JohnMo

As @JamesPa says mixing. A 4 port buffer will have little or no temperature difference between top and bottom so you get a different temperature profile compared to no buffer, as seen in the test. I run my 180L buffer as a two port, so only excess flow goes into the buffer, this can still lead to a 4 or 5 degree temp spike for 10 mins or so (gas boiler). I get a bit of a sine wave +/- a few degrees either side of the weather comp set point.

It depends on how you operate the system, if all the other loops have thermostat, the thermostat will call for heat. The actuator will open the loop at the manifold and if the pump and boiler aren't on, start them up. Another alternative is a single thermostat can control one loop or multiple loops, depending on how the actuators are wired up in the wiring centre. As I see it, you would have one loop in each room, as you only have two ports free on your manifold. Each could have its own thermostat. An alternative control, is you leave a loop on a manual valve and always open, then when anywhere else calls for heat, that loop is always heated. No call for heat elsewhere, this loop is off also. Lots of differing ways, depending on what you want. I just have a single thermostat for the whole house, that starts the pump, all loops are always open, as I run on weather compensation.

Looks like two loops to me (one in each room), you could put a thermostat on each loop. Or you could just balance the loops and do a single thermostat or none if on weather compensation. Doesn't fill you confidence when they state zones instead of loops.

Report says The buffer tank pump is set to run at 30 l/min and a delta T of 5. The flow rate coincides with max heating demand of 10.63kW.

You don't need a buffer to defrost, as long as the open capacity in the heating system is acceptable. Yes soft starts of inverter put less strain on things, but that not the whole story. Short cycling just uses way more power that it needs to, doesn't matter if it a heat pump or a boiler. Short run time use loads of energy just heating up steel work, it stops, everything cools, starts, heat the metal work -cools, repeat. 10 stop starts is 10x heating up up stuff that not inside the house. 3 stop start 3x heating stuff outside the house. Long runs good, short runs bad. The worst case is lots of start, no heat put in the house to any useful degree.

SFUF Will give you a u value of 0.8. so units then become, w/m2k. So if you are flowing 40 degree water. And under floor ground is 6 degrees. You are loosing downward 0.8x(40-6)=27W per m2. Say your ground floor is 50m2, that 1.3kW or nearly 900kWh per month. Or £90 a month plus the cost of heating the rooms. Well insulated floor would be loosing way less, my floor has a u value of 0.09, but my typical flow temp is 28. 0.09x (28-6) =2W per m2. 100W per 50m2., 66kWh month or £7 per month Someone will correct if I've messed up the calc. Poor insulation under UFH heating is a big running cost.

I build a whole house without, but it depends you risk aversion. Friends helping out may not be covered anyway. Just would need to carefully check policy wording. It maybe your current home insurance covers you, maybe worth asking. We had a rental once, we spoke to insurance people they just asked for a small premium while work was ongoing. You will have tell them anyway, if you don't they may void the insurance anyway, if you tried to claim.

500L buffer. Let's use a working range of 28 to 50 degs. So 7kWh an average nighttime temp of 5 degs. Flow temp of 55 and outside air temp of 7 should give a CoP of 2.5. Say you pay 10p kWh, that 4p per kWh delivered. Using the same hp as above it puts out 5kW. So you dump in to cylinder and floor for 6 hrs at 5kW, so 30kWh in total, so that's cost £1.20 Doing a similar 30kWh but slowly into the floor at 30 degs across the day at normal 34p per kWh. Let's spread heat dump into floor at hp min output at 7 degs, so that takes 13hrs but CoP is 5.9. 34p/5.9=5.8p per kWh. So total cost is now £1.74. Let's say heating season is 180 days, that's £90 per day saving. But you would need add in extra day time rate and increased standing charges etc. By a way of comparison for gas. Currently getting 110% efficiency out of the boiler so about as good as you can. 10.7p kWh / 1.1 =9.7p 30 x 9.7p =£2.90. Gas is a lot more expensive!

Not sure I really agree. A modulating heat pump can only modulates so far. So let's use an example a media 4kW monobloc (staying low energy house) So my house at -5 needs say 3.5kW and at +10 needs a heat input of 0.5kW, flow temps equate to 30 and 25. -5 HP output ranges from 1.3 to 5.2kW, so all ok with the world, I need 3.5kW so mid range. +10 output is just under 2kW, I need 0.5kW. So on/off operation. To allow the hp to run for 10 mins you need just over 50L of water engaged for the heat pump to work on. So in this case the heat pump runs fine without a buffer. But it only works if I don't split the system in to zones. If zone down the system capacity a buffer would be required as I only have 56L of water in the floor. A 6kW hp would need a buffer in my case.

£10.5k - I now see why I did it myself. If you are thinking heat pump in the future, why not think about it now, the government will pay for the heat pump, new cylinder and possibly some of the UFH? Your UFH will be much of a sameness for gas, oil or HP. W/m2 is just a matter of flow temp and spacing. You should be aiming to get low flow temps what ever route you go. Biggest thing to consider is insulation below the UFH loops, without it keep the the money in you pocket.

