Domestic UFH - Weather Compensation

rampantbunny

Did a search through the forum (albeit quickly) but couldn't find an answer to the following. Conceivably I could post this in the construction forum/renewables or maybe even electric but I've seen weather compensation discussed on this forum before.


The question:
If UFH is controlled by weather compensation system, and works in conjunction with a heat-curve you tailor over time to suit your house, what happens if heat is injected into the building from another source e.g. solar gain, or that gas fire that's lit once a year for 'effect'.

In my situation I see a decent variance in internal temperature between those days that are cold but sunny and those that are cold and dull.
The heat curve is tailored for those cold days with little sun (because I find they're most common) but when that yellow ball appears the house can get warmer than is comfortable.

Is there any concept of a hybrid approach out there i.e. cold outside, but before telling heat pump to run, an interior temperature check is done. Or is this too 'smart-home'.


EDIT:
Have just read a web article re. how weather compensation works for UFH. The article suggests that external and internal temps are monitored. Is this the case with all systems (and if so, what is the reason for a heat curve if the unit has real internal temps to compute with).

rampantbunny

Interesting article if anyone is browsing:

http://www.chapmanplumbers.com/home/weather-compensation-or-room-compensation/

Dardania

With proper controls, the internal temp of each room is checked. Not necessarily smart home-esque - could be as simple as a TRV.

So, if you have an additional heat source, the room control (e.g. the TRV) switches off, whilst the rest of the house gets heated.

The problem with this strategy with UFH is the time constant is long - the thermal mass continues giving out it's heat for a long period after the room is satisfied. Which is possibly what you're experiencing?

Nest have this concept of "True Radiant" which I think tries to address this: https://nest.com/uk/support/article/What-is-True-Radiant

Also, note the concept of optimising start: https://www.heatmiser.com/en/2016/02/26/always-feel-comfortable-and-save-energy-with-optimum-start/ - in simple terms it looks at the internal temp, it looks at the setpoint and what time the setpoint should be achieved, and starts the boiler when it figures it needs to in order to meet the setpoint at the right time.

And with the concept of weather compensation, your description / understanding is spot on, but it shouldn't be relied on too much - it serves to adjust the heat input to the house in order to match the outside temperature (relating those two parameters to account for heat lost due to insulation losses) - it still needs decent internal controls

rampantbunny

Dardania wrote: »

With proper controls, the internal temp of each room is checked. Not necessarily smart home-esque - could be as simple as a TRV.

So, if you have an additional heat source, the room control (e.g. the TRV) switches off, whilst the rest of the house gets heated.

The problem with this strategy with UFH is the time constant is long - the thermal mass continues giving out it's heat for a long period after the room is satisfied. Which is possibly what you're experiencing?

Nest have this concept of "True Radiant" which I think tries to address this: https://nest.com/uk/support/article/What-is-True-Radiant

Also, note the concept of optimising start: https://www.heatmiser.com/en/2016/02/26/always-feel-comfortable-and-save-energy-with-optimum-start/ - in simple terms it looks at the internal temp, it looks at the setpoint and what time the setpoint should be achieved, and starts the boiler when it figures it needs to in order to meet the setpoint at the right time.

And with the concept of weather compensation, your description / understanding is spot on, but it shouldn't be relied on too much - it serves to adjust the heat input to the house in order to match the outside temperature (relating those two parameters to account for heat lost due to insulation losses) - it still needs decent internal controls

