Sizing hot water cylinder for ASHP and solar thermal

[embra]

We are about to install an air source heat pump and associated pipework throughout our four bedroom / two bathroom home, replacing an existing gas boiler. We already have solar thermal panels (two Worcester FK240 solar collectors routed to a vented twin coil cylinder) which will be replaced with an unvented twin coil cylinder as part of the ASHP works.

 

I'm interested to know how best to size the new cylinder? I understand that typically a cylinder for solar thermal is sized up compared to a "normal" DHW cylinder, to maximise amount of energy stored from solar collectors. Equally this creates a larger volume of water the ASHP would be left to heat during winter.

 

Our heating engineer has suggested a 200 litre cylinder, however when I this guidance (Table 2), something in the region of 275 to 300 litres is recommended.

 

Appreciate any thoughts and advice on this topic. Equally, any recommendations for cylinders optimised for ASHP and solar thermal would be most helpful.

Edited July 24, 2023 by embra

[ProDave]

Work on a minimum of 300 litres.

 

The big issue is the ASHP will not heat the water in the tank as hot as a gas boiler would.  So in use, the hot water from the tank will be diluted with less cold water, so for the same hot water usage as before, you will need a bigger tank.

 

Look at Telford stainless steel unvented cylinders, they will do any combination of input coils.  You will need a high capacity heat pump input coil as well as a separate solar thermal input coil.

[sharpener]

19 minutes ago, embra said:

Our heating engineer has suggested a 200 litre cylinder, however when I this guidance (Table 2), something in the region of 275 to 300 litres is recommended.

 

Sounds a retrograde step! Will he not just add a plate heat exchanger to the exisiting setup? Or at least keep the solar tank as a preheater if you have the space.

 

Our installer wants to put in a new 300 l tank for 4 bedrooms, saying this is required for an occupancy of 5 people which he gets from some table calling for [#bedrooms +1] so if this is correct you would need 300 also as @ProDave suggests - but I have not given up on the heat exchanger approach.

 

Assuming reasonable insulation the HP will only be called upon to replace the HW actually drawn off, so if your usage pattern does not change then an increase in tank capacity does not matter to the HP as it will not necessarily being heating from cold.

[embra]

6 hours ago, ProDave said:

Work on a minimum of 300 litres.

 

The big issue is the ASHP will not heat the water in the tank as hot as a gas boiler would.  So in use, the hot water from the tank will be diluted with less cold water, so for the same hot water usage as before, you will need a bigger tank.

 

Look at Telford stainless steel unvented cylinders, they will do any combination of input coils.  You will need a high capacity heat pump input coil as well as a separate solar thermal input coil.

 

Thanks, have found this item which looks suitable: TEMPEST HEAT PUMP SOLAR

 

The only specification for the coils that I can find is the statement: "Purpose-designed renewable coils with a 3.3m² surface area". Any rule of thumb or specific points I should check to confirm it's suitable? I'm thinking of coil position in the tank etc.

 

Do you have any reference I can share with our heating engineer for the 300 litres sizing? 

 

6 hours ago, sharpener said:

 

Sounds a retrograde step! Will he not just add a plate heat exchanger to the exisiting setup? Or at least keep the solar tank as a preheater if you have the space.

 

Our installer wants to put in a new 300 l tank for 4 bedrooms, saying this is required for an occupancy of 5 people which he gets from some table calling for [#bedrooms +1] so if this is correct you would need 300 also as @ProDave suggests - but I have not given up on the heat exchanger approach.

 

Assuming reasonable insulation the HP will only be called upon to replace the HW actually drawn off, so if your usage pattern does not change then an increase in tank capacity does not matter to the HP as it will not necessarily being heating from cold.

 

We're looking to move from the existing vented cylinder to an unvented cylinder so we cna have showers at mains water pressure. Also we want to relocate the cylinder anyway, so makes sense to get the optimum equipment setup in place.

Edited July 24, 2023 by embra

[sharpener]

30 minutes ago, embra said:

Thanks, have found this item which looks suitable: TEMPEST HEAT PUMP SOLAR

 

The only specification for the coils that I can find is the statement: "Purpose-designed renewable coils with a 3.3m² surface area". Any rule of thumb or specific points I should check to confirm it's suitable? I'm thinking of coil position in the tank etc.

 

Do you have any reference I can share with our heating engineer for the 300 litres sizing? 

 

MCS guidance here, you will see they suggest a bigger HP can get away with a smaller cyl because of greater reheat capacity, not seen this on cyl mfrs web sites (try googling hot water tank sizing, you can choose any answer you like over a wide range!).

 

Conversely if you are trying to minimise the HP size then a bigger tank will help avoid the need to reheat it when space heating is required. With the solar thermal as well you might be able to heat only at off-peak times. 3 sq m surface area will be fine with a 5-7 kW HP.

 

39 minutes ago, embra said:

We're looking to move from the existing vented cylinder to an unvented cylinder so we cna have showers at mains water pressure. Also we want to relocate the cylinder anyway, so makes sense to get the optimum equipment setup in place.

 

Makes a lot of sense.