JohnMo

I have a Polypipe UFH controller (rebranded Tech Controller/Roth controller) which is Opentherm and or on/off. My basic issue is the boiler is a combi, but I use it as a system boiler. The Opentherm knows it's a combi, so the control is tailored to the combi.

The on off thermostat and temperature ramping up is discussed here on page 5.Hints Tips & Fault Codes.pdf PDHW will work with any configuration for heating, so on/off thermostat, Opentherm controller (Atag controller for example) or load compensation (same as weather compensation but using inside temperature for reference) Another read h too big to upload https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.atagheating.co.uk/wp-content/themes/atag/hubfs/Appliance%20Technical%20Guide%20Pre%202019.pdf&ved=2ahUKEwi1xfGl4biLAxVkVEEAHQFyBAwQFnoECB8QAQ&usg=AOvVaw1gl0y29TTeKTIoqVVZ3xGC

Something isn't correct here. 2.5m³/h would indicate about 12kW heat pump, and that isn't a low energy 150m² house. Why all the complications of floor and ceiling and A2A heat pump? If you need dehumidification that is the role of ventilation, what is you ventilation system? Delete the need for a buffer. Do you need floor and ceiling heating and cooling and A2A? Do you need zones? The efficient design would be UFH (heat and cool) run as a single zone, or with fan coils in bedrooms for heat or cool (no UFH in bedrooms). Correctly sized heat pump, this would depend on proper heat loss calcs. It could be sized for batch charging floor or WC. No buffer No ceiling system

@SimonD Got the boiler running Opentherm and as you found getting the flow temperature wrong and you get plenty of cycling. In the end the Opentherm ran no different to my simulated weather compensation (resister instead of outside sensor). So just reverted back to on/off, this also gave me PDHW which didn't work on Opentherm. So now just operate at a fixed flow temp of 37.8, control via a resistor, WC is on setback when thermostat is satisfied (setback will never allow boiler to refire). To do PDHW, a relay switches resister out of circuit and the boiler ramps to about 56 degs. This was last night, wobbly bit up to 3am was the heat pump on at the same time, doing defrosting. The peak around 5.30 was cylinder heating. Charging a floor via UFH (no mixer, but boiler is behind a CCT). Basically runs once per day (if outside is below 5 degs for 6 hours) maybe twice if super cold.

Atag, great boilers. Return temp drives condensing. Most Atag boilers have modulating output and have modulation of circulation pump, so almost manages dT itself. They work very differently depends on running against a thermostat or weather compensation. On WC the flow temp is almost rock solid, on thermostat operation it will keep adding to flow temp approx 10 mins after starting and will go up to max temperature set in the controller Poor advice, flow temperature does not directly affect condensing, return temp only does. Avoid if you can. One big zone or two large ones are the way to go. The bigger volume the better, you also need good flow, so a nice open system give the boiler plenty of opportunities to move the heat and it be absorbed. With an Atag you just need the Atag controller, and TRVs on the radiators. Just keep everything simple

If your pretty airtight go that route otherwise... My view is dMEV done well or MEV. Both working conditional demand - CO2 and or humidity, with self actuating air inlets either through wall or trickle Video https://youtu.be/CGP_1rsOzDI?si=37tNvRjRVN1sBFNQ

@Stevenwithav Back to my first question?

Just giving information based on doing that and paying the huge bills while understood what was happening. Been there, got the tee shirt. Big zones good, small zones bad for the bank balance.

Sounds like work, to make more work.

Back to my original post, looks like someone has stopped the boiler being called for heat, just to satisfy UFH on purpose. Looking at your photos of the UFH manifold the only loop with flow is the lounge. So assume each room with UFH has its own thermostat also? If so your boiler will just short cycle (use bucket loads of gas with little or no heat being delivered), if you do get the wires sorted to fire the boiler with just a single UFH circuit calling for heat.

