Tank or Instant Heater for Hot Water

RhythMick

Octopus are visiting us this week to quote for replacing our 10 year old gas boiler with a heat pump.

We actually don't use a lot of hot water and it's always seemed odd to me to keep a tank full heated and ready. Aside from shower (which is a thermostat mixer type so we need a hot water feed) and washing up we quite rarely need hot water from the taps.

I'm wondering about whether to look into an instant on-demand hot water heater instead of a tank. I think that would remove the need for a tank (heat pumps don't actually NEED a hot water tank for central heating purposes do they ?). 

Anyone else go down this route ? What are your experiences ? I don't know if Octopus would be able to quote for one or even discuss the options but I will ask them.

Netexporter

A halfway-house solution might be the Heat Geek "inside-out" tank, which is designed to fit in a standard kitchen cupboard, of the space vacated by the gas boiler, and acts like an instantaneous heater but still allows time for the heat pump to fire up to provide a larger quantity of water, for a bath etc.

The Worlds SMALLEST Cylinder is here!

Heat Geek chose not to patent it, so it is available to any installer.

wrf12345

Not sure if you can get the £7500 grant unless the heat pump can heat water or is that only if replacing gas boiler?

Nick_Dr1

Octopus fit systems that meet the defined requirement for the size of the house, not the current owners needs. I'd expect them to fit a water tank of their recommended size for the property. Up to you whether you turn it on or not!

matelodave

As said above, if you want the £7.5k you have to have a system that conforms to MCS standards for your house - you may not live there forever

NedS

We have a 160L tank, and at the moment no shower fed off it as bathroom is being redone and we're using an electric shower for now. Reheating the hot water tank daily is using less than 1kWh, and is covered by solar so costing maybe 10-12p per day. If you don't think you'll use much hot water, ask Octopus for as small a tank as possible (see the MCS guidelines on DHW tank sizing below, general guidance being 45L per occupant)

How many occupants?

How many bedrooms/ bathrooms?

Do you have a dishwasher, or wash up in a bowl of water?

Any other notable hot water usage?

The main thing to remember is that a heat pump is hugely efficient - maybe 250-300% (COP of 2.5 to 3) for a hot water cycle. So even if you heat 3 times more water than you need, it would still cost the same as a direct water heater which is only 100% efficient.

https://mcscertified.com/wp-content/uploads/2019/08/Domestic_HW_cyl_selection_guide.pdf

For example, based on the above we have a 3bed/2bath property with a larger 12kW heat pump, so would fall under the 35L per occupant guidance. One could reasonably expect 4 occupants in a 3 bed property (2 adults plus 2 children), so 4 x 35L = 140L using the 35L per occupant method, or 150L to 180L using the tables, so our 160L tank sits in the middle of those recommendations.

There are other solutions for where a DHW tank is simply not an option due to space constraints, but in the vast majority of circumstances a DHW tank will be the preferred solution. I think with 2+ showers per day and regular washing up / hand washing, you will not have a huge amount of hot water left over at the end of the day.

stripling

I have the 180litre Daikin tank and it gives me 2 days of high pressure hot showers & washing up. It's a good piece of kit that costs me a 1Kw - 1.8 Kw to heat to 50° from around 38° . It sits in my cellar. 

matelodave

stripling said:

I have the 180litre Daikin tank and it gives me 2 days of high pressure hot showers & washing up. It's a good piece of kit that costs me a 1Kw - 1.8 Kw to heat to 50° from around 38° . It sits in my cellar. 

Ours is also a Daikin 210 litres and like you we only heat every other day but we only heat to 45 degrees, plenty hot enough for showers - takes just over 1kwh in the summer and a bit more in the winter. If we really want extra hot water for washing up then half a kettleful does the job.

We've got an Eco shower head which still gives a more than adequate flow and pressure but only uses 6 litres per minute

Alnat1

We're 2 people in a 4 bedroom house. MSC requirements (in order to get grant) are that the tank is suitable for the size of the house, not for the people currently living there, so our tank is 185L.

We have it set all year to heat for 1 hour a day to 47C at 10pm, no reheat, plenty for 2 showers next morning and some left over for use in the day. This uses 1kWh of electricity in summer and 2kWh in winter. The old tank, heated by gas, used 3-6kWh so probably cost a tiny bit more than the heat pump.

In winter the heating is setback by 2C from 9pm so we heat our water at 10pm when the house won't need warming and the Cosy tariff is cheaper. The tank may have dropped 1C by the time we shower next day but this heat is helping keep the house temperature up a bit overnight and the shower is plenty warm enough.

My only "complaint" is that the cupboard doesn't get as warm as it used to because the new tank is so well insulated. It takes way longer for my bread to prove!