marshian

It's a conversation being carried out elsewhere on here - Smart is not always "smart" sometimes it's pretty (expletive deleted)ing dumb

Sorry meant to also say it does show that there is an impact on efficiency until 60 mins of run time (OK so it's the outer edge of a bell curve) IMO it's still over 1% (but less than 2%) loss at a 3 min run time which is why I like to set the system up to minimise cycling as much as possibe

That is an excellent result but maybe way outside the abilities of most people All I’m trying to do with this boiler is get it running at it’s optimum when matched to the house needs using the parameters that i have control over - I think I’m fairly close to where it’s doing decent length cycles at low flow temps and maintaining the house at a comfortable temp without burning excessive amounts of gas.

I've seen that before and I reckon it's flawed because it doesn't take into account the increase in modulation on start up - my old glow worm boiler took 4 mins to drop down to 10 kW from it's fire up at 85% of max (which is actually fairly typical of many boilers)

Absolutely correct but it's not hard to log the totals by day and as a result get the daily number of starts Or perhaps that's just me being typically OCD

I had mine set to an "oil boiler" before I went 24/7 that tempered the Wiser unit from intervening as often

So twice the size of mine 😉 and with 21 rads a much bigger CH circuit. Any heat loss calcs been done or have you done one yourself? This company link below have an online one that I used and found it fairly straightforward - I made a few mistakes - so check and check again before submitting to get the report - the detailed report at the end of it was pretty comprehensive and the few mistakes I made I could manually correct for. https://heat-engineer.com/en/ You really only need a tape measure and a reasonable understanding of the fabric of your house and can match it to the selections available - took me a couple of days to go thro the house room by room I wouldn't ever make a boiler purchase without knowing or having a damn good idea of my potential heat loss so I can size the boiler correctly (I made that mistake with the previous boiler - not enough research and just thinking that can't go wrong with bigger cos bigger is best right!!!!) For the new boiler I could have got away with an 11kW Heat only Viessmann but as the 16kW version had the same min output I chickened out - I was also still heating to a schedule rather than 24/7 and you need a more ooomph to get the house back up to temp quickly) In fact all the Viessmann Heat only boilers from 11kW to 32kW have the same 3.2KW min output (only the 11kW to 24kW fit inside a 300 depth kitchen cupboard - the 32kW is much deeper) Turns out that the 11kW version can't be range rated and the 16 kW version can so I got lucky there by accident rather than good research

Warning - I may or may not be a little OCD on data collection and evaluation Todays statistics House Internal temps 18 - 20 deg Outside temp C Lowest -1.9 Highest 2.3 Average 0.2 Highest Boiler flow temp on CH 34.4 Highest Boiler return temp on CH 27.1 Currently 34.3 deg C flow temp CH has been on for 14 hrs, HW for 30 mins so far today so total mins 870 mins Total boiler burn time 600 mins Number of starts 11 So boiler run time is 69% of the heating on time and average boiler burn time is 54 mins (Would be higher but it did 30 mins of HW and it always stops and restarts after doing the HW due to the 80 deg flow temp mandated by the HW Demand box)

I can accept a level of cycling - less than 3 per hour would be my aim It's not just the efficiency loss from excess cycling it's the wear and tear on the boiler components, expansion contraction, gas valve, igniter etc etc Old boiler peaked at 24 burns in an hour and I got a cooling house as an added bonus (don't over zone and don't try to achieve the biggest drop you can on each radiator flow and return were lessons learnt there) New boiler set a record of 53 cycles in 8 hours per hour but that was down to 13 Wiser smart TRV's all having the ability to individually fire the boiler They don't get to do that now