A modulating boiler needs to have hit it's target flow temperature before modulating down. Many boilers do not modulate well on thermostatic control, they try to get the thermostat to meet heat demand as quickly as possible. But may not be in condensing mode depending on flow temp and return temperature. A long run time helps stabilise the whole boiler and maximise efficiency. All modern boilers (since the 1990s) have been designed to condense. This extracts the latent heat from the flue gas, by using return water and flue gas heat exchange. The condensation of the flue gas adds to the energy balance. You need a return temperature of 53 degs or below, and the lower it is the better the efficiency gain.

@James Logan why not post a drawing or sketch of your system, showing what you currently have. Low flow could be a number of reasons, but should be easy enough to sort out. Can you drain down some of the fluid being circulated through heating system, what does it look like?

Definitely does not reverse flow water. The refrigeration 4 way valves moves from heat to cool mode effectively. Putting hot refrigerant into the condenser instead of cold. UFH defrost cycle looks like this Note the timescale, nearly every heat pump does the same. Looks dramatic but whole defrost is about 3.5 mins.

There is zero need to bring DHW in to play. You then bring the thermostat for cylinder into play, it does defrost, cylinder cools. Heat pump heats cylinder, goes in to defrost, cylinder cools repeat. Zero advantage lots to loose. If you are concerned (no need to be) you add a volumiser.

Nearly S plan, but S plan is just one flow temp, so S and Y plan, becomes W and X plan.

But your sad, the rest of us are just dull and boring and like to watch TV.

You may need to get the conductivity up a bit, I think it needs to above 100 for a few reasons, heat geek VDI 2035 mentioned a few. This is what my ASHP manual says for the spec.

PDHW allows boiler to run at different temperatures, one for cylinder heating, and a other for central heating. Weather compensation allows boiler to run at the lowest practical temperature for long periods. Running S plan boiler runs at a set temperature, which is generally not ideal for either cylinder or central heating. The analogy being, you cruise down the motorway acceleration is very gentle, etc, your fuel consumed very good (PDHW). The alternative is you only run at high revs and at full acceleration and then slam your brakes on, accelerate... (S or Y plan) You get to the end of the journey at nearly the same time, one uses lots of energy the other uses way less. Low boiler flow temperature can increase efficiency by 10s of percent. S plan is lucky to get 80% efficiency well set up PDHW and WC is nearer mid 90s. I actually dropped my consumption by closer to 50% year on year. If you are worried about wasting energy insulation and airtight are the way to go.

These should all be detailed on your drawings, otherwise you have no idea what you are going to get. Not even sure why you have planning without this level of detail? If this level of detail is not being used to quote against, then everything will be a variation and you best have a huge contingency fund. Stop the process get proper drawings of what YOU want. More details you put on the drawing, the less stress you will have and the less wiggle room for being wrapped off.

Sorry to say - why? Seems a bucket load of work and cost. If you have a gas boiler, just get an UVC with big coil (heat pump one) and do PDHW (priory demand hot water) and weather compensation and top up with the stove as needed.

There are certainly some bonkers rules with respect heating. Should be 20% now in UK

You need to sort the losses from ventilation really also. You would also size for 21 in the main rooms and 18 I believe in bedrooms under MCS install. Not 18 to 19 overall. But sounds like a 6kW unit not an 18kW. So a big miss match of information. Are you sure you have not missed something in the handover of information. Wall, floor, roof and windows buildup? Do you have an air test certificate? MVHR?

Do you have depressed the UVC depressed and therefore the EV? Prior to pressuring the EV? Maybe the bladder is at the end of its life, allowing air across but not water?

For anyone scratching there head about gas boiler and heat pump cylinder, this is a plot of one. Note the boiler temperature, the dip at the end is the heat cycle going off

Unless you have cold water running continuously, you do not get much condensation. What does form just evaporates off when the cold is switched off. The main pipe coming in to the house (blue plastic) never seems to even get damp, as nothing really runs long enough. Haven't even bothered to insulate our pipes and very unlikely I ever would.