I have the smallest heat pumps in the Valliant range. It seems to be working really well after 1 year The space heating is not an issue in our small house ( 2 to 4 adults). However, it was originally recommended to purchase the 5kW machine by the supplier for domestic hot water reasons. The 5kW machine would heat the DHW faster. I have seen the same advice in many community blogs. Since the 3.5kW machine is a “capped” version of the 5kW machine. So go big!
However, I found the answer, like most things in life, is a little more nuanced.
The 3.5kW machine (capped at 85 rps) seems to out-perform the 5kW machine when it is set on “Eco” mode ( capped at 50 rps) for cost savings. I suspect the 5kW “balance” mode may be closer to the 3.5kW performance. Unfortunately, I need to walk the dog now and that calculation can wait!
The 5kW machine in “normal” mode (120 rps) will out perform the 3.5kW machine, but not by that much for a typical household. It is far more important if you have a large hot water tank >200 litres and/or a big family or are a couple who love long hot 40 minute showers ( call out here to my kids as teenagers).
Time difference: To heat 190 litres ( 1 full bath) from 10C to 50C in a A7/W45 setting takes 67 minutes with the 5kW machine and 97 minutes for the 3.5kW machine. So 24 minutes longer.
Cost difference: The cost saving on the 3.5kW machine with a 27.7 p/kWh electricity tariff ( or less at night) is really small at 8 pence per tank cycle. The 5kW “eco mode” saves 8pence per tank but takes 170 minutes to heat the water! That is way too long in my house, since the house heating is off during the DHW cycle.
In the end it is a personal choice for your own household and lifestyle. The “go big” advice towards the 5kW machine for DHW only makes sense to me when no subsequent restriction is placed on the DHW below the “normal mode”- it may make it worse. Of course you can always “go big” and have future functionality. Food for thought.
See the plot below.