Just bear in mind the modulation range of the boiler for the CH side of the House if going combi Old boiler Glow-Worm Flexicom HX a 24kW boiler with a min modulation of 10 kW My House (Stupid shape 4 Bed 2 Bath 113m2 floor area over two floors) has a calculated heat loss of 4.7 kWh at minus 2.4 Deg C So even in the depths of winter when it's minus 2 my old boiler was only capable of throwing more than double the energy required by the heat loss of the house. For the rest of the winter and the shoulder seasons it was probably 4 to 6 times the energy required. It cycled - it cycled a lot!!!!! Each time throwing energy out of the flue before it had chance to do anything to the water circulating in it Replacement Boiler 16kW with a min modulation of 3.2 kW Running 24/7 heating at a target 32 deg C flow temp at -2 deg C outside - return temp 27 deg and over a 24 hr period running a total of between 14 to 19 cycles (Outside temp never stays the same for long)

Quoting myself I've had to revisit the Range Rating setting as my initial impressions was that it wasn't actually working at all Actually appears that it did but it's not what I expected............ I've got that menu sussed now - I write this stuff down because the boiler manual is pretty poor - see urban plumbers review of Viessmann Boilers and the sketch at the end - I'll link it below just for squits and giggles So you would think with a range of settings from 9 to 100 it would be a % of output - It's clearly not!!!! When I set it to 25 that is actually 15.7% of the output (after the initial fire up where it can go to around 60% and then stabilisation before modulating down to manage the flow and return temps) Background to the discovery I needed to increase the flow temps because water heating was 40 mins and the house temps had dropped a bit much to recover - Fridays is my legionella cycle and it's also Mrs BC's bath night so water in the morning is only heated to 45 for the showers - we have a fairly cold tank to re-heat to 60 to 65 deg C but that takes about 40 mins So I raised the target room temp to 25 from normal 20 - this should have given me a flow temp of 39 deg (at 0 Deg outside) but it didn't it gave me 34 I backed the room temp down to 20 and raised the WC curve to 0.5 but plus 8.0 on the slope - this should have given me a flow temp of 39 deg (at 0 outside) but it didn't it stayed at 34. The boiler modulation was 15.7% nailed there. Ahhh lets look at the range rating I implemented Menu b.2 Option 7 is setting max heating output Quick experiment and I've got the table below Setting % Modulation kWh 20 10.6 3.2 25 15.7 3.9 30 20.4 5.0 35 27.6 6.8 40 32.3 8.0 45 39.1 9.0 50 47.2 10.0 So the setting is not a % at all but just a number but now I've got a good guide for the % I might need when things warm up Mean time I've left it at 40 because that gives me head room to accelerate house warm up if I need to and when it warms up again I can try a lower value (or faster pump speed) Now last a link as promised (jump to 11 mins in for a summary of Szymon's experience during the install and watch to the end) PS I still wanted a Viessmann boiler even after seeing that review because very few boilers come close to it's turn down ratio - I mean it can't be that hard to get it performing at it's best can it (Don't answer that..........) Anyway house has responded to a burst of heat and I'm back to 0.5 and 1.0 level and I've learnt another element of control on the boiler

Careful here - you need a really good understanding of how your system is set up and wired Manually opening the valves may not result in the micro switches sending the signal to the boiler In my system the single CH zone valve is a normally open one (it only shuts when there is a demand for HW ie the tank needs re-charging and at the same time the HW zone valve opens) - the boiler isn't wired to fire on the confirmation of a CH zone valve opening ie microswitch made)

Keep them as temp limiters is what I've done (set them fractionally above the desired room temp) - especially when you are in the getting the heat balance right stages

Oh absolutely that is the case - drip feeding information takes ages to get to a root cause or resolution

PS off topic but just to say I fabricated (welded up) some hanging brackets for the pit ladder so that in the winter when the pit can get a bit wet (high water table) the ladder can be swung up and tied off - That was just a check it fits in the hole shot

You can always open the lockshield end of the rads to allow a little more flow on a lower pump speed - but I can understand why you wouldn't want to leave them with a cooling house Your tank level already looks a heck of a lot lower than mine - you don't want to end up with the opportunity for the top up (cold) to allow air to be sucked in to the circuit Oh I'd definitely be raising the vent pipe even if I had to run it up following the angle of the roof truss and then bring it back down I relocated and lifted my cold water tank and my F&E tank a couple of years back - it was in a bloody stupid place right by the loft hatch and I used to catch my back on a support beam every time but I kept the height of both HW Tank Vent and CH circuit vent the same above the tanks New frame at the back of the loft Old one at the front by the hatch being dismantled and removed I hate waste so I may have built a ladder for my mates pit out of the removed timber

Why didn't you try PP1 ?

