180l tank for 2 people

barker77

I need a slimline tank for my install (just me then daughter every other week too) and octopus want to install a 180l tank that’s 475cm x 1880. This seems massive and I wondered if anyone had a thin tank installed that’s smaller size ? Im worried about the costing of heating water I won’t use 

tacpot12

You didn't say whether you are having a heat pump installed by Octopus. I assume you are. Do you shower, and does the shower run from the tank or is it an instaneous electrical shower?

Also, are you receiving a grant towards the installation of the heat pump? If so, you might have to accept the larger tank to get the grant. You can pay to have it changed later and sell the tank on eBay. 

If you shower and the shower is fed from the tank, then you probably need a larger tank than you have currently as the water won't be as hot. Probebly 50C vs. 60C. 

NedS

We have a 160L slimline tank (Joule Kodiak) as we were limited by size. The tank has a rated heat loss of 79W.

Tank size is normally based on the property (number of bedrooms) rather than the number of people currently living there. 180L would be about right for a 3 bed property.

barker77

Sorry yeah octopus install. It’s a 3 bed house but as mentioned 2 occupants and that’s only every other week … shower will be fed from tank like it is now. I shower daily as does daughter when here every other week but hardly have any baths. @NedS how much does that cost you to heat? I would have preferred a tank in the loft but apparently my roof doesn’t have the height or something 

NedS

How much it costs will depend how hot you heat the tank and it's starting temperature, how warm it is outside and thus your COP on the day, and on the rate you pay for electricity. For us, it's always significantly cheaper to heat our DHW than it would be with gas or oil as we reheat on cheap rate electricity, generally with a COP of around 3.

Take a look at this water heating calculator:

https://www.omnicalculator.com/physics/water-heating

Lets assume you heat your 180L tank to 48C every day (a typical temperature for DHW with an ASHP system), and that the tank had cooled to 30C. The calculator above shows that requires 3.77kWh of energy, and with a COP of 3, that would require 3.77/3 = 1.25kWh of electrical input at the ASHP. I pay 12.86p per kWh cheap rate on Octopus Cosy tariff, so it would cost 16p per day to reheat the hot water tank (or £58/year). You should have plenty of hot water for 2 showers. The key here is to schedule DHW reheating at cheap rate times.

Alnat1

Ours is a bigger tank, 2 of us in a 4 bed house and we had similar concerns. Hubby showers every day but I shower at the gym several times a week.

We've got it set to 47C and heat it once a day, not set to reheat. In the coldest weather (incoming water colder) it uses 2kWh a day, sometimes less. If the hot water doesn't get used it only drops 2-3C over 24 hours. There's plenty of water for 2 showers and we've not run out yet.

FreeBear

barker77 said: I would have preferred a tank in the loft but apparently my roof doesn’t have the height or something 

Better to keep it within the thermal envelope of the house. Up in a cold loft (assuming you can get a tank up there), the heat loss would be greater. Having it inside the house, the wasted heat would help to keep at least one room warm - Not a lot mind you, but it all helps to save energy.

matelodave

The advantage of a larger tank is that you can have a lower water temperature rather than heating it it up to 60 degrees or more and then diluting it back down with cold. This not only reduces the amount of energy required to get it to a higher temperature but significantly reduces the heat loss from the tank.

We have a 210  litre tank for the two of us and only heat it to 45 degrees (it gets a legionella cycle once a week *). This give us more than enough water for our daily needs (at a pinch we can get four showers out of it) and we can use the hot water straight out of the tap.

*The legionella cycle runs on Saturdays and we then have enough hot water to inhibit the tank heating cycle on Sunday so it doesn't really add much the to cost.

Exiled_Tyke

I'm in agreement with the previous comments.   There are two of us with a 240 litre tank.  I can get away with heating this to just 44 degrees on cheap over night electricity and no need to reheat during the day.   Lower temperatures are massively more efficient for an ASHP so this works out very well for us.   

barker77

Thanks for the advice everyone! Can I ask what the dimensions are? Im after something thwt would fit in a 52x59 x 2m cupboard if possible 

FreeBear

barker77 said:

 I'm after something that would fit in a 520x590 x 2m cupboard if possible 

Don't forget you'll need space to get a spanner on the fittings and room for an expansion vessel - Much easier to build a cupboard around a tank (making sure it is removable should the need arise) rather than trying to install a tank in an existing cupboard.