marshian

Mine gets that low every damn day.............. Cold water in is less than 12 Deg C right now

If my mental maths is right I'd say your water heating is going to need more poke than your space heating so get a boiler which will go as low as possible for space heating (Viessmann 200 will go down to 2kWh) Then size the boiler to to recharge a 400 litre cylinder with a heat pump coil in the shortest time possible (so 400 litres from 20 to 55 needs 17 kWh of energy) to do that in ~30 mins would need a boiler capable of throwing 32 kW at it)

Can always "upgrade" to ASHP later?? What do you think your heat loss will be at design temp because that's going to determine what min boiler output you need and the HW cyl size and HW demand is going to determine the maximum boiler output? (unless HW is lower than the heat loss)

As a result you should see a lower return temp at the boiler, longer cycles and hopefully a little more efficiency - it was effectively a short circuit for water to return to the boiler I have an ABV valve on my circuit - it's fully shut - all the rads have TRV's set to act as temp limiters in the event of solar gain on the south facing side of the house - North facing side will only get solar gain in the middle of summer (when the heating will be off anyway) It was only in use when I had "smart" TRV's on all the rads acting as temp controllers when I was doing scheduled heating - once I transitioned to Weather Compensated flow temps I removed all the "smart" TRV's and controlled the room temps by a combination of rad flow rates and WC flow temps

From this thread https://forum.buildhub.org.uk/topic/41910-balancing-rads-help/page/3/

I have a suspended wood floor on ground floor - 1980's house It is insulated now but UFH was never an option so rads was the choice I had Low flow temps (max 35 deg C at -2.5 Deg C OAT) T22 and T33 rads - heat 24/7 and the house is really nice and comfortable all the time I think your best option is decent size rads, heat pump cylinder and a boiler that can do DHWP for fast recharge of HW cyl and WC for the CH

I went to see a friend in Essex a couple of weekend back - passed a new development just outside a village - all 4/5 bed detached houses - all the houses had different aspects north, south, east and west facing - normal for a small dense estate except all the PV was on the front facing roofs so some of the north facing houses it was pretty much window dressing

Just to expand on this (and give a real life example) I upgraded all my upstairs rads from T11 to T22 in order to run lower flow temps from a gas boiler As soon as we hit the heating season proper I had a nightmare with upstairs overheating and downstairs being not warm enough - so I throttled back the flow thro the upstairs rads - this had a big downside I effectively shrunk the avaliable circuit and had short cycling issues burning lots of energy and ending up with a house that was too damn cold all over unless I wound the flow temps up I learnt a valuable lesson - since then all the rads are now upsized and sized to meet the heat loss of the individual rooms I replaced the boiler with one that could modulate down to 4 kWh and could cope with low flow temps for CH and do DHWP for the cylinder recharge house is super comfortable and I'm running flow temps between 25 and 35 deg C under weather compensation and can re-charge the HW cyl with 70-80 deg flow temps in 30 mins once a day.

If I can help I will

Outstanding........................ Bravo

Adding to this - after just under a year of working perfectly I suddenly had an issue with the bathroom room temp - it was sitting at 16 deg C rather than the 20 to 21 that it normally was. Classic short circuit symptoms - only the bottom horizontal section was warm rest of the rad was cold (I'm running flow temps from 26 to 34 Max so rads are never what you would call hot). I isolated the rad and drained it down and looking from the opposite end to the diverter it was clear it had inverted and water was passing by. So I've piped it up TBOE like the rest of the normal rads and normal service has been resumed as far as bathroom temps are concerned. I don't like the visual appearance of a TBOE set up on a tall vertcal rad so I'll re-do the plumbing in the summer to hide the pipe behind the rad Having had the diverter failure I'll re-do the identical one in the ensuite as a "planned activity" rather than an activity provoked by Mrs Alien saying "it's bath night the bloody bathroom is freezing what are you going to do about it"

Gutted for you - good luck with removal and replacement - hope it goes perfectly

Yes

Good trick for pinch points for vented systems is to put a magnet against the copper pipework - if it sticks to the pipe then you've got a magnetite build up - how much you can't tell but you've definitely got an issue in that location

I had to do this a few years back when I air locked the boiler - it works quite well But if you aren't prepared with people at F&E tank and any bleed points it can get a bit damp