Soap dodgers - ASHP quote hot water assumption?

michaels

Got my quote from Octopus, don't really like it.

One odd factor seems to be the hot water assumption - despite based on bedrooms them assuming we need a 300l tank, they also assume 2000kwh per year output heat demand for hot water

We have about 8 showers a day between the 4 of us, at 80 litres per shower, water temp increase of 35 degrees (7 to 42) my calcs come to 25kwh per day so 9,000kwh per year.  This ties in well with the 20-25kwh per day we are currently adding via immersion heater.  2000 kwh from Octopus seems to suggest one shower each every 2 days?!

Thing is hey base the cost comparison vs gas on this which would seem to me to be tantamount to mis-selling

Netexporter

At a COP of 4 you'd need 2250kWh of electricity to to put 9000kWh of heat into the water. Not sure you'd get a COP of 4, at that flow temperature, probably 3 would be nearer the mark.

DougMLancs

Have you taken into account that modern heat pumps will produce 3-4 times as much hot water per kWh of electricity than a standard immersion heater? Our ASHP averaged a COP of 3.81 for DHW at 45C. Our kids aren’t at the super long shower stage yet thankfully; your DHW usage is much higher than average- the cost of that on an immersion must be eye-watering!

koru

I think the assumptions on DHW are stipulated by the MCS rules (which must be met to get the BUS grant).

koru

michaels said:

Thing is hey base the cost comparison vs gas on this which would seem to me to be tantamount to mis-selling

If you are currently using an electric immersion then you are going to get much bigger cost savings than Octopus calculate, because gas is about a quarter of the price of electric, per kWh.

Reed_Richards

If you use 640 litres of hot water per day then you will be reheating your cylinder quite frequently and using more hot water than is assumed.  Time spent doing that is time not spent heating your house so you might have to use water that is a bit hotter than Octopus estimates in order to heat your house in less time than they calculate.  If so, the SCOP they calculate for heating will be a bit of an over-estimate.    

matelodave

Try being a bit more frugal with your showers - we have an Eco shower head which delivers at 6lpm and I can do the full works, hairwash and all the important little places (including between my toes) in around 2 minutes. SWMBO can do it in three, That comes to 5 x 6 = 30 litres a day at 45 degrees C. We can get enough hot water for a couple of showers a day for around 1kwh or 25p. In fact we've got a 200 litre tank and just heat it every two days in the summer although the second shower on the second day is a bit cooler than I'd like but suits my wife.

If you have 8 showers day and get them down to three minutes each with a 6lpm Eco shower head or flow restrictor then you'll get through 8 x 3 x 6 = around 150 litres, to heat that by 35 degrees will take roughly 6kwh with an immersion heater and around half that or less with a heatpump assuming a COP of around 2.5 for water heating

so if you reckon you use 25kwh a day at your present rate of consumption, halving it and then heating with a heatpump you could probably save 20kwh a day in energy costs, say £5 a day at the current cap which isn't to be sniffed at.

michaels

We don't want to change our lifestyle, i suspect the shower is 10-12 l per min and 8 to 10 mins gives the 'standard' 80l per shower.

Good news is that we heat using overnight electricity at 8p per unit or 9-10p if it has round tripped through the battery. A bit more per unit than the gas boiler but we think the 100% efficiency makes up for the slightly higher cost per unit.

Reed_Richards

michaels said:

We don't want to change our lifestyle, i suspect the shower is 10-12 l per min and 8 to 10 mins gives the 'standard' 80l per shower.

Good news is that we heat using overnight electricity at 8p per unit or 9-10p if it has round tripped through the battery. A bit more per unit than the gas boiler but we think the 100% efficiency makes up for the slightly higher cost per unit.

But you use 80 x 8 = 640 litres of water each day just on showers, lets make that 700 litre a day total use!  If you heat your current cylinder to 65 C and dilute with cold at 7 C then a 50:50 mix will give you 36 C (a bit too cool maybe?).  But then you would only need a 350 litre cylinder to get the 700 litres you need.  I'm assuming that the water in your cylinder remains stratified so it doesn't run cold until all the hot water is gone.

How big (in storage capacity) is your current hot water cylinder?

If you get a heat pump installed, make sure your new hot water cylinder has at least the same storage capacity as your old one.  Run the heat pump at night to heat the cylinder to 55 C (probably the maximum it can do) but then use the internal immersion heater to raise the cylinder temperature to 65 C.  All heat pumps can do this because it's a Legionella protection feature (but check that you can specify 65 C).  Let's be pessimistic and say that the combined heat pump plus immersion gives you a SCOP of 2.  Then you will halve your current hot water heating costs.

Have you tried explaining to Octopus that your household is a heavy user of hot water and they need to quote you for a bigger cylinder?  That will increase whatever cost they quoted you but get what you need. 

michaels

Can't quote but they have suggested 300l tank. 

Current tank is 180l (180l at 60C = 300l at 40C) and our 20kw boiler can reheat to 40 degrees in 25 mins so probably effectively limitless hot water.  With the immersion we run the tank to 70C to maximise storage

TBH I don't have an issue with the tank size but do with the assumption on which the annual cost is calculated - I guess it is some sort of MCS standard but on the one hand they are saying the house size/bathroom tank means we need lots of hot water available in a short time (at least 4 showers worth) but on the other hand the annual usage suggests that only 1.5 showers are taken per day.  inconsistent much.

In reality I know that we use about constant 8kw (boiler modulation) to keep the house at 20c when it is 0 outside (50c flow), not sure what the design parameters are for a heat pump but I would be most comfortable if could achieve a slightly higher output at say -5 and of course would also need to support hot water production although against this the heating demand is lower at night when we only need about 15c

DougMLancs

Which heat pump are they proposing? The new one they developed with Alpha uses R290 as a refrigerant which can easily heat to 65 C without an immersion. Mine never uses the immersion for the legionella cycle at 60C and still achieves an average COP of 2.8 for that cycle. So when Octopus are saying DHW supply will use 2000kWh a year then energy supplied into the cylinder will be much higher than that so they aren’t being inconsistent. 

The design temperature varies by region but is commonly -2 to -3 (-5 in Northern Scotland). Our heat loss needed 7.1kWh of heat @-3 C so our 9kWh unit comfortably covers that and can cope with DHW demand much quicker. It modulates down well though plus we’re open loop so less risk of short cycling.