SimonD

The basis for my calculations came from the Handbook of Domestic Ventilation by Rodger Edwards. It's a book that's getting a bit long in the tooth and could really do with another edition, but the fundamentals, including various formulas are there. Its section on PSV is very comprehensive and shows that it works, and if facts works better in air-tight houses because they have more control - e.g. studies in Denmark show a mean ACH of about 0.45 with PSV with no detrimental effect on air quality. In terms of understanding implications of hygroscopic materials, I did a lot of research looking at studies across Europe and North American to gain an understanding of how these materials reduce ventilation rates due to buffering moisture load - so, for example in cases of PSV, with a house that does not have moisture buffering peak RH can reach 75% for short periods (but not periods long enough to cause issues with condensation etc, in the building) and with moisture buffering, this can easily be reduced to peak loads of 60-65% during peak moisture load, like showers, baths and cooking etc. However, the area is still pretty poorly research tbh, so there is some guesswork required and a careful attention to detail. I have made sure I choose materials through the whole of the fabric, including paints - so everything is finished with clay paint. I also use gypsum plaster as opposed to lime because gypsum is both vapour permeable and hygroscopic whereas lime is only vapour permeable - which is why lime can survive damp stone properties and gypsum can't, but gypsum is excellent in a newly built of deep retrofit property (but it's also been used in Italy for a good couple of thousand years because of its moisture buffering properties). We also have clay bricks on the ground floor, which are brilliant for moisture buffering. Yes, it's bouyancy and also wind - if the PSV is designed and installed properly it should benefit from a negative pressure zone above the roof. It's actually been shown that if ventilation at the bottom of the stack is increased, the flow through the PSV increases, which is one reason why PSV performs better in airtight buildings that have controllable ventilators - in leaky houses you can easily get over-ventilation with PSV.

You've also got the option of just buying a second hand unit too, which you don't usually get with a gas boiler. Or at least I wouldn't install a second gas boiler for any of my customers.

Not really. Theoretically it should be easier than swapping out a gas boiler. Even connecting the flushing equipment is easier as you can do it off the isolators outside.

I bet you were 😊

MCS and the consumer protection companies don't really like this structure any more. They prefer the BUS grant to be subtracted from the quote.

The installer will make an application for the grant to confirm eligibility as soon as you sign on the dotted line for a contract. It's normal practise to ask for stage payments first and then apply for the grant after commissioning. Once the initial grant eligibility has been confirmed with OFGEM the installer has 120days to finish and apply for grant redemption.

There always lots of content about why not to get a heat pump installed, but I would like to know why you chose to get a heat pump installed instead of a fossil fuel boiler? Doesn't matter whether it's for retrofit/renovation or new build, I'm just interested to know. And what were the pain points that were most difficult to overcome? Was it just the price or is it in line with so many posts on BH that it's difficult to find a decent system designer and installer?

We are 4 plus dog in a volume of above 900m3. But as I mentioned above, ventilation requirements change dramatically once a hydrophilic fabric is introduced into the equation. So, to quote from an earlier study I read when deciding on my design: https://web.ornl.gov/sci/buildings/conf-archive/2004 B9 papers/002_Simonson.pdf Now, it does acknowledge that consideration regarding air polutants is probably separate, but other studies using MVHR show similar reductions on ventilation requirements simply because moisture drive such a significant proportion of those ventilation requirements. Yes, indeed. At some point when I actually find some spare time, I might draw together my collection of research into this and building physics just so there more readily available reference.

SS is used for traditional metal standing seam roof clips and screws.

There are some articles out there that talk about this. If using over-lay, then it must be one that uses an insulated backing such as pir. In effect, this creates a decoupling with the slab and as it increases thermal resistance to the slab, it pushes it upwards instead. However, you need this to be properly calculated by someone who knows what they're doing because floor down losses also depend on the shape of the floor and external sides.. It's probably going to be better than over-lay on a suspended floor, but neither are that ideal into uninsulated floor. The other thing you need to consider is that you'll be raising the finished floor height and this will have an impact across your entire house and will also impact your stairs, which may need modification to still comply with building regs.

Not necessarily, it depends on the ventilation strategy or mixture of strategy, of which buoyancy, is just one aspect. Mine mainly utilises pressure which is either vapour pressure difference or wind pressure. I do have a stack effect used for when it gets warm, and a limited stack from ground floor to first floor. But one of the things often overlooked is how building a house which uses materials that buffer moisture significantly reduces the ventilation requirements of the space which in turn reduces the energy requirements, whether through heating incoming air, or running mechanical ventilation. The calculated ventilation rate we have is about 0.38ACH and complies with building regulations minimum vent rates. Thermally, we now know that the building outperforms the calculated heat losses too. IAQ is fine. We've on a few occasions had raised CO2 but nothing to worry about at all. All the other measures like particulates etc. are always good, other than immediately after doing a load of dust inducing building work, but that clears pretty quickly.

This whole thing is a bugbear of mine ever since I did the detail design of my house, which is so fundamentally different to most building methods that I've even had extensively long debates with the powers at be on the green building forum and the understanding here on BH is also narrow when it comes to the full understanding of ventilation in relation to moisture management and building physics, and particularly the differences and relationships between vapour permeability and hygroscopicity in the building fabric. This is why people still think I'm mad for designing my house to be low energy while also naturally ventilated. But suffice it to say that when I not long ago had some people planning a self-build round to look at my house, they asked why would I seal the whole building up and then introduce air supply - the simple answer is of course so it's then properly controlled while also adequate for good indoor air quality. When explained it was good that they accepted this having seen it in practise. Well, that is the million dollar question when people try to calculate the roi of upgrades that will provide them with a lifetime of better comfort in their home (even if it costs a few quid, versus spunking goodness know how much on crap quality tack on extensions, kitchens and bathrooms every 10 or 15 years and trying to mask the problems with dehumidifiers, over-sized boilers and radiators etc.

Thank you, you're on my tester list ☺️

Just to be aware. SAP calculates heat loss slightly differently to the industry heat load calculations as defined by BS EN 1283-1:2017 and CIBSE Domestic Heating Design Guide. SAP is interested more in energy consumption and energy efficiency whereas CIBSE is about sizing the heat source and system appropriately for peak loads. SAP is absolutely fine to use to gain an understanding of the thermal performance of the building, but just be aware of the differences and you might need an alternative calculation for final system sizing and design. For BH, please keep and eye on a free design tool coming soon that can do both, which is something I've been working on since the latest implemented changes to heat load calculations last summer, but I need to test it on a couple of real world projects first 😉 I might be able to make it available sooner if I can receive bug feedback as a beta test kind of thing.....?

I completely agree. Investigate first. For example - when we bought our shell for development, the bungalow built in the 1920s, which we had to live in for a few years was extremely damp - I installed a pretty heavy duty extractor in the bathroom as a first step because there wasn't one in there. We found problems in a few of the rooms that had to be addressed and when we stripped out the old kitchen, we found black mould completely covering the wall behind the cupboards.