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Night Rate Immersion Vs New Boiler

  • 07-11-2022 10:19pm
    Moderators, Science, Health & Environment Moderators Posts: 23,201 Mod ✭✭✭✭

    Tried searching this and googling.

    what is the cheapest option for heating water? Night rate immersion vs a new oil boiler?

    From what I read, in the summer it would be cheaper via immersion (with the added bonus of solar) but in the winter it would be the oil?

    is this correct?



  • Registered Users Posts: 1,353 ✭✭✭DC999

    @graememk crunched the numbers on it recently. Can't recall what thread. We don't use the hot water tank so I can't recall the detail.

  • Moderators, Home & Garden Moderators Posts: 5,458 Mod ✭✭✭✭graememk

    Depends on how cheap your night rate is!

    Oil has 10kwh per litre.

    New condensing boilers should be able to get 90-95% efficiency. New boilers should have good control methods too where you can priority the DHW only. Say 9kwh/L

    (At lower temperatures it will be in condensing mode, but once the return passes 55c it isnt condensing anymore)

    Electric is 100% efficient, 1kwh electric in 1kwh of heat.

    Oil is currently sitting at €1.24 (at 1000L)

    1.24/9 = €0.143

    If your night rate is cheaper than 14c night rate is cheaper.

  • Registered Users Posts: 1,353 ✭✭✭DC999

    Adding to that, electric is much lower in co2 emissions than oil. So that helps reduce the amount of co2 your house produces by a lot.

    I'm actively cutting down on using our gas boiler for that reason. Plus it's 15 years old so less it's used should extend it's life I hope.

  • Registered Users Posts: 64,281 ✭✭✭✭unkel

    Same here. Trying to heat water and house almost entirely from electric now.

  • Moderators, Science, Health & Environment Moderators Posts: 23,201 Mod ✭✭✭✭godtabh

    Looks like my night rate is 0.1879 with all the hikes. Must look at changing

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  • Registered Users Posts: 49 ForzaForward1

  • Registered Users Posts: 64,281 ✭✭✭✭unkel

    Gas is much easier as you consume in cubic meters, but you are billed in kWh equivalents, so the same way as you are billed for electricity

    The only difference is the efficiency of your gas boiler. Say if it is 90% efficient, then add 10% to your unit price, if you know what I mean

    If you pay 12c for gas and 13c for night rate electricity, then night rate electricity is slightly cheaper to use for room and hot water heating

  • Moderators, Science, Health & Environment Moderators Posts: 23,201 Mod ✭✭✭✭godtabh

    Who supplies 12c night rates?

  • Registered Users Posts: 3,205 ✭✭✭cruizer101

    I'm on a smart plan with ev charging rate of 10c from 2am-5am so can use that to heat immersion and its plenty warm enough for showers in the morning, even in the evening it is still warm.

  • Registered Users Posts: 64,281 ✭✭✭✭unkel

    Was just an example. Some people pay more, some people pay less.

    Personally I'm paying 7.9c per kWh incl VAT for nightrate at the moment with Energia, I switched at the end of August and this rate is fixed for a year. When I switched gas this summer I went to pay 8.9c / kWh incl VAT but Flogas upped the price by 56% 😯 last month, so now I pay 13.9c / kWh

    So in my case it is not only far cheaper to use as much heating as I can on night rate electricity and via battery (and from the sun of course) and also far better for the planet

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  • Registered Users Posts: 4,406 ✭✭✭Gerry

    Its easy when its cheaper, and better for the planet. But for most smart tariff folks Gas works out cheaper. I would like to go to Energia's EV plan and use the few hours to heat the immersion, but right now that works out significantly more expensive than SSE for the rest of the day.

  • Registered Users Posts: 1,353 ✭✭✭DC999

    There’s deffo an education piece needed for the masses wherever that comes from. A huge majority of Irish people are wedded to only a single type of energy for a single purpose. So only use electric for X. Use Gas for X… Though Ukraine has forced us all to think differently, which is good. And I’d include myself on that until this year when we got solar and an EV.

