marshian

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

If the house is 70’s or 80’s my money says It’ll be expansion and contraction on a HW pipe cut into a joist and pinned with a nail folded round it.

I'd avoid a combi for a lot of reasons They are over spec'd for space heating but frequently not powerful enough for multiple showers or a show and a bath Because they have a huge demand for gas the gas line frequently needs upgrading to a larger size If your water pressure is low on a combi you get what you get same with UVC Without a tank options for solar HW or even HW via PV and immersion are out 2 bar mains is going to be about 15 litres per min - need @SteamyTea

Often very ancient pipes and hot water My house a lot of the underfloor pipes are 42 years old - System was filthy when we bought the house - probably 9 year old inhibitor - bleed a rad and the water came out black indicating a lot of corrosion in the system. It's clean now but I was surprised at the level of muck in the bottom of the last original rad that I cut open when I replaced it

Oh that's a good call - I guess they did isolate when the pump was changed or maybe a partial draindown was carried out so no need to close the isolation valves??

Where is your parents place - rough location? It's really not a massive job to replace that H section - I don't like compression fittings but it could be done with those - I much prefer solder fittings but if you can find a plumber that has press fit in 15mm and 22mm that section would be easy to replace with minimal drain down and I'd very much suggest that you do the combination vent and fill that @John Carroll recommends I going to do that on my system fairly soon to solve a different pump issue I have with the new pump being a bit too meaty for the application compared to the old selectric I had.

Oh lovely - replacing it all is pain free process apart from the financial impact and he's on the receiving end of that 5K and some uplift in the process due to a stupidly large combi being a big draw gas wise. On top of that you are going to end up with a presurised system - weren't you worried about pipes buried in the concrete floor? My advice find a better gas enginer/plumber or go with your plan B Maybe consider @John Carroll suggestion to combine Feed and Vent

How open is the lockshield valve on that rad? I don't think drop legs from a flow and return circuit above will make any difference but heat will always prefer to rise Ages ago I got into a discussion with a chap who had a type of reverse return circuit for flow and return in the loft of his bungalow - all the pipes dropped down from ceiling to the rads The last radiator fed with the flow was the first on the return leg really high primary balance and all the lock shields were pretty much fully open

Thanks for that explanation - it's an expression I've never heard of and it didn't translate well without the background My rough calcs of radbots market penetration wasn't far off I wonder how many smart TRV's (accross all the manufacturers) have been sold - They are quite an expensive purchase Ahh the "energy saving" combi boiler where water heating demand means hang the biggest kW white box on the wall that has a piss poor modulation rate so totally unsuitable for space heating without cycling but saving on water right?? Crazy situation promoted by industry with a lack of education in installers and homeowners. In my view we've learnt something from the combi process but unfortunately I don't believe the UK is fully prepared for ASHP introduction - I don't believe the grant process is helping either - it's just inflating the cost of the change from gas to electric

29.9 million homes in the UK majority using water based heating systems - for shits and giggles lets say average number of rads per home 6 500,000 radbots (Never heard of them but google educated me) so a market penetration of less than 1% I'm not sure why you would be eating your own dog food - strikes me as petulant comment adding nothing to the discussion which was going rather well I though as a pro's and con's of TRV's I'm 100% OK with your views on TRV's but I'd hope you can understand that there are ways in which they can drive up energy usage.