marshian

Down side (from a quick look at the specs) whilst it does modulate down to 3.5 kW the min flow temp for CH is 40 Deg (max is 90 Deg) With UFH throughout seems a shame to have to blend the temp down rather than get a boiler that can run at temps low enough for UFH but I'm pretty sure that'll just me me that's concerned about that

I don't agree I've got T22 rads - they replaced T11 rads in a 1980's house - they are the same size as original - just thicker - They are not silly sizes. My WC flow temp starts at 25 deg C and ends at 35 Deg C at -2.5 OAT It's perfectly possible to run UFH flow temps thro rads and still keep a house at temperature

And he has a multitude of suggestions He can pick the bones out of the thread

Couple of times I had to email Octopus because my bill has fallen into limbo land - it ends up in some weird stasis situation and they don't know. Each time it takes about a week. I wouldn't mind betting that's the reason you've only got a bill now

Because at that point he will be missing (hopefully) two x 30 min shower times and rattling around in the house without the need for "unlimited" HW........ Plus lets not forget - not having a tank rules out any PV assist to water heating during summer months if that is later added....... Combis are fine for small houses, apartments and flats - madness IMO in a house with 3 bathrooms and a requirement for multiple showers at the same time

No entrapment there at all - Not Guilty is my plea I don't think there will be 1. A ban on sale of gas boilers or 2. Gas will run out But I do think cost benefits to convert from gas and oil to ASHP will be incentivised by methods other than a grant and price of the fuel is one

I refer the honourable gentleman to my reply to @JohnMo I specifically did not say anything about a ban on gas or it running out The economics of ASHP conversion are currently being bolstered by a £7500 BUS grant (Ignoring the grant harvesting impact) it wouldn't take a lot of movement in gas price or more importantly the standing charge to further drive the move to ASHP's - especially when the BUS grant scheme ends and equipment cost falls

I feel you need to read the first post again My point which I will make again is that if the space is not made for a tank due to a hoofing great combi and no tank - it's going to be much harder at a later date to fit in a tank into a house that wasn't designed to have one. Not disagreeing with you - more than one way to skin the proverbial - but it's a forum which shares different opinions - the discussion of pros and cons - will hopefully lead the OP to a decision that he has to live with preferably happily!! As far as fossil fuels go (and I have recently replaced a gas boiler with a gas boiler) it might not be a ban on fossil fuel boilers it could as easily be the increased cost of fossil fuels as a result of ASHP becoming the norm - the costs of the gas grid would then have to be covered by the remaining gas customers............... This could result in the "economics" of gas driving further conversions to ASHP....... JM2pW

OP ruled out ASHP on his new build - downside of fitting a bloody great big combi is when he does “have to go” to an ASHP he’s then got to find space for tank which was never allowed for in the build makes no sense to me at all to go down a combi route

Wasn't the point being made........................

80 - 20 rule for me If I can get 80% of the benefit for 20% effort I'm comfortable with the compromise I'm not going to put 80% of my effort in to getting the 20% extra - big wins for min effort every time for me I could have fitted 100mm of PIR under my suspended wood floors v 75mm the cost would have been 40% more 75mm PIR U value 0.33 W/m2K 100 mm PIR U Value 0.23 W/m2K When I considered my actual floor losses once I was over 50mm the bang for buck was done. Payback on 75mm was 4 years (Russia helped me out - I was projecting 8 years based on energy cost increases)

And there in a nutshell is the root of their demise................ Enjoy it whilst it lasts How many Energy "Distributors" have gone to the wall in the last 9 years?? The answer according to google is 51 Quite disappointed the answer wasn't 42..........

I have politely asked @Nickfromwales To do a quick clean up and move non relevant conversation that related to my boiler set up to my Viessmann Journey thread

I was under the mistaken belief that oil fired boiler heating was the cheapest of the fossil fuels? I’ve always assumed the downsides were storage, higher cost of oil boilers and paying up front for the oil (compared to paying after usage for gas) clearly I was wrong?

I'm A OK with all that.............. Doesn't change the need to use math.................

It's OK - don't worry - mistakes I've made a plenty You know the nice thing about making a mistake is you learn................... Observation skills are paramount - math is everything Oh and to keep @SteamyTea away from the conversation don't mention thermal mass and above all don't get units wrong - PS if this seems like a dig at ST it's not - we all need the voice on our shoulder pointing out the error

I think you have a miss-understanding of DHWP....... I think this is partly due to the word "priority" in the description DHWP is about maximising CH run times at low flow temps, accepting a loss of efficiency when doing HW - the priority is getting it done promptly - ie don't muck about trying to go low and slow - hit the HW with higher flow temps - stir the tank up with higher temp differentials and get it done I'm definitely in the camp of not trying to re-heat a stratified HW tank when HW is being demanded (ie showering) that's a recipe for getting lynched by my partner....... Lets say the evening shower time - tank was heated to 50 deg at 7.30am - we've had two showers and taken ~50 Litres out - tank is stratified into 48 deg C for top 50 litres and 20 (ish) deg for the remaining 60 Litres - I try to recharge the cyl and the first thing that happens is the first bit of heat that the coil gets starts a mixing process - top of the tank that was 48 deg drops rapidly to mid 30's OK Gas boiler not ASHP - maybe with ASHP the process would be much more gentle and less stiring the pot...............

