ASHP Buffer configuration for both UFH & radiant ceiling cooling

[siletto]

Dear Buildhub'ers!

 

I am quite new here, and I've already learnt a lot from you regarding the proper heat pump design principles, which I thank You very much!

 

One thing that comes to my mind again and again, is that how would you place the buffer tank for a zoned UFH and zoned radiant ceiling cooling.

 

For the "traditional" 4-pipe configuration, it is quite straightforward, you need several ball valves(6 or 8 IIRC) manual or motorized on the primary side to change that the cooler or warmer water goes to the top of the buffer.

 

The more modern, 2 or 3-port configurations really caught my mind, but I simply cannot imagine how would anyone pipe these layouts for both buffered heating or cooling.

 

The Caleffi article describes a 2-port config with cooling, but it does not reverse the cool-warm ports of the tank, so the cooler goes to the top. 

 

 

What would you advise in terms of energy efficiency, but with realistic costs at the same time? Should I stick with the traditional 4-port method? Perhaps a 4-port with custom internal baffles to prevent mixing? What is the way to go ?

 

 

 

My home will be about 150sq.m newly built low energy type, with only UFH in all rooms, and radiant ceiling in bedrooms with A2A split heat pump in the living room for dehumidifying.

According to the initial plans, the heat pump may be 2-2.5m3/h flow on full load, while the house needs 1.5-1.8m3/h all zones engaged, at 35°C flow temp.

 

Thanks in advance!

[JohnMo]

5 minutes ago, siletto said:

home will be about 150sq.m newly built low energy type

 

6 minutes ago, siletto said:

the heat pump may be 2-2.5m3/h flow on full load,

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?

 

9 minutes ago, siletto said:

What would you advise in terms of energy efficiency

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

 

[sharpener]

2 hours ago, siletto said:

What would you advise in terms of energy efficiency, but with realistic costs at the same time? Should I stick with the traditional 4-port method? Perhaps a 4-port with custom internal baffles to prevent mixing? What is the way to go ?

 

Avoid buffer if you can. I have a custom thermal store with an internal baffle but the results are disappointing as the two volumes mix to a much greater extent than I was expecting.

[Marvin]

We have a buffer with our TRV (Thermostatic Radiator Valve) regulated radiators. It seems to help to avoid the ASHP 'cycling' (going on and off in short bursts because, for example, only one radiator demanding hot water so little to do). As I understand that the ASHP takes about 10mins to reach optimum efficiency.? 

[joth]

Have modelled heating and cooling load for the property?

I've not got experience of radiant cooling ceilings, is it common in Hungary?  Like JohnMo, fan-assisted cooling has been very effective in my experience. 

 

In my house I drive ASHP to the UFH directly, and have a separate zone for fan coils that has a volumizer tank inline with the FCU to reduce short cycling.

The UFH is entire ground floor, but the FCU generally just services one room at a time (office or bedroom) hence much more prone to short cycle without the extra buffer, which is plumbed for cooling. (It does work OK for heating anyway... it has the intended effect of increasing run times even if it's not the optimal way up).

 

 

[siletto]

4 hours ago, JohnMo said:

 

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

 

 

Here we size the heating system to -15°C, so the heat loss of the building is around 8.5kw net, maybe 9kW with losses in the distribution system.

Frankly, a modulating condensing boiler is a way better approach in our climatic area, because in winter it is often foggy and around 0°C.  ASHPs are way better in UK and warmer climates. Many heatpumps struggle with defrosts and keeping the performance at low ambients, that is why we need to size a 10-12kW ASHP that can still provide 9kW at -15.

Due to EU regulations, condensing boilers cannot be installed in new homes alone, only along with 3-4 minisplits or PV panels(with very low return of investment). Buying a heat pump only for heating, and installing air conditioners for cooling seems to be a waste of money.

 

There isn't enough space for floorstanding ultra-quiet fancoils, but the looks and noise of the high-wall, minisplit-like fancoils are very disappointing. The radiant panel ceiling heating and cooling becomes more and more common here. It has very good thermal comfort without any noise.  Its only weakness is the humidity factor, because we use 15-18°C flow temps to cool, either with humidity sensors, or condensations switches.