Just been looking at this out of interest, just looked at the datasheets for Comfopost Things noted The smallest unit is the one to go for your 217m3/h is mid range in the datasheet. Requires some high water flow temps to a get a small heat output. Could lead to dreadful CoP for a heat pump Water flow rate is 10L/min, which will be too low for a heat pump if this is the only use for the water. So a buffer would be required The heat exchanger differential temperature is very low again rubbish for a heat pump You could get A2A heat pump for the same/less and it would do a better job

I would think Scenario 1 represents quite a lot of real life situations, where you read threads on here, and people want to zone the house with third party thermostats. Many will have the controller isolated from the house and then on/off thermostats controlling the heating zones. Grant for example show the installer to do that in their manual. The OEM controller hidden away so the end user doesn't mess with it. Scenario 2 although it states weather compensation the system is frigged to give a fixed flow temp, which in effect is the same as Scenario 3 based on a fixed fixed temp also. So really the only difference becomes the buffer.

Do you have a structural engineer involved, as they will have to issue a certificate to cover all aspects of the structural design. This forms part of the warrant, but is also required for the building sign off.

What does your warrant say about no DPC, not sure they would accept that? The concrete slab is a good cold bridge, would you be better, to run a few theromolite blocks in line with the outside edge of the blocks below to form an upstand up to finished floor level. Then inside that have a further insulation upstand in PIR or your preferred solid insulation, then cast the concrete floor?

You have calculated for the best case, which is the coldest day. Most days will have a much lower heat demand down to about 1kW. And you need to size for 62.5L, if the radiators are on a different zone and not always switched on.

Couple of different formulas Buffer sizing – from ASHP book 6[kW=kJ/s] x 60/4 [minutes/number of starts per hour] x60 [secs/mim] = 5400kJ Mass of water required [kg] = 5400 [kJ]/(4.18 [kJ/degC kg] x 5 [degC] = 184.5kg or 184.5L Buffer sizing – from Hydronics design book - you need to convert US imperial - not SI units V= t(Qhsmin – Qloadmin)/500(delta T) V = minimum buffer tank volume (US gallons) t = minimum heat source on time (minutes) Qhsmin = minimum stable heat output of heat source (Btu/hr) Qloadmin = minimum concurrent heating load when heat source is on (Btu/hr) ΔT = change in average tank temperature during minimum heat source on time (ºF)

It may be easier keeping most of you heating system open to the boiler, so it has plenty to work on.

The original post you referred to stated Vb = (Pmin-Dmin)*Tmin/(Hmax*4.2)-Vs (or zero if this yields a negative number) You got a negative number so buffer size is zero litres. As you add thermostats your system volume decreases, the number then becomes different, if you have a small radiator on its own thermostat and the rest of the house a another zone, the small radiator could be the only part of the system on for the boiler. So that is the volume you use in the formula. The idea of WC is keeping it simple, you don't need thermostats with WC, you can add them but you set them as limit stops a couple of degrees higher than required room temperature

Obviously didn't try to hard, first hit on datasheet for the above. 10.7kW min while condensing. https://www.baxi.co.uk/our-boilers/system-boilers/baxi-platinum-plus-system

Don't need TRV's if you balance the radiators, this limit their output without switching them on/off. On heat Geeks there is a write up on how to balance. One thermostat can be ok, but it takes a while to get everything balanced. If you go one thermostat, go weather compensation, so you are always on the lowest flow temp possible is best IMO. If you are coming up with negative numbers you system has a large enough capacity. If you do install a buffer let the buffer float on system temperature, don't install a thermostatic control on the buffer, otherwise your HP/boiler will be firing at a higher temp than required, just to heat up the buffer.

I used Pert-Al-Pert, because there is almost zero memory in the pipe. Unroll, straighten by hand and it stays straight, clip it down and bend away, super easy to use. I did 300mm centres in 100mm of concrete, flow temps at -9 were 33 degrees. Designed it myself, instead of relying on a sales man to tell me I needed to buy several km's of pipe (which you don't) and a couple of manifolds to cope with the loops. Works fine and dandy.

Just to be pedantic - normally water, depending on vessel material and end user this water is treated to remove certain minerals etc. Large vessels are infrequently tested with air/nitrogen, because the stored energy within a compressed gas. Went to a heat exchanger manufacture once and the heat exchanger (printed circuit heat exchanger) was for a gas service and the company would only perform air tests. The test were completed ex WW2 bunker with 2m thick wall in case of mechanical failure. You had to witness the test via a camera in another building. The walls had big chunks out of then were a heat exchanger had failed in the past.

What are you trying to find out? Our last house was subject to building control inspection, internal doors had round knobs, no comment from them. This time we have lever handles, not expecting any comment on them either. If there is no defined regulation, you just fill your boots and do what suits the application.

Boiler runs on weather compensation, so can deliver anything from 25 to 35 degrees. Boiler just monitors the return temp and gives more heat to system as required. Buffer is just more system capacity, plumbed as a 2 port buffer, with an oversize tee at the buffer ports to act as a hydraulic separator. Buffer sits at about the same temp as the heating flow temp, hotter at the top than the bottom.

If you have room for a buffer and allow it to float on boiler output temp (no thermostat), min modulation isn't the end of the world. Currently with a min modulation of 6.7kW at 30 degree, against an under 3kW demand, my efficiency gas into boiler to heat at UFH manifold is around 110%. So go with the boiler you are comfortable with and is at the right price and can run X plan or priority hot water and a different CH flow temp.