ruggers

Theres too many figures and a bit too confusing for me to calculate. MVHR ACH, Heat loss survey ACH which very depending on the room use from 0.5, 1, 1.5 ACH. The lower the number the more it reduces the heat loss for the room. then theres the air permeability score too. Are you saying 2m³/m² @ 50 Pa is much higher than a natural windy day? If so, why are they testing at such a rate? The MVHR I'll be installing, but it will be designed by ADM and commissioned by them. They calculate it all to comply with English regs and satisfy B.C MVHR isn't to be considered in any heat loss calculation I've been told by a lot of heat engineers and read it online. Reasoning being, the building has to be be sized to heat from cold when theres no heat to recover. Including MVHR in the calculations would result in smaller heat emitters and it would take much longer to reach desired temperature from cold. It's 5.1Kw without exposed area, I can't see it being lower than this. You have massive amounts of insulation compared to what I'm putting in. Mine will be: 100mm T&G PIR in the cavity, taped and self installed. 0.17 W/m2k 400mm loft insulation 0.11 W/m2k 170mm PIR floor 0.12 W/m2k And intermediate floors and stud walls too which I've added, but it makes no difference when all rooms are sized the same for 21C at -4.4C with a MWT of 37.5C A friends family had an identical house size & layout, built in 2006, it never had MVHR, will have had 50mm PIR vs my 100mm, and probably poor air tightness, they always had the heating on, but said it cost a fortune to run.

Heat engineer I'll have to look through my private messages, I think I was sent it a log time ago but couldn't get my head around it, the software seemed easier to use from memory. I'll be parge coating blockwork, setting windows and doors back, air tight tape, blower paint, minimising thermal bridging with as many places as possible, not having a door letterbox, masonry hangers, internal thumb lock doors etc. But my insulation won't be to the levels you'll have in yours in Scotland, the budgets too tight now too. I'm not sure how I'm approaching it wrong? I've detailed I aim to build to an airtight standard, minimise thermal bridging, Insulation is the most I can get in with adding excessive amounts of cost for small gains or screwing up the thermal mass adding internal insulated plasterboards. I'm just wanting to ensure my heat loss results are based on the correct ventilation info, and then I can obtain all of my quotes for the everything heat related.

Thanks for the answers. Using the rule of 14, at the old air permeability lowest pass rate of 10m3/m2.h would mean that the ACH would be 0.7, which would have increased the property heat loss compared to 0.5 ACH. If I achieve 2m3/m2.h@50PA / 14 it would mean using a value of 0.14 ACH instead of 0.5 default. Which rooms can I apply 0.14 ACH to, the wet rooms (Kitchen bathroom toilets) all require higher ACH of 1 or 1.5, as listed in my OP infiltration losses should be based upon: (air permeability (m³/m².h@50Pa) / 14) + (MVHR flow rate (m³/m².h@50Pa) * 10%) Do you have a example for this formula using my predicted air permeability score of 2 for the house, and also, what's the *10% for at the end? The heat loss survey has 1.5 ACH for the kitchen, so I need to know how to change this result?

Very true this. I know that my air test will be under 3 thats a start, and doubt it will be under 2, if it is its a bonus. The UFH can be installed once fully weather tight since it's going on top of the pir on the beam and block floor. I'm going to try loop CAD but have found a company who are willing to design snail pattern to match room by room heat loss and consider floor finishes, pipe spacings and usable floor space including weather comp and maximum MWT. I'll be installing everything except the plant room. @Conor a 5Kw heat pump can output 6.3Kw so it's more than whats required, but slower reheating water. I'm focussed more on the days when its warmer outdoors and requires minimum modulation, but do want the heat survey to be accurate because I don't want to size my radiators too small.

Just to try it, I've removed the exposed area 10% increase option and halved all of the ACH for each room and it's reduced the total heat loss from 5.7Kw to 4.22Kw. It's such a difference and if wrong left with a cold house or an over sized heating system. One of my larger UFH rooms has more glazing, so it's at the higher end of scale for maximum W/m2 floor output required at low flow temperature. I can't add more floor insulation, so it requires better pipe spacings, reduced heat loss or higher flow temperature. That's digressing from the OP though.

