taken care of. Heating is the main cost of your gas bill.
    Invest in jumpers and knock the room stat down to 19.
     
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  15. oldbutnotdead

    oldbutnotdead

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    I've not bothered with solar- as above the ROI is very poor. I've got the option with my setup- rads either from the store (wet woodburner also feeds the store) or direct from the gas boiler. Not really hammered the dhw with heating on the store (the place is undergoing a very slow renovation)- my suspicion is when I'm here alone it'll cope, a houseful might strain the dhw provision in winter. Only had the gas boiler fitted last month so not really tried it yet...can see me going back to wood if gas price gets silly (just takes time cutting & storing it, buying firewood is v pricy per kwH, coke is far more sensible but still pricey)
    EDIT Can't remember if I mentioned it elsewhere, mine is 300l
     
  16. neilht

    neilht

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    I've just checked the mains water pressure and its a touch under 4 bar.

    would it be best to add PRV to the DHW pipe after the mixer valve and set to 3 bar?

    need to buy a thermometer and check the temp also.

    whats a good brand of thermostatic rad valves? are there any to avoid?

    Thanks!
     
  17. oldbutnotdead

    oldbutnotdead

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    If you do put a prv in you need to make sure it regulates the cold feeds to any shower bars ..at 4 bar I wouldn't worry too much
    TRVs- I've chucked a load of Toolstation contract ones in but put Wiser WiFi heads in then- if you have the valve in the return they're a bit noisy but apart from that seem to work fine
     
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