I'm the same on bedroom temps - unfortunately my better half is not - she like to sleep in a warm bedroom so I compromise 19.5 deg bedroom temp - she's happy and I don't overheat at night Another very good reason why not to over zone and why a lot of TRV intervention means a hard life for the boiler. I'm pretty confident I've got about 30 L in the CH circuit (I did a drain down a while back where I closed all the rads and just drained down the circuit - BTW it's not a good idea if your system has a tendency to air lock Oh well "experience is what you get when you don't want it" I always say) Quick bit of maths shows 16% of my system volume is subject to set backs (ie TRV intervention) Reason for those rooms being on setback Front Hall - North Facing 3 external walls and door and glazing Study - As above and it is directly above the front hall - if I use it I just boost the rad temp as when the PC is in use it'll add to heat input Utility - Fridge and Freezer contribute to stable temps with minimal additional heat needed Toilet - tiny room - only heated morning and evening (when it's likely to be used) responds quickly to heat input 84% is therefore available all the time as the TRV's are set to act as limiters not temp controllers and it's a circuit size of 110 litres and with a pump pushing 0.5 m3/hr (8.5 Litres per min) and a boiler min heat input of 3.2 kW system is quite happy

OK so pump over can be reduced but even on lowest pump speed/pressure it's still there. @John Carroll for the cheap seats just explain to me how lowering the water level in the F&E tank would help Is it that you lower the level in the F&E Tank and as a result you lower the level in the vent pipe so potentially reduce the opportunity for pump over and you are effectively lowering the overall head - I'm struggling to get my head round it (also my F&E tank when system is running is a few mm below the overflow pipe)

Agreed If I'd have left my WC settings as the gas engineer set it up I'd be forever bouncing of the TRV limits 1.4 was the initial set point at 20 Deg Target Room Temp (Not measured) I'm now on 0.5 at 20 Deg Target Room Temp I do have an easy way of compensating for room factors - the boiler Target Room temp can be adjusted from 3 Deg C to 30 Deg C Changing that by one or two degrees either way has a significant effect on flow temp and it changes the WC curve

Yes they are - but it's only a few quid to replace them and if you are doing a partial drain down it's the ideal time to swap them out. One of mine wouldn't close when I replaced it (wouldn't fully open either) it did free off with a lot of persuasion but I didn't know that until after replacement - they were both 40 years old which is a pretty good innings I thought

I agree ^ don't need 4 pipe unless you want to get a Viessmann that goes down to 1.2 kWh (which is nice but I needed a boiler that fitted inside a kitchen cupboard Heat only Viessmann with HWDB (HW Demand Box) and external Weather compensation sensor CH set up with a NO zone valve - clean and simple it just runs HW set up to close NO CH valve and open NC HW zone valve Get a decent tank with a 3m coil and you'll have really fast HW recovery This set up with DHWP and what is really a small tank knocks a combi out of the bloody park!!!

It's like a bloody good whodunnit

Not disagreeing with you because you are absolutely correct but the same result can be achieved with a pump set to too high a flow for the circuit - the main issue with this thread is no one knows if the situation was the same before the pump was replaced

It’s not the only option because it’s clear the HE isn’t totally blocked as OP still gets HW and CH (albeit CH with some pump over into F&E tank) OP hasn’t fitted magna clean yet but in preparation he could put a decent quantity of system cleaner in the circuit and see if that puts any restriction due to magnatite or sludge back into suspension Personally I’m still leaning towards a pump that is over powering the circuit And as @John Carroll has said they aren’t a popular choice amongst gas engineers/plumbers (not sure how the benchmark brand for circulation pumps gets itself into that situation but it seems they have) If @Peaklander gets it resolved I’m hoping I can have the old selectric pump (if it has working speed selector) and I’ll cover the postage