JohnMo

Big coil same as a heat pump, run at lower temperature in the cylinder same as a heat pump?

No the biggest reduction would be better insulated houses. Including your own. Also in your calculations you use the cost of a boiler as £2300, I paid £1000 for mine. Looking at Screwfix the prices vary from £500, upto about £2500, so you have chosen top end to suit your calculations. If you are looking to retrofit a HP the comparison is do nothing and keep gas (then only gas consumption is included), or replace like with like then a similar gas boiler would be chosen. Or replace gas with a HP, life style choice, most in this country cannot afford.

That's because they are, either hot or cold water powered

The hardest parts for me were and are Fitting the insulation into the floor, there seemed like a mountain of it. Then after the house is livable taking another 18 months getting the land scaping, decked areas etc completed - this is still ongoing.

Really depends on the level of insulation you have and how long you have the floor heating running for. Long run times let you have super low flow temps. Our flow temps at the moment are around 26, so floor temp is 21, no perception of a warm floor. As it cooler than your body.

Interesting that it's a water based system

If you have water without antifreeze in the UFH, it could freeze, and if did you wish you had blown it out or filled with an antifreeze. I remember a grand designs where a self builder had water in the UFH, a few freezing days, next thing was water escaping and the floor being dug up.

Not a good idea to block these ventilation paths, as they are there for ventilation. When you say they replaced the trickle vents on the windows, what did they replace then with, like for like or something different?

Fill the depression with sand, lay insulation, job done.

Why would anyone sane do that? Do it via G99 and export limit to 16A. The bigger the house the less the incremental contribution to heating you get from humans and appliances, unless you cram the house full of humans. So the more likely you are to need some sort of heating. Even the best insulated house is always loosing some heat to the outside. If you have a screed floor its a no brainer to install loops, cost is small I only have 500m of pipe in the floor over 193m2. So a small cost adder overall and flexibility for heating you can't install later. Cheap to install would be a Willis heater (more costly to run).

So to answer the question. I picked a number of 1 for ACH. This then demanded I had MVHR (or balance supply and extract ventilation) to comply with Scottish building Regs etc. Your worst case is you pick 4 and don't install MVHR and then get 1 and have to then retrofit to comply with building regs. Choosing a low figure also demands attention to detail - which is a good thing, but not something people are used to on a build. You have to set the ground rules (some times several times to the same person). If you are using a builder ask them what they usually achieve, if the answer is 'I don't know', look for another builder who does know, then at least they understand the concept.

My showers all came from eBay, a shower base on gumtree, another on facebook market place, another at full price. Everything else basically ex display, from local show rooms.

Passivhaus say you can satisfy a heat demand of 15w/m2 based on their ventilation rates. Anything more ends up with a burning smell as dust carbonises. But you still need to heat the air from somewhere? So if your heat demand is below this, you could go this route. Other options could be A2A heat pump, heating cooling with a good CoP. Nothing is easy all have downsides. UFH doesn't need to be complex, we have 300mm centres, with 7 loops over 193m2. Manifold with pump and mixer. You need a buffer and heat pump, three way valve and hot water cylinder, a few other bits and bobs, but not a lot. Run on Weather Compensation, you don't even need thermostats.

Merge your zones in to a single zone, so everything is on or off. Areas at different temperatures require higher flow temps in the warm area as the heat tries to get cold areas at the same temperature. Set up your Weather Compensation and use that. Set a 2 degree set ack at night. Simple is best, long slow run times at low flow temps.

I think they are two different things, MVHR will ventilate the interior space, walls buildup is subjected to interstitial condensation, so you really need to treat it differently. I think?

I'm really operating my gas boiler exactly the same as you should a heat pump. Operating at very low temperature flow all the time and with weather compensation, and monitoring gas consumption and reheat times. Have 100mm concrete screed with UFH 0.09 u value for floor. If you let things get too cold reheating time can be 24hrs. As the heat input is closely matched to the heat loss. You also use way more energy in that 24 hrs reheating, than you would over a couple of days at steady state, and very small night setback. Also noticed it cost no more to have the whole house at a similar temperature, than it does with warm and cold rooms, maybe slightly less, as the flow temp can be lower. So low flow temps, on for long periods, small nighttime set backs, no thermostats, if you hitting a thermostat your flow temp or DT across the radiator or floor loop is wrong and needs adjusting. Lots of on/off thermostats, expecting quick heat up times, equals expensive to to run, true for oil, gas and heat pumps.

You should have a temperature setting knob on the UFH manifold with numbers in it, that is what your flow temp is set to. You can also check the menu on your boiler and check the flow and return temperature.

As suggested get a better plumber. Proper heat loss calc is important, not this is what normally do. I have gas, so will give you an unbiased view on both. A well insulated house with UFH throughout will have very low water flow temps, circa 26-29 on weather compensation. Limit the number of thermostats to one per floor and balance flows to match room temps needed. Install a small buffer 50L. This will act to give hydraulic separation from the pumps, as well perform a duty for the heat pump. A decent heat pump should then give a CoP of around 5 for central heating at those flow temps and about 2-2.5 for DHW. So running costs will be lower, but if you have loads of thermostats and a low loss header, the efficiency will be lower. If you have solar PV some of the running costs can be offset. Gas, no cooling as mentioned. Gas can be efficient, but I suspect in most cases they are badly installed to get a high efficiency, they also suffer from to many thermostats short cycling.

What flow temperature is the UFH set at?

There is no reason not to use a split system, a couple of things to think about. You generally supply to dry room and extract from wet rooms. The extract and supply rates should be equal. But I am not thinking of any reason why you couldn't supply or extract from bedrooms the important thing is having a movement of air, sweeping stale air away and replacing with fresh. So if you have only bedrooms upstairs, supply 45m3/h to two bedrooms and extract from the other two. Get a unit that has a holiday function and run a timer so the unit ticks over during the day, and goes to normal duty at night. Get an oversized unit so it's near silent. Another thing you could do with MVHR, if you have a cupboard upstairs is run a single supply similar to PIV, through the cupboard to a downstairs ceiling and then an extract in each bedroom. To get a through ventilation, but with heat recovery. The supply use a coanda effect terminal, so the air clings to ceiling, a move out as far a possible. Just need to think a little out the box.

The nearer you get the smaller the cone gets. I used white masking tape also. But if you have a calibrated thermometer even better.

Just to jump in, If you are overshooting the thermostat, this could indicate the flow temperature is hotter than it could be. If you want the room/house at 21.5, set the thermostat slightly higher than that. If you exceed 21.5 deg tweek the flow temp down, using the information about the WC curve provided by @ReedRichards. Just do 1 to 2 degrees at a time. getting the heat curve correct takes a bit of trial and error. WRite down the changes you make, then you can back track easily.

Just get an infrared temperature gun, and adjust the mixer so inlet flow and the supply header temperature are the same. Seemed to work ok for me.

Leave the valve and pump, just do what I did to set mixer valve. Use this spreadsheet to calculate your heat loss Use sheet to calculate +20 and -20 heat loss, this will give you the heating curve, convert to W/m2 at the extreme ends. Use this this sheet to calculate the flow temps at the the different outside temps from the W/m2.

I have managed to use a 1k kWh of gas since 01 June to the end of Oct, which followed a series of modifications to heating and DHW system. Summer DHW mostly being heated by excess solar PV. Overall I am hoping for a sub 5k kWh annual consumption. During Oct we had a number of days from 3 to 7 degrees, so am hopeful. The 2 to 3kW input is at -5 so hopefully won't have that many days at that temp.