Out otf interest, wha construction was your build to achieve 1.5? I'm aware that the building airtightness and MVHR flow rates are separate things, and them flow rates also have to factor in around 90% of the ventilation will have the heat recovered from it. I've attached an MVHR pre design estimate from one company some time ago showing flow rates, it does have a mistake on it for at least one room. If it's so negligible, I'm happy to ignore that part. MVHR is excluded from the heat loss survey. But the airtightness of the building which won't be known until it's built and tested, will make a big difference in reducing the total heat loss if the correct ACH are input, rather than the softwares default ACH used which assumes the worst case air tightness result of 10. Sizing a heat pump should be based on an accurate heat loss survey otherwise you face short cycling issues in the shoulder months around 12 to 15C outdoors. The 5 KW heat pump can modulate down lower, Ive had mixed answers of 1.2 and 1.5Kw. 7Kw model is around 1.7Kw

I'm unsure if this is best posted in Boffins corner or MVHR being ventilation heat loss related. When planning my self-build heating and ventilation system, I've learned a lot of things, but some are a little out of my grasp to fully understand. It's very theoretical and I'm unsure how some of it plays out in the real world. So, I'll post what I have, and see if anyone with a good understanding of it all can advise me or correct anything I'm wrong with. You can over size or undersize a heating system, but undersizing is a more of a concern to me, especially with under floor heating pipe spacings. The property will be 200m2, masonry cavity wall construction. Without going into every sealing detail I've researched, I'll be applying a lot of attention to airtightness and aiming for the lowest score I can. I'd like to achieve 2ACH @50PA if possible and will have MVHR installed. I've completed a heat loss survey using some popular software, & tried various things to see how it impacts the end result. Being a self-build, I've been lucky enough to obtain the exact U values for all of the construction materials I'll be using, from the manufacturers, but there's still a couple of things that could have a large affect on my total heat loss. My maximum heat loss will be 5.7Kw, this includes me selecting the exposed area tick box to all rooms with an outside wall. I believe this adds 10% on, so if I removed it, the property could be 5.1 Kw I selected it to air on the side of caution with it being detached and 3 miles from the NW coast in the lake district, where the wind and rain can be heavy at times. I was told that this has less impact on properties with good levels of airtightness, should I leave it on or remove it? Airtightness: The heat loss software defaults most rooms to 0.5 ACH, except, Kitchen 1.5 ACH Toilet 1.5 ACH Bathrooms 1 ACH I believe 0.5 ACH derives from the worst case air pressure test result to obtain a pass. Building regs lowest pass limit was 10 ACH@50 PA, (Now apparently 8 ACH) which is terrible, I'd expect an average new build to be around 5, higher standard 2 ACH, and passive house 0.6 ACH. 10 ACH @50PA / 20 = 0.5 ACH. Is 20 the correct figure to divide by? If I achieved 0.2 ACH @50PA / 20 = 0.1 ACH. This lower air change per hour, input to the heat loss software would severely reduce my property heat loss. Without trying it yet, I'd guesstimate to be 3.5Kw which I'd not feel confident was realistic. If I should reduce the ACH from 0.5, how much do you scale the other rooms by with higher values? Do I play it safe and leave the ACH at their defaults, and if wrong, at least it's consistent for all rooms. Once the MVHR design is complete, their room by room air flow rates will likely affect this again, which I've glanced at, but one step at a time. The MVHR designer doesn't factor in heat loss and heat engineers don't factor in MVHR ventilation rates.

I can't comment since I'm not experienced with stoves, all I've done is look at all the regs and drawings associated with the burner I want to install. I had a meeting with an installer who recommended stepping up to 6" in most cases for various reasons, but best to check for yourself. I'm of the same opinion as you, install for 150 not 125 and you're not limited then.

@G and J maybe future proofing was the wrong term, but a lot of 6" twin wall requires 50mm clearance, so 60 and the odd few 70mm. So try to base it on the maximum and then if you ever have to replace the flue, you have enough clearance for any brand.

https://simplystovesuk.com/product/125mm-230mm-high-temperature-red-rubber-flashing/ bit cheaper

It's one of those case by case, area by area things I think. I don't think it's a strict rule, but B.C seem to like it boxed in to prevent fire spread from floor to floor through the vented plates or people resting things against it.

@G and J You need to have the twin wall boxed in after it leaves the room with the burner in, and in a cold loft it requires boxing or a cage around it so i believe. For future proofing leave at least 70mm clearance between the twin wall and combustibles and ensure the twin wall starts lower than 450mm from the ground floor ceiling if using single wall before it. Make sure there's no more than 4 bends or offsets in the complete flue. I priced building control to sign mine off at £300 but they were really trying to put me off explaining theres an 8 month waiting list due to only 2 inspectors for a large area.

Thanks. I thought these were just for the external roof outlet but probably provides a better seal than a DIY job. At least theres a way forward with it. Do you have a name for this so i can check if they do a 200mm I.D for the twin wall.

Great solution. Thanks.

@marshian thanks for the info. I understand what you mean and realise in PDHW is looks to minimise the down time of the CH so gives it max power to reheat the water quicker, but I thought there'd be an option like John suggested to change this. What flow temperature does it provide in PDHW mode, is it 70-80? What do you set the cylinder hysteresis to so that it doesn't cycle too much when showering, or do you operate it based on time only?