 

Btw a fancoil "indoor" unit costs alone as much as a complete minisplit set, with more noise, less eficiency and more pipework.

 

[siletto]

1 hour ago, joth said:

Have modelled heating and cooling load for the property?

I've not got experience of radiant cooling ceilings, is it common in Hungary?  Like JohnMo, fan-assisted cooling has been very effective in my experience. 

 

In my house I drive ASHP to the UFH directly, and have a separate zone for fan coils that has a volumizer tank inline with the FCU to reduce short cycling.

The UFH is entire ground floor, but the FCU generally just services one room at a time (office or bedroom) hence much more prone to short cycle without the extra buffer, which is plumbed for cooling. (It does work OK for heating anyway... it has the intended effect of increasing run times even if it's not the optimal way up).

 

 

Yes, almost all new homes, apart from the very basic, low-cost ones use these panels. It is either a metal sheet with small heating pipes embedded, with drywall overlays or drywall boards with pipes embedded from the factory.

It is fixed to the ceiling, or cast in the concrete ceiling. 

They work very well, but only with ventilation and dehumidification included.

 

 

[JohnMo]

10 minutes ago, siletto said:

we size the heating system to -15°C,

Didn't look at your profile, now see you are in Hungary. Very different from the UK.

 

13 minutes ago, siletto said:

ASHPs are way better in UK and warmer climates.

Many different climates here. High external humidity drives a lot of defrosting also.

 

If you are doing a buffer, 2 port would be my only choice. Set heat source to only manage buffer temperature. Heating system to only draw from buffer, via a second pump.

 

Reversing for heat or cool, shown as a 3 port but same is true for a 2 port

 

 

[siletto]

1 hour ago, JohnMo said:

Didn't look at your profile, now see you are in Hungary. Very different from the UK.

 

Many different climates here. High external humidity drives a lot of defrosting also.

 

If you are doing a buffer, 2 port would be my only choice. Set heat source to only manage buffer temperature. Heating system to only draw from buffer, via a second pump.

 

Reversing for heat or cool, shown as a 3 port but same is true for a 2 port

 

 

 

 

Thanks for the reply!

 

By 'Managing buffer temperature' you mean the start/stop of the HP?

 

Do you think the 2 port is better instead of the 3-port?

[JohnMo]

47 minutes ago, siletto said:

 

 

Thanks for the reply!

 

By 'Managing buffer temperature' you mean the start/stop of the HP?

 

Do you think the 2 port is better instead of the 3-port?

Thermostat or sensor on buffer is only used to stop and start ASHP. The house thermostat for heating starts and stops the circulation pump on the heating side.

 

2 port buffer - Heated water is always available for heating straight away. If flow either side is the same all water bypasses the buffer. Only if a zone closes does any excess water flow into and out of buffer. Acts to increase cycle time, more efficient than continuously mixing water in the buffer.

[siletto]

On 10/02/2025 at 18:44, JohnMo said:

Thermostat or sensor on buffer is only used to stop and start ASHP. The house thermostat for heating starts and stops the circulation pump on the heating side.

 

2 port buffer - Heated water is always available for heating straight away. If flow either side is the same all water bypasses the buffer. Only if a zone closes does any excess water flow into and out of buffer. Acts to increase cycle time, more efficient than continuously mixing water in the buffer.

 

 

What do you think, how much energy is lost in the 4-way tank reversing valves? I mean the flow and return essentially contacts the same brass body, heating/cooling each other.

 

I am still confused with the 3-port vs 2-port configurations, many articles describe that the 3-port method is the neater, because at least the lower portion of the tank is engaged at all times.

 

 

Edited February 25 by siletto

[JohnMo]

49 minutes ago, siletto said:

many articles describe that the 3-port method is the neater, because at least the lower portion of the tank is engaged at all times

3 port is better than 4 port. But a 3 port would need a pretty big buffer to stop mixing. The 2 port ideally will be at least 50L, ideally more if you have the space.

 

Different running profiles for 3 and 2 port buffer.

 

The 2 port has a thermostat this calls the ASHP to start and stop. So mixing of flow and return and the distortion this causes is minimal. Low demand heating the ASHP more or less runs to keep buffer at temperature, thermostat switching on and off. High heating loads the 2 port is likely to be by-passed fully.