    That (overly simplistic) ‘ruleset’ is:

    1. Heating rooms = gas / oil / whatever burns in the fireplace (but don’t use electric for home heating)
    2. Heating water: see above (but don’t ever, never…. use immersion. Remember all the immersion jokes from parents in the 80s panicked when the immersion was left on)
    3. Car = petrol / diesel
    4. Everything else could be electric

    But as a lot of us here now know, electric can do all above. Though you could end up losing your shirt if you don’t know the rates you’re on for different services / time of use tariffs. Or if you treated electric heating like a gas boiler. A WFH office or part of a house is perfect for electric heating to supplement the overall house heating. Start small and see if it works I'd suggest.  

  • Registered Users Posts: 64,281 ✭✭✭✭unkel

    @DC999 - great post. And it can't be said enough that it is crucial you know which rates you are paying. For the people who pay less (or about the same) for a kWh of electricity (night rate) than for gas / oil it is important to use that electricity as much as you can. Personally I use a lot of electric heating now, haven't had my gas central heating on yet, only for a few minutes to see if it was still working. Also heat all my water with immersion now.

    Nice side effect is that it is much better for the environment to do so too...

  • Registered Users Posts: 1,453 ✭✭✭Evd-Burner

    I've been thinking about this for a while, not only heating water but heating the entire house with nighttime electricity and would it be possible.

    Currently I have 3 hours of just less than 9c for electricity at night and after I take out boiler efficiencies etc I use about 40kWh of heating during the coldest months, DHW only does the odd bath for the kids and this would be included in the 40kWh.

    In the future (pending more capital and prices to come down) when we extend the attic the plan is to:

    1. Install large water tank for hot water, I was thinking 300L (2 adults 6 kids) with mixing valve to allow 75C internally in the tank temperature. I could potentially go to a 500L and use for DHW & Heating, at 75C that's a potential 35kWh of storage.
    2. Spray foam and PIR in attic extension, improving on current insulation values and airtightness.
    3. Install outer insulation (saving on erecting scaffolding twice) and new triple glaze windows (already have new doors, all DIY) which will improve the building fabric.
    4. DIY 6+ kWp solar array SE.

    If the above improvements brought the heating and DHW for the coldest months down below 35kWh the thermal store could last a full 24 hours. If I was to heat this off nightrate only at 9c I would save 7c per kWh, plus I could get rid of the gas standing charges for gas.

    If we ever see smart meter pass through pricing we could potentially see sub 5c per kWh, at that rate for at least 5 hours it wouldn't make sense to install a heat pump in my opinion.

    Food for thought.

  • Registered Users Posts: 1,353 ✭✭✭DC999

    Fair play 1st off for trying to cut back. Is 40kWh of heating a day during the coldest months?  I’ve 2 plug-in oil filled electric rads at the moment (small house and it’s mild still). I’m only using under 6kWhs a day for the whole house. We’re only heating downstairs and heat travels itself upstairs (we don’t like it warm upstairs anyway). Not saying that to make us sound great, but it’s a benchmark. And that’s single glaze windows and lots of air leaks. We don’t use the DHW - use an have electric shower. No bath.

    Our setup:

    • 3kWh a day on one that does the sitting room and kitchen. 400w rad. That runs 24x7 but on a thermostat at 17c at night. And 18.5c in the day (which is colder than some would have it I know, but our thermostat might be a little off of course). Reason I’m testing running the heat for 24x7 is to let the solid walls heat up so they become a thermal mass. So far it seems to make a difference. When we open doors or windows the room temp bounces back quicker that it did before, or so it seems to me
    • Then same again on a north facing cold WFH room. It’s much smaller but much more leaky and gets no solar gain
    • Both on a thermostat plug to turns the rads on and off when hit's planned room temp. Cost 25 quid each on Amazon
    • Set the dishwasher to finish when we get up in the morning. It's the kitchen 'heater' for that time

    If we don’t close the doors downstairs, we can’t heat the space. Heating just runs and runs. So I got a door hinge on Amazon for ~25quid. Auto closes the door. That’s made a massive difference. Some days the sitting room only needs 1-2kWh the whole day (over a 24 hour period). 

  • Moderators, Home & Garden Moderators Posts: 5,458 Mod ✭✭✭✭graememk

    Couple of notes, more things to think about.