Absolutely this ^ before I had DHWP and WC for CH - I was heating the water the previous evening because I was managing the flow temps for CH manually - it worked from an energy usage in CH but having a tank sitting there overnight meant standing losses were much larger and a 117 litres of water didn't manage 4 showers a day Right now with DHWP - water is heated at 7 to 7.30 am based on a schedule for showers at 8am - typically recharged from mid 20's to low 50's - by 11pm same day tank temp no longer will support another two showers at a comfortable temp (up to 11pm it's fine) energy usage is typically 4 kWh to 5 kWh for a recharge Like @JohnMo when I was starting out with DHWP I set the schedule to allow HW cyl re-charge whenever the tank stat triggered a re-heat. Absolute bloody waste of energy - it massively increased the standing losses and increased energy used for HW by 50% for no real benefit. From memory it recharged 4 times in a day - each time taking the tank back up to 50 deg C - some of those losses were due to heating up the primary circuit and firing up the boiler but it didn't work for me. Most economical way for me is heat the water I need to the temp that covers the for the whole day in one hit but I'm sure if my HW usage was different and not fixed other ways may work better.

If I did that to my other half I’d be dead - HW re-charge is done well outside of any shower reqts

Yes comfortable - being 5’ 13” in my socks helps with stuff lije that

I think to bring this thread back round to topic and thickness of screed - to me it’s just making your “heat emitter” aka radiator as big as possible with the ability to soak up temp changes in the environment around us whilst treading lightly with it’s energy consumption. As @JohnMo stated so well you need to optimise whatever heating process you use to cover your needs based on heat loss, occupancy, HW needs and cost - there isn’t a one size fits all - careful thought, some maths, and considered purchases as far as kit/rads, UFH, heat source and HW storage (volume and method of recharge) is required and we all have a duty of care to the planet and it environment to do as little damage as we can - maybe even leave it in a better shape that it was when we arrived on this earth with de-carbonisation. I look around the estate I live on only 67 houses (mix of 3 and 4 beds) and see plumes from every flue first thing in the morning and early evening - so much energy wasted that could be harnessed/recovered with low temp heating. multiply that out on every housing estate in the country and our emissions and energy consumption would drop. oil and gas companies wouldn’t like it much but there is little they could do about it except cut their cloth accordingly………. I’ll get of my soap box and get back to painting fences on what has turned out to be a very nice afternoon

I was being a little tongue in check with my response I was however being serious about the two tank scenario - would mean the re-heat for each tank is much shorter and finding space for two smaller tanks is a heck of a lot easier than finding a space for one 400 or 500 Litre tank (which would then require an extended period to recharge) Heating one big tank quickly is going to need a much larger boiler than is needed for CH which could potentially compromise CH efficiency if the boiler is not chosen wisely with both HW and CH requirements in mind. Caveat to that is if you go for something like a Viessmann or similar high modulation boiler then large doesn't matter when the whole range has a turndown to anything from 3.2 kWh to as little as 1.4 kWh depending on type/model of boiler. You'd need a good gas engineer to work out the logic of managing two independent tank heating processes but I'm sure it's do-able Watch the video linked by @JohnMo 120L tank heated by a 30 kW boiler in 10 mins that 120L tank at 60 deg C would be blended down to max 38 Deg C so effectively incoming cold water 20 deg, HW 60 deg for every 10 litres of shower water used 4.5 litres would come from the HW tank and 5.5 litrs from the cold As a result 120 Litre tank supplying the shower would be blended to 266 Litres of shower water - at 10 LPM at the shower head that's 26 mins of shower time without any re-heat. Caveat - I'm not saying 120 Litres is the tank size you need - just that you need to do some maths around your needs and how best to meet the demands of the house - if it's a new build then consideration to where the tank/tank could be situated and the pipework feed and return as well as proximity to the boiler to minimise losses I'd also want to know what the mains incoming flow is for the property because ultimately that's going to have the biggest impact on being able to run multiple showers at the same time (or any activity involving HW usage)