 

A 3 port is normally just a vessel with ports the heat pump doesn't know it there. If the heating demand is low some flow from ASHP will migrate to the the lower ports and cause distortion to the way the ASHP runs.

 

Distortion in simple terms is an elevated return temperature caused by flow and return mixing. This gives an elevated return temperature and causes output temperature to increase affecting CoP.

 

But all the above said, try to a do the heating scheme as open as possible, no zones, no buffer. Run the UFH and ceiling loops fully open, this will allow the heat pump to flow what it wants and manage dT and flow rates.

[siletto]

Thanks for the reply  John! 

 

In our climate, almost every installer use some kind of buffering technology. 

It's because we have to size the HP to a greater span of heat loss. 

It needs to be sized for winter loads(-15°C sometimes), so heating without cycling is almost nonexistent. 

 

In my example: I need about 8-9kWatts of heating at 35°C flow temp for the winter. A 9kW unit might be a good choice, but even a good quality unit can only modulate down to say, ~4kW which might be exactly the load you size the machines there at the UK -2°C.

 

Above freezing(about 5kW for me) the heat pump will eventually cycle. 

 

With this "oversized" approach, a second problem comes. 

If I set the WC curve to supply 30°C water in 0 ambient, The 5kW power with 5 deg deltaT results in so little flow that with your "all open" approach, the secondary(UFH) flowrate surpasses the heat pump's builtin pump, thus severe mixing occurs. Mixing can be avoided only if you fix the flowrate at a constant maximum, but it increases deltaT in low-load conditons. 

 

 

I've been learning about sizing hydronic equipment for quite a few weeks, and I am more and more convinced that part-load calculations are basically nonexistent. Everyone talks about inverter-driven heat pumps, but no one about the fixed speed secondary circuits. 

[ProDave]

On 10/02/2025 at 13:22, siletto said:

Here we size the heating system to -15°C, so the heat loss of the building is around 8.5kw net, maybe 9kW with losses in the distribution system.

Here in the Eastern Highlands, -10 is not uncommon so I sized my ASHP for that.  Heat loss at -10 and +20 inside is just over 2kW so at -15 it would still be under 3kW  so a 5kW ASHP does the job nicely.  I think -18 is the coldest night so far but that is not a sustained cold.

 

So your heat losses are coming out about twice mine for a similar sized house.

 

Defrosting is not usually an issue well below 0, there simply is no moisture in the cold air.  Defrosting is mostly an issue around and a few degrees above 0.

[JohnMo]

Expanding on what @ProDave says.

 

We are in similar area, but closer to the coast so get a bit more defrosting. Slightly higher heat loss due to form factor of the house. I have a 6kW heat pump, but run at a slightly elevated flow temp - the heat pump doesn't really modulate, this afternoon it was 10 degs and with excess solar the heat pump ran for 1.5hrs, putting out 5.5kW the whole time. Decent screed thickness is the buffer, no house overheating either.

 

A 9-12kW unit, will ideally have around 180 to 200L of water. Or a huge lump of concrete in the floor to dump heat too.

 

Choice a buffer if you really don't want, but you just don't need it.

 

I would really eliminate the buffer. Just use a fully open system, a suitably sized volumiser, fully open UFH flow meters, and then let the heat pump circulation pump modulate as it wants.

On 10/02/2025 at 08:05, siletto said:

heat pump may be 2-2.5m3/h flow on full load, while the house needs 1.5-1.8m3/h all zones engaged, at 35°C flow temp

Let it run slightly lower dT (fully open system will do this for you). Then make sure you have a decent screed thickness.

[siletto]

11 hours ago, ProDave said:

Here in the Eastern Highlands, -10 is not uncommon so I sized my ASHP for that.  Heat loss at -10 and +20 inside is just over 2kW so at -15 it would still be under 3kW  so a 5kW ASHP does the job nicely.  I think -18 is the coldest night so far but that is not a sustained cold.

 

So your heat losses are coming out about twice mine for a similar sized house.

 

Defrosting is not usually an issue well below 0, there simply is no moisture in the cold air.  Defrosting is mostly an issue around and a few degrees above 0.

 

You should have very good insulation!

