marshian

Boiler min output is quoted as 3.2 kWh (at 50/30 flow / return) 2.9 kWh (at 80/60 flow / return) However couple of points Viessmann technical have not been able to confirm min output kWh at 32/25 (I am assuming that it is more than the 3.2 kWh quoted at 50/30) At min modulation the best I have seen is 4.0 kWh over a period of 1 hour (initial ramp up and period where it modulated down to min was not included) During this period I did not see any increases in modulation as it was 0 Deg C OAT Data if you wish to check the calcs Meter Reading at Start 633.766 m3 09:00 Meter Reading at end 634.130 m3 10:00 Time 60 Mins Usage 0.364 m3 Calorific Value 39.1 kWh 4.04 kWh I am also waiting for the Viessmann engineer to replace main board and WC sensor because despite having range rated the boiler to min I occasionally see the boiler ramp up to 15 to 20% modulation with no assignable cause - problem is more frequent at warmer air temps (5 deg to 10 deg range)

Model info It’s currently running at 58w on speed 2 Dab Evosta speed 2 - 15w at 3.0 m head and 0.5 m3/hr (system was noisy but stable from a cycling perspective - boiler flow 32 return 25) speed 1 - 8w at 1.8 m head and 0.3 m3/hr (system was quieter but cycled more and rad deltas not really any better - if anything mean water temps in the rads were lower - maybe 1 deg better diff between flow and return)

Well it was a good plan - but speed one doesn't want to play not sure why - probably a result of building one pump out of two. Running on the grundfos on speed 2 for the next couple of days to see if it reduces the number of cycles per day compared to the DAB pump

I have that down as my first port of call from previous mentions you've made I don't want to thread jack this thread

Also on my spec list

Useful thread I've added the bottom of the tank immersion to my new HW tank criteria (we've only ever had top mount immersions as it's only ever used in the event of HW/CH circuit drain down but with a plan to install PV at some stage in the future it makes sense to avoid the issue of not fully utilising the excess electric.

I’m going to struggle for this week - working late shift because of someone else on holiday

I'd start by cleaning up the plug prongs with scotchbrite

As I said before - Poor connection between the male of the plug and the females of the socket causing resistance to current flow - resistance generates heat

On a PC with a proper keyboard it's a lot easier than a phone 😉 Good idea on the autocorrect but again not 100% sure that's easy on a phone either

Poor connection normally

His pedantry over units can be a little tiresome but I do admire his persistence

Your contract was with the installer - I'd say the manufacturer was being rather reasonable (unless they warranty the windows for longer).

Nit picking: more heat than the Circuit can accept and dissipate. All the rooms at temp and TRV's have shrunk the circuit does not mean summer

^ this probably said it better than I have

It helps to describe why cycling is a waste Boiler fires up at between 70 and 80% of max output (so in the case of your boiler 24kWh) it only does that for a couple of mins before modulating down - but in that time it's impossible for all that energy to get into the heat exchanger water so a lot of energy ends up going out of the flue This loss is offset by the time the boiler actually runs and the level of modulation - a little loss at the beginning of the cycle is fine if the boiler then ticks along at minimum modulation Problem comes when those 2 mins of wasted energy is repeated every 5 or 10 mins - the loss is not offset by the time the boiler runs for To use a car related comparison Scenario A You are on a slip road at the bottom of the M1 - you need to be doing 70 mph to safely merge into the motorway traffic and getting the car from 0 to 80 mph is going to result in 7 mpg consumption - however you have 100 miles to cover at 80 mph and your fuel consumption is going to be 35 mpg at those speeds The fuel used in acceleration is offset by the cruising consumption Scenario B You are on a slip road at the bottom of the M1 - you need to be doing 80 mph to safely merge into the motorway traffic and getting the car from 0 to 80 mph is going to result in 7 mpg consumption - however you have 100 miles to cover and it's a bank holiday monday and a 80 mph cruise is impossible - your progress is sporadic and requires frequent acceleration and braking because of the numpties on the road don't understand keep left unless overtaking and don't bother to look in their mirrors before any change of lanes. Your fuel consumption at the end of the journey is 25 mpg even if you average speed and journey time isn't much different.

