JohnMo

Don't think any thermostats are that good with a slow reaction heating system. We are in 100mm concrete screed, thermostats just cause over and undershoot. Smart controls cannot deal with the timings. You just have to balance the energy going into the slab, low flow temp for long periods, or slightly higher flow temps for a set period. Then balance the system room by room, to get the room temperature you want.

Mitsubishi SRK20ZSX-W was the A2A I referred too.

A window is good.

A2A and everything ventilation, euro certification database. https://www.eurovent-certification.com/ The one I quoted was a Mitsubishi heavy industries A+++ rated for heating and cooling. Model number is on my other computer.

Does look a bit half arsed kind of a fit

Bit baffled why you need (from your figures) 6 ACH. We have a fresh air environment with under 0.5 ACH. Think you are making things way more complex than needed, for little or no real life benifit. Your house will be in very noisy and full of big pipes. Think you could run a split Aircon unit for less than the cost of running an MVHR unit at 600m3/h, plus the extra brine pumps etc. A 2kW A2A heat A+++ rated, at 30 degC outside temp would give you up to 35kWh of cooling per day at an EER of 9, increasing to an EER of 14 at 25 degrees outside air temp. With an EER of 9 generating 1.47kW of cooling power, is 163W. 600 m3/h at an installed SPF,with duct losses is circa 0.8 W/l/s. 600 m3/h is 166l/s, 0.8x166= 132W. Plus pumps etc digging boreholes etc. Little or no saving, same would be true in the winter.

How old is your house? It may already have some floor installation? 5mm XPS isn't very much insulation. Ideally you would want 10 to 40 times that, but that would not be practical. I would go online and look up a floor u value calculators and play with that. That will tell you the floor u value, which is the amount heat you loose downwards in watts per m2 per degree. So if you have a 30 degree mean flow temp, the ground is 8, you have a delta T of 22. If your U value was 1.0 your downwards heat loss from your UFH water flow will be. 22x1=22W per m2, so over a 8hrs a 30m2 area would loose. 22x8x30= 5280Wh or 5.2kWh The same area with radiators and a room temp of 20degrees would loose 20-8=12 delta T room to floor. 12x1=12 12x8x30=2880Wh or 2.9kWh As a real example my floor u value is 0.09. my flow temp is 27. Delta T is 19. 19x0.09x8x30=410Wh or 0.4kWh.

UFH doesn't care if it's a new build or old house. So the basics are Pipes in the floor circulate water from a manifold, which will include a mixer valve and pump. The mixer valve determines the flow temperature through the pipes. The pump circulates the water around the UFH system. The higher the room heat loss, the higher the flow temperature has to be to get and keep the room at the required temperature. This is where a good or poorly insulated floor comes in to play. Couple of things to remember, the ground below your house is always at or around 8 degree C. Heat likes to travel towards cold, the colder it is, the quicker it tries to get there. The water flow temperature can vary between around 25deg for a well insulated house and above 40deg for a not so well insulated house. A well insulated floor will ensure most of the heat travels upwards in to the room. As the insulation will slow down the downwards travel. A poorly insulated floor, will do the opposite, most the heat will flow down towards the 8 deg ground, rather than the 20 deg room. The delta T, delta just means the difference between two values. So the delta T in UFH is the difference between the water going in to the UFH pipes and the temperature of it coming out. The energy your UFH gives to the room is managed by setting a mean flow temperature, which is the mid temperature between the flow and return temperature going into the UFH pipes. So as an example, your mixer is set at 40 degrees, the return temperature is 34 degrees. So your delta T is 6, your mean flow temperature is 37 degrees.

Don't get me wrong I do stop, but I can stop at a farm shop or similar where there are no charging points, or fuel points and get a decent meal also. Literally 4 charging stops, assuming where you want to stop is working, then you are looking for a charge point, obviously no range anxiety going on. Then you need a fast charger, otherwise you are stuck there for a couple of hours. Why bother. When they sort the battery tech out and the charging infrastructure I will be first in line, but not till then. Current situation, no forward planning required. Jump in car, fill up at first filling station, 10 mins, drive as far as I want, stop for a coffee and bite to eat, go again, repeat, if I want. Automatic everything in the car, 500+ mile range at any speed I feel like going at.

Whatever system you go with ensure your insulation below the UFH is good. Otherwise you will be pumping plenty of heat into the ground, which be costly. If your floor isn't well insulated, UFH is not a good solution. Basically the rapid response is cut boards with a reflective foil and your floor covering is attached directly to the board. The pipes are therefore close to the surface, so heat to room is quite quick. Screed system is buried in a screed layer, the thicker the screed, the slower and more stable the heat is.

If you can get a free heat source why waste it heating the air, which is already get pre heated by the outgoing air. Can you not use it directly for UFH heat source? Or are you looking at 8 to 10 degrees temperature source?

As Dave says. But we have a combi, with preheat cylinder upstream, if you go this direction, you need to install a PV thermostatic valve between the cylinder and boiler. Since I installed the PV thermostatic valve (2 months ago) I have halved the gas consumption month on month. The valve stops the boiler firing needlessly when you have a cylinder above 45 degrees. The risk of an unvented cylinder upstream is lots of Luke warm water for prolonged periods if solar PV is low performing. I am just using a cheap vented thermal store. As no drinking water is stored.

As your leaving things in place, you really need to do some experiments. Get your heat loss sorted out, then slowly ramp down your boiler heating flow temperature (leave a couple of days, and make sure you meet the thermostat setting). If you can get it down to 40ish, then a heat pump is doable with your existing pipework. You will need a decent buffer to allow you to get the circulation rate the HP demands. Almost any heat pump these days will do in excess of a CoP of 3, it's normally the implementation that screws things up.

It's just an external plate exchanger kit that's added, that makes them suitable for anything.

Don't get me wrong I do stop, but I can stop at a farm shop or similar where there are no charging points, or fuel points and get a decent meal also.

That job was all done by contractors, as I was working overseas at the time and my wife managed the house renovations.

We go down south a couple of times a year couldn't imagine doing it in an EV, would need at least two charging stops, (charging point(s) either full or not working) would add loads of time to journey; which is already way to long. ICE car get each way without a stop.

B&Q normally compete on best price, look at both their websites to see which is cheaper. Also take a look at insulation hub website. All the builder merchants around us are way more expensive than B&Q.

Our last house we moved a similar aged boiler and the plumber didn't even comment. If it working fine and suits your needs keep it, swop the plumber.

What's the reason for dumping gas?

If you need a 13kW, I would be looking to spend the difference between a small and large heat pump to reduce the heat load for the house. May be worth posting your build spec, insulation etc, lots of good advise will be coming your way. With energy prices the way they are, you need to do anything to reduce heat demand.

Your going to do what you are going to do, and asking the question will not change, what you are going to do. Dave says, make safe, you say it not worth the hassle, we will just wait.

Sorry not related to the question, but if your doing a new build, why do you need a 13kW heat pump. I have a 194m2 and was looking at a 5kW. Heat demand circa 3.5 kW at -5 deg. Have you done the heat loss calculations? Just seems huge.

I really wouldn't ask the question.

We didn't need to use conduit, just a bed of sand below and above, then the marker tape. Only bit of conduit was where it went into and up through the foundations.