    You'll not get 35kwh out of a 500L tank, more like 20-23 (cycle between 75 and 35). Very hard to cycle below 35c.

    How are you thinking of using it for DHW.. I have some ideas but plate heat exchanger + pump sort of thing for "on demand hot water" like a combi boiler.

    I am wary of spray foam. Hearing very mixed opinions of it regarding roofs, (could be an issue when going to sell).

  • Registered Users Posts: 1,515 ✭✭✭Gooser14

    Is that the ex VAT & does it take the standing charge into account?

  • Registered Users Posts: 64,281 ✭✭✭✭unkel

    @Evd-Burner - "I've been thinking about this for a while, not only heating water but heating the entire house with nighttime electricity and would it be possible."

    Yes. But you will need large storage for heating. And a very large solar PV install will help too. A large well insulated cylinder will store your hot water needs for 24h, heated from night rate. That's probably the easiest. Then you could use some very large old skool storage heaters, particularly if you need to heat the house during the day with people WFH, you can pick these up cheap as chips. Then of course a large chemical battery for electric heating later in the day (before night rate kicks in again).

    I have done or am in the middle of doing all of the above myself, should be finished very soon. I plan to not replace my gas boiler when it is end of life. I do accept that even with all that in place, on very cold days, I will have to use some very expensive day rate electricity.

  • Registered Users Posts: 1,453 ✭✭✭Evd-Burner


    Under 6kWh seems very low to me. I've the house set at 20C with the TRV's set to 18C for upstairs, the upstairs usually sits around 19 when downstairs is on, the rads tend to only come on first thing in the morning. I've fixed nearly all of the air leaks in the house and that has the house comfortable with 40kWh a days from about now till mid February. Heating is on from 6am till 10pm, I work from home and always wife/kids in the house so its always needed.

    How many do you have in your house?


    40kWh is what we are using now, with the upgrades (and extra attic space) I'm hoping it would go below 30kWh per day at least, but wanted spare capacity.

    Your right on how much storage is possible at that temp difference, I worked it out wrong. Not sure I would get more than 500L in, could go to 90C in the heat store depending on the unit. That'd give it closer to 27kWh, also might be able to tweek it by having the downstairs rads on during the cheap night rate to maximize efficiency of the cheap rate.

    We've also always wanted a stove, and as its a heat store we could easily put a back boiler in, the store can take multiple sources.

    Mixing would be done with 2 x 3 way Thermostatic mixing valves. DHW set to 50C with the attic cold water feed on the cold side and the heating set to 60C with the cold side being the radiator returns.

  • Registered Users Posts: 1,453 ✭✭✭Evd-Burner

    I was replying and never refreshed the page when you commented, hence the skip. We are upgrading the tank anyway as its starting to become not fit for purpose as they kids are getting bigger, will have 2 teenagers this time next year and then 5 teenagers within 3 years! It isn't hugely more expensive going from the 300L i had looked into vs a 500L.

    Your going to have to excuse my ignorance I haven't heard of these old school storage heaters, what are they?

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  • Registered Users Posts: 5,231 ✭✭✭con747

    Don't expect anything from life, just be grateful to be alive.

  • Registered Users Posts: 64,281 ✭✭✭✭unkel

    We're in a similar situation with 5 person household and 3 people working from home in different rooms and large garden shed most of the time and elderly granny over a lot in the evenings, so 19C in the rooms is not an option. Also with 4 adult and teenage females, we use a lot of hot water. Our cylinder is 360l (biggest that physically fitted) and already have 40 solar thermal tubes too, fitting an eddi in the next few days. Very large PV setup helps a lot on days in winter with a good bit of sun (like today)

    Those storage heaters are the way most cheap apartments in Dublin were heated in the 90s. Basically a metal enclosure with ceramic / concrete bricks in them. You heat them up with a simple element (like in a hot water cylinder) with cheap night rate and you open the vents on a timer later in the day to release the heat. Simple 100% efficient heating but the timing is slightly dodgy as they don't keep the heat in all too well. Not suitable or really expensive if you are in work all day and only need the heat at night, but perfect for when people are in the house all day.