It's not going to be optimal and that sometimes is a problem - boiler cycling is a huge hit to efficiency from memory you ideally don't want a boiler to cycle more than 5 times per hours (even that would be a bit shit) My old 24kW with 10kWh min output was cycling more than 10 times per hour in autumn and spring and at best 7 times per hour when house was up to temp in the winter and chewing thro gas My new boiler in winter is doing 22 to 24 cycles per day!!!!

Quick search online seems to indicate that that boiler has 11.3 kW minimum (30% of max) It's also not compliant with low temp heating so in my opinion it's going to be in conflict with your rebuild/insulation improvement plans Whilst the boiler might only be 1 year old the install manuals were written in 2013/2016 so it's not a "latest" boiler

pretty sure I’m right @SteamyTea I can’t feel much convection above a radiator at a 30 deg flow temp and come to think of it does UFH convect or radiate 😉 re the OP happens occasionally 🙂

I’m not actually sure why anyone would think a radiator is more or less suited to low flow temp I’m sure surface area is a factor for heat transfer but the lower the flow temps go the ratio of convection to radiation changes At a higher temp most radiators convect more than radiate At a lower flow temp the radiator may convect less than it radiates I know that from my perspective I’m more comfortable at a lower room temp with less convection creating air movement but that might just be me!

Major renovation I assume is to a higher insulated standard than the house is currently - maybe with increased house footprint if boiler capacity has been assessed as good enough to support it and with limited space to site a AHSP unit As @JohnMo said the decision needs some data gathering or decent workings to make that decision. Current property heat loss calcs first - then with that as a baseline look at what the renovations are going to do to the heat loss - it could be that the renovations dramatically reduce your heat loss and as a result the current boiler is then oversized Taking my own example as an illustration of how quickly a suitable boiler can become unsuitable when house is upgraded 4 bed detached built in early 1980's - Single glazing with wooden windows, open cavity between inner and outer brick skins, suspended ground floor no insulation and 25mm of loft insulation = 25,000 kWh of gas per year (heat loss 10.5 kWh at -2.4 OAT) Throw in sealed unit double glazed windows and doors, CWI (cavity wall insulation) as well as 70mm of loft insulation and despite a two storey extension annual gas usage is down to 15,000 kWh and already the 24kW boiler is 3 x oversized for the new heat loss 7.5 kWh at -2.4 OAT Add in underfloor insulation, replacing all the rads for lower flow temps into the mix and annual gas usage is down to 8,500 kWh with a heat loss of 4.0 kWh at -2.4 OAT and the boiler is now 6 x oversized and worst still it's minimum modulation 10kW (output is 2.5 x the heat loss) I considered ASHP but it didn't fit well with the silly shaped house so I replaced the now totally unsuitable boiler with one that was only 16 kW but can modulate down to 3.2 kWh meaning when it's -2.4 or lower outside it happily runs 24/7 at close to min modulation. Probably help if we had make and model of the boiler to see what modulation or turndown ratio it has because if you are improving the house having one with a really good modulation would mitigate the improvements.

not to take this thread off topic - I guess my next research job will be how many metres of pipe do I need to put down at 133mm centres with a flow temp of 32 deg at 0 deg OAT to match the heat loss of the room and can I fit that in the open area because I’m guessing no one puts it under units??

That sounds terrible - have you got a "heat geek" accredited heating engineer to do a survey and see if it can be improved - as @JohnMo frequently points out the best way to make them work well is keep the circuit simple - weather comp for the flow temp - ditch buffers - big zones and have minimal thermostat interventions I predict that poor installs of ASHP will be the next miss-selling scandal (Like PPI and Diesel cars)

First time I've seen UFH done in that way so it doesn't result in an increase in floor height - looks quite neat Given me an idea for when I redo the kitchen - unit space is at a premium and I would need to relocate the existing rad and so underfloor would be a much better solution - I now wondering if at my normal flow temps (32 Deg C at OAT of 0 Deg C) it would be enough to run underfloor at the same temp as the rest of the house rads Now googling Alu Spreader plates

If I had my time again I'd definately sink an IBC (or two) into the ground - with probably a small moss/leaf litter catch tank before them If putting two I'd link them together so whilst the level might be lower in the summer the two would share the level But as I'm not digging up my patio to do that now I'm thinking of an alternative plan when I re-do the 16 year old decking - I'm pretty sure I could get three low profile 400 litre tanks under the decking and collect the rain water there without it even being on show (I'd still have to drain in winter which would be a pain but manageable)