My house is perfectly good with the latest insulation and lifetime emission regulations, although not a certified Passive House.

The heat loss per calculations greatly depend on the filtration number between 5.5-7.5kW.

11 hours ago, JohnMo said:

 

I would really eliminate the buffer. Just use a fully open system, a suitably sized volumiser, fully open UFH flow meters, and then let the heat pump circulation pump modulate as it wants.

If you open all flowmeters, and you don't use zoning, how do you balance the system for "equal" room temperatures?

 

The "standard" UFH layout per my studies uses a Constant Head circulator, but according to your suggestion it is not needed without zoning, right?

[JohnMo]

19 minutes ago, siletto said:

how do you balance the system for "equal" room temperatures

If a new install you can balance by design, so room loop sizing. Or can tweak room flow down on any overheating rooms only. 

 

22 minutes ago, siletto said:

The "standard" UFH layout per my studies uses a Constant Head circulator, but according to your suggestion it is not needed without zoning, right?

Yes drive the whole heating system direct from heat pump circulation pump. You just don't need mixing valves and additional pumps. One thermostat/controller by heat pump manufacturer.

 

System is then

 

ASHP 

Diverter valve (to cylinder or heating)

DHW cylinder 

UFH manifold and loops

 

29 minutes ago, siletto said:

filtration number between 5.5-7.5kW

I would

Address airtightness 

And look at ventilation system.

When cold the house air is naturally dry, so if house is heated nearly no risk of mould or condensation, but over ventilation can cause health issues as the air can get too dry. Sustained cold use an enthalpy MVHR or a demand based (CO2 and humidity) ventilation like MEV or dMEV with suitable controls

[siletto]

4 minutes ago, JohnMo said:

If a new install you can balance by design, so room loop sizing. Or can tweak room flow down on any overheating rooms only. 

 

Yes drive the whole heating system direct from heat pump circulation pump. You just don't need mixing valves and additional pumps. One thermostat/controller by heat pump manufacturer.

 

System is then

 

ASHP 

Diverter valve (to cylinder or heating)

DHW cylinder 

UFH manifold and loops

 

I would

Address airtightness 

And look at ventilation system.

When cold the house air is naturally dry, so if house is heated nearly no risk of mould or condensation, but over ventilation can cause health issues as the air can get too dry. Sustained cold use an enthalpy MVHR or a demand based (CO2 and humidity) ventilation like MEV or dMEV with suitable controls

 

I downloaded the LoopCAD trial version, modeled the house, so I got very useful results, I really suggest everyone doing this, it is really straightforward, albeit i studied building physics for a year.

Basically the program suggested 100mm CTC piping for rooms with many outside walls, and 150mm CTC for rooms with few outside walls.

Even using this variable floor piping, there is about 30% difference in flow quantities accordig to the program, that is why I asked if this "all open" method works.

 

How would you combine it with zoned cooling (perhaps only one room at a time with ~0.15m3/h flow)?

 

The manufacturer of the slab/drwall radiant cooling panel suggested a separate cooling buffer tank. The radiant panel needs about 18°C water, so filling the buffer with 5-7°C flow, then up-mixing it may provide the buffering solution. At least it is their idea.

 

 

 

 

 

 

[JohnMo]

1 hour ago, siletto said:

Even using this variable floor piping, there is about 30% difference in flow quantities accordig to the program

I believe the program just apportions flow based on loop length and then assumes a fixed dT across the loops. Think all my loops expect one were pretty similar in length. The short loop is in an ensuite, and that never overheats.

 

You can play with outputs in loop cad by having different manifolds, set the flow temp and manipulate dT and loop lengths to get the room output (or room temp) you want. It's quite a while since I used it though.

 

This is how mine is sitting, it's 10 degs outside, full cloud cover so no solar gain and heating came on from 00.30 to 6.00 this morning.

 

Heat pump circulation pump runs 24/7, when not heating or cooling the circulation rate is at minimum set speed.

 

So this where flow rates settle to at min speed, all flow meters are open 

 

1 hour ago, siletto said:

How would you combine it with zoned cooling (perhaps only one room at a time with ~0.15m3/h flow)

I just do cooling exactly the same way as heating, just set the flow temp to maintain between a high of 19 and low of 14 through the floor pipes. It generally only cools when the sun's out, it sorts it's self out, I have no input as it doesn't need any. I don't zone anything in heating or cooling there is no point. It add complexity for zero net gain. Heat pump is very happy with fully open circuit.