Over shoot is an interesting subject - I can only assume by observation that the in the initial purge and burn on start up the flow temp exceeding the set point is ignored as the flow temp can typically hit 40 deg C in the start up phase but the boiler ramps down to min and the flow temp stabilises at very close to the target flow temp. just by way of illustration have a sequence of screenshots on fire up initial fire - boiler modulation is 58.2 % (58.2 % = ~12 kW) flow temp is 40.9 Deg C 30 secs later modulation has dropped to 30.9 % = ~8 kW flow temp hasn’t dropped yet (due to initial heat up in HEX) 1 min later modulation has dropped to 11.5 % (so ~5kW) flow temp is dropping rapidly to 30 Deg C 2 mins after start up Modulation is at min 10.8 % (so ~4 kW) flow temp stabilised at 27.7 deg C It’s 14 deg C outside on North side of the house where the OAT is taken so the boiler will do a burn of around 8 mins before shutting down for around 60 mins Point to note - the boiler isn’t heating the house - it’s just replacing the heat that is being lost - all rooms are at or very close to target temp. fires per day are around 15 right now The burn and coast process is the same in shoulder season as it is in depths of winter (the only change is to the time the boiler runs for) at -5 outside last winter the boiler was running for 55 mins followed by a 5-10 min coast fires per day were 22 - 25 probably very similar cycles to an ASHP would operate in this house

Absolutely ^ that - S or Y plan is middle ages approach - X or W plan enables boiler to run two independent temp profiles depending on what it's heating (CH or HW). Or even a basic "Heat Only" boiler set up to run DHWP - doesn't need to be a system boiler - just needs to know if it's doing HW or CH and adjust the temp and modulation priority accordingly. Out of the box my Viessmann "heat only" boiler is a dumb boiler - runs one flow temp for everything - for HW I need a min of 64 Deg C so I would have to run the CH circuit at that same temp except that's high temp heating and I have mostly K22 rads or verticals in the house and one K33 rad. They don't need 64 deg C flow temps they need 35 Max. So the solution was to "Add" HWD (Hot Water Demand) box and an "outside temp sensor" and tell the boiler it's no longer a dumb boiler capable of only running one flow temp for everything but to run WC flow temps when doing CH and when it's asked to do HW to get it done in the shortest possible time with the flow temp selected for HW - in my case 64 deg for summer and 70 deg for Winter * see note Note due the variables between summer and winter 1. Tank losses being higher to surrounding temps lower - 25 deg v 19 Deg 2. Incoming water temp being 10 deg in winter compared to 20 in summer 3. I want to heat the water in 30 mins max during the heating season (so I can use Octopus download to provide exact water heating energy consumption - Outside the heating season I'm only heating water or cooking and water heating is done in the morning and cooking in the evening I can see my consumption clearly 😉)

Sorry I've broken your quote down into bite size chunks to make it easier to reply too You'll have to link me to those statements because it makes no sense - the whole point of a condensing boiler is to run them in condensing mode for that to happen the flow temp needs to be 55 deg or lower to ensure the return temp is below the dew point Temp of the boiler can be set from 20 Deg C to 80 Deg C in non weather compensated mode In weather compensated mode the Curve sets the flow temp and that will be between 20 deg and 80 deg Discussion is good - it's not an argument and I have no problem explaining where I'm coming from As earlier link me up to where makers are saying stick to a 55 deg Return temp Nope not a combi - I have a HW Tank heated by the boiler - however flow temp for CH and HW are not the same - I'm running DHWP so if there is a call (or Schedule) for HW to be re-charged the boiler flips from low temp CH running WC based on OAT to HW mode - where it runs an elevated temp and modulation rate to get the HW done asap. I heat the tank to ~50 every day except one where I heat to 60 (Legionaire cycle) Mrs Alien likes a long, hot and deep bath once a week so as we only have a 117 Litre HW tank I take advantage of HW requirements on that day to do a Legionaire cycle - If Mrs Alien didn't want a bath I don't bother heating the water to 60. Over shoot is an interesting subject - I can only assume by observation that the in the initial purge and burn on start up the flow temp exceeding the set point is ignored as the flow temp can typically hit 40 deg C in the start up phase but the boiler ramps down to min and the flow temp stabilises at very close to the target flow temp. The only time the boiler "hunts" is on HW cycle for a min or two where it's trying to balance the modulation rate against the target flow temp (22mm primaries from tank coil to the boiler means at 0.6 m3/hr flow rate the rate of change is quite rapid and the boiler can get caught out by it - I really should close the gate valve a little to trim it. I don't have a ASHP to compare but I can tell you costs House is 4 bed Detached 113m2 Floor area across 2 floors, It's a stupid T shape so has a few rooms/areas that have 3 external walls - SUDG windows and doors - Blown CWI - 325mm of loft insulation and 75mm of PIR under the suspended ground floor Calculated Heat loss is 4.5 kWh at -2.5 Deg C OAT (pre loft insulation improvements) Annual Gas consumption is 9500 kWh at £0.05 per kWh (Again this is historical consumption and does not take into account the recent improvements to Loft insulation - I am expecting to get back to my previous schedule based consumption of below 9000 kWh whilst heating 24/7) That is broken down into HW 19% CH 80% Cooking 1% So just under £600 per year for all gas usage (including the standing charge - Take off £100 to the figure if you don't want it included) I'm sure an ASHP would be cheaper (especially if favourable tariffs were involved) but how much cheaper I don't think there would be much in it However capital costs (under the BUS grant) would have been much higher compared to the boiler swap/system tweak from Y pan to X (or W plan). IHTH