Heat pumps for larger household (more than 8 people)

SamK2022

Hi all,

Has anyone had a heat pump fitted in a large household?  Do they actually save money?  What about the tank capacity needed for hot water for everyone?  I'm thinking of a system without any additions such as an electric shower or wood burning stove, just in a moderately well-insulated 1930s semi.  This would be a move from a gas boiler with no immersion.  I don't mean the set up costs, just the actual running.

Many thanks!

Reed_Richards

At current fuel prices an Air Source Heat Pump will cost you more to run than a gas boiler.  You can reasonably hope for a coefficient of performance of 3, electricity costs about 21p per kWh at present so the heat pump would cost about 21/3 = 7p per kWh to run.  That's more expensive than gas.

Heat pumps are more economical heating your house than heating hot water so the hot water component of the bill might work out at a bit more than 7p per kWh.   

matelodave

Heatpumps dont save money compared with mains gas and possibly not even oil at the moment, so dont be tempted to rip out a gas boiler and replace it with a heatpump in the hope of saving money.

As R-R says they are less efficient at heating hot water than heating because the hot water temperature is usually higher than the rad temperatures and heatpump efficiency drops off rapidly when running at higher temperatures

We can get away with heating our 200l hot water tank for less than two hours a day to just 45 degrees and that gives the two of us enough hot water for all our needs which I doubt you'd manage for eight of you.

We have a heatpump because we don't have mains gas and although it satisfies our requirements, our heating and hot water costs would probably be 30% or so lower if we had mains gas.

markin

With the new cap likely to be 7p for gas and 30p for electric the numbers don't get any better.

tacpot12

Heat pumps have a real problem producing water that is hot enough to avoid legionella. They can just about produce water that is hot enough for washing and showering (say 50C) reasonablly efficiently, if the incoming water isn't too cold.  So in the winter months you might need a gas boiler, solar panels or immersion heaters to suppliment the heat pump for hot water production.

You also need hot water tanks to be heated to 70C occassionally to kill legionalla. All of this complicates the installation of a heat pump. 

For a large house today, I would either install a heat pump for heating and have the large emmitters that allow the heat pump to operate at high efficiency, and have a gas boiler for all the hot water needs, or just have a gas boiler as this will be much cheaper. Having both a heat pump and a boiler woudl give some redundancy, you could arrange to be able switch over from the heat pump to the boiler for heating, and use immersion heaters as a back up for water heating in the coldest months.  

QrizB

tacpot12 said:

You also need hot water tanks to be heated to 70C occassionally to kill legionalla.

<Citation required>

According to https://www.wcs-group.co.uk/wcs-blog/temperature-affects-legionella-bacteria-in-water-systems :

To kill the bacteria, the water temperature needs to be above 50°C – as at this temperature the bacteria will begin to die off. To ensure a rapid and certain demise, the temperature needs to be above 60°C.

shinytop

tacpot12 said:

Heat pumps have a real problem producing water that is hot enough to avoid legionella. They can just about produce water that is hot enough for washing and showering (say 50C) reasonablly efficiently, if the incoming water isn't too cold.  So in the winter months you might need a gas boiler, solar panels or immersion heaters to suppliment the heat pump for hot water production.

You also need hot water tanks to be heated to 70C occassionally to kill legionalla. All of this complicates the installation of a heat pump. 

For a large house today, I would either install a heat pump for heating and have the large emmitters that allow the heat pump to operate at high efficiency, and have a gas boiler for all the hot water needs, or just have a gas boiler as this will be much cheaper. Having both a heat pump and a boiler woudl give some redundancy, you could arrange to be able switch over from the heat pump to the boiler for heating, and use immersion heaters as a back up for water heating in the coldest months.  

From Wikipedia

Temperature affects the survival of Legionella as follows:[3]

• Above 70 °C (158 °F) – Legionella dies almost instantly
• At 60 °C (140 °F) – 90% die in 2 minutes (Decimal reduction time (D) = 2 minutes)
• At 50 °C (122 °F) – 90% die in 80–124 minutes, depending on strain (D = 80–124 minutes)
• 48 to 50 °C (118 to 122 °F) – can survive but do not multiply
• 32 to 42 °C (90 to 108 °F) – ideal growth range
• 25 to 45 °C (77 to 113 °F) – growth range
• Below 20 °C (68 °F) – can survive, even below freezing, but are dormant

I think most go for 65 degrees every 2 weeks.  The ASHP will easily achieve 50 degrees, leaving the immersion to top up the final 15 degrees.  Depending on the size of tank, that's a couple of quid or so a time.  Hardly a fortune and not complicated as the ASHP's control system does it for you.

If I had 8 people in my house people and wanted an ASHP I'd ban baths outright for anyone over 5 years old, and showers as a leisure activity.

I think a hybrid system is over complicating things.     

Reed_Richards

tacpot12 said:

Heat pumps have a real problem producing water that is hot enough to avoid legionella. They can just about produce water that is hot enough for washing and showering (say 50C) reasonablly efficiently, if the incoming water isn't too cold.  So in the winter months you might need a gas boiler, solar panels or immersion heaters to suppliment the heat pump for hot water production.

You also need hot water tanks to be heated to 70C occassionally to kill legionalla. All of this complicates the installation of a heat pump. 

I think this issue is way overstated.  My heat pump can control the immersion heater.  I have it set to take the water temperature up from the normal 50 C to 60 C once an hour for a week.  In my old house the gas boiler did very much the same thing; the tank temperature was set to 50 C and once a week (on a Sunday morning) it would take the tank temperature up to 60 C.

matelodave

My heatpump heats our hot water tank to 45 degrees every morning with a boost to 60 every  Saturday lunchtime for one hour. 

The heatpump does most of the work getting it up to around 55 degrees, then the immersion heater kicks in to do the last 5 degrees or so.

It really isn't the dreadful situation that some think or make out.  The immersion only has to raise the water temp by 5 degrees whereas the heatpump does most of the work with a much better COP than an immersion heater.

According to my calculations it requires around 900watts to heat 150 litres from 55-60 degrees (although its a 200 litre tank and the immersion is around halfway up I'm guessing that it doesn't get the whole tank up to 60) TBH 1kwh a week isn't really going to stop the world rotating.

As it happens, because the tank is heated to 60 on a Saturday it's temp is usually still high enough on a Sunday morning that the heatpump doesn't need to heat, so it's even less of a burden.

markin

If its 8 adults could the numbers for solar thermal be cheaper?

michaels

WE only have 5 in the house but use lots of hot water, we put the 200l tank at 60 degrees so we store enough energy that along with our 19kw system boiler it supports 4 showers and a bath within about 60 minutes every morning - I suspect 200l of hot water at 45 degrees recharged by a 10kw output heat pump would not be sufficient.  I think modern tanks lose very little energy even at higher temps - almost certainly the pipework is much more lossy.