  • Registered Users Posts: 5,231 ✭✭✭con747

    The link above on done deal shows the inside workings for people who never seen them. I know you did! For others that haven't.

    Don't expect anything from life, just be grateful to be alive.

  • Registered Users Posts: 1,353 ✭✭✭DC999

    Only 2 rads on at mo, both downstairs. Both connected to a thermostat plug so they never overshoot on temp. I've no idea how much kWh they will need once it's cold. And wind makes the house very cold - very leaky house. Only trying it this year for 1st time as have solar now.

    Well, lots of stuff can heat a small WFH office. I've 200w of heating and that keeps it just about warm enough. Won't be enough when it gets colder, but grand for now. I'm grand in 18c for much of the day. I've an 80w heater - puts out very little heat but stops the room getting cold. Then 140w dehumidifier actually heats too. Both are deffo 'low and slow' so need to start on a timer well before the work day. I'm kinda enjoying seeing how little watts I need to keep the room warm. Hot water bottle on my legs and heated gilet (that runs from a usb power bank) mean the room can be a lower temp. I've a 700w heater but that makes the room go up in temp a lot in a few mins (perfect at start of work day) but which doesn't feel comfortable for me beyond that. Am sort of messing around to see what works and then I'll get the right sized heater. Might get an infrared heater but wanted to use what I owned for the start to test stuff.

    We humans put out about 100w. Computer and monitor put out heat too. So sitting rooms heat up once kids are home and they are watching telly.

    Then 400w heater in sitting room which does kitchen too.

  • Registered Users Posts: 64,281 ✭✭✭✭unkel

    Ha yeah, no I never saw the inside of one before, but that's exactly what I was expecting 😁

    You'd need to do some research but it's possible you could bring that insulation more up to modern standards if you want to keep the heat in it longer for release later in the day. For me they would be perfect as is, as I would need the heat from the second night rate finishes

    BTW you should be able to pick them up much cheaper than that. Basically they are going into skips every day of the week.

  • Moderators, Home & Garden Moderators Posts: 5,458 Mod ✭✭✭✭graememk

    I have a 500L tank plumbed in as if it was a oil boiler. (It has radiator water in it)

    Can be heated via immersions(night rate & solar) or via the stove.

    You'll not really get much past 75 as it trips the thermal cutout on the immersions.

    Trying to do both DHW and heating will be tricky. How handy are you with plumbing?

    Another method could be is to get one with coils, but use the coils as a way to pull heat out of the tank instead of putting heat in.

    Then tank then does DHW and some heating.

    Depending on how the coils are situated you'll always have a top 1/3 of the tank always hot, and not disappearing into the house.

  • Moderators, Home & Garden Moderators Posts: 5,458 Mod ✭✭✭✭graememk

    Interesting seeing the inside, it's what I had guessed too.

    I was gonna suggest replace the insulation inside but don't know how hot the bricks get.

    I think that's the theory behind the nezb boiler, it is just a really big storage heater with better control.

  • Registered Users Posts: 64,281 ✭✭✭✭unkel

    Yep exactly. But it costs £4000, and these heaters can be got for free 😁

    If you upgraded the insulation that it became so good that it could hold say 70-80% of heat in for at least 10 hours, these could work until the evening too. Also of course very interesting if you have a very large PV setup. Couple of eddis and you could power 4 of these 1500W lads for heat throughout your house for the whole evening (or from night rate on not such a good day). All of a sudden this is then by far the cheapest "battery" per kWh you could possibly get!

  • Registered Users Posts: 5,231 ✭✭✭con747

    Yeah, you should be able to pick them up for next to nothing for the older ones, that link was just to show the poster what they were. A much cheaper one close to you up there too. I can see them in your shed by the weekend. 😂

    Don't expect anything from life, just be grateful to be alive.

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  • Registered Users Posts: 64,281 ✭✭✭✭unkel

    Not quite by the weekend, although wouldn't totally rule it out. And not in my shed. But the biggest ones are 3kW output (with 15kWh storage) and just the one of those would be perfect for my living room. Hooked up to my new eddi that I'm picking up tomorrow.