 

Heat pump running last night, putting out just under 6kW, for 5.5 hrs without stopping, getting a CoP of 5 while heating. House doesn't overheat, floor will keep the house temp stable, until tonight and it will do the same again. (Big spike is DHW heating). 

 

[siletto]

4 hours ago, JohnMo said:

just do cooling exactly the same way as heating, just set the flow temp to maintain between a high of 19 and low of 14 through the floor pipes. It generally only cools when the sun's out, it sorts it's self out, I have no input as it doesn't need any. I don't zone anything in heating or cooling there is no point. It add complexity for zero net gain. Heat pump is very happy with fully open circuit.

 

But the cooling does not only take place in the UF slab, but also in the suspended ceiling radiant panels, which needs approximately the same flow as a slab UFH, but has zero mass. (Uponor Thermatop system) 

 

 

Edited Tuesday at 16:19 by siletto

[Nickfromwales]

2 minutes ago, siletto said:

but also in the suspended ceiling radiant panels

Who has calculated the condensation risk, and do you have panels which have drainage points to collect and manage the condensate?

 

I would be very cautious about putting the same temp cold (defo not cooled) water through the system that heats the ceilings....

 

Do you have a design or specification that states the product can cool via these panels? Any product link / info you can post?

[siletto]

3 hours ago, Nickfromwales said:

Who has calculated the condensation risk, and do you have panels which have drainage points to collect and manage the condensate?

 

I would be very cautious about putting the same temp cold (defo not cooled) water through the system that heats the ceilings....

 

Do you have a design or specification that states the product can cool via these panels? Any product link / info you can post?

Sure, you can check many manufacturer's website:

Uponor Thermatop M

Look for Wavin Comfia CD-4, it is not yet released in the UK

Rehau chilled ceiling

 

Somehow these radiant cooling systems are not widely introduced to your Country, idk why. As far as I know, it is now widely used across europe, even in our almost-third-world county, compared to the UK or Germany. Interestingly, underfloor cooling is strongly not recommended in Hungary, because you may slip on wet floors if condensation occurs, cool floor is unhealthy to the feet, "warm air rises to the ceiling", etc.

 

Btw, condensation is prevented by using RH metering thermostats. On simple systems, you can set a maximum RH level that shuts down the cooling in the room. With advanced bus-type controllers, you set the buffer tank flow temp to a low temperature ( e.g. 5-7°C), and use a motorized mixing valve to constantly adjust the flow temp to be above the dew point for the highest RH room. It maximizes cooling effect, preventing condensation, while acts as a further "buffer" when it uses up the 5°C water to mix it up to 16-18°C.

 

Yes, it works. It has moderately high costs, but can provide the peculiar cool "church feeling", if done right.

 

To better understand its problems:

Combining radiant heating and cooling makes the system not ideal to either tasks. As many of you advised, UFH should be as open as possible, and use the heat pump's inbuilt circulator without any buffers. You basically heat the whole home, every room. This radiant cooling system always use some kind of room thermostat, and unless you have every ceiling full with these panels working all the time, you don't necessarily have the necessary flow rates to prevent short cycling. I planned to install these panels only in the Bedrooms, but it results in less than 9l/min flow all bedrooms engaged.  A single room uses about 2-3l/min, so you either cool the whole house, or nothing.

We are starting to be a Mediterranean country in terms of summer weather, but still, cooling is not used for more than 2 weeks at a time, and it can be switched off at night.

 

Why am I bothering with all these unnecessary hassles? Because a very good, silent fancoil(Like the Innova Airleaf that Panasonic sells rebranded to their Aquarea lineup) costs more than these radiant panels for a given square meters of room. The fancoil costs as much as the panels, but you must buy overpriced modulating thermostats, specific comm. boards, drain pans, etc. Still, the small flow problem persists if you plan to cool 1-2 rooms at a time, and a buffer is needed.

 

If anyone has any engineering data regarding radiant floor cooling, I could awfully thank if he/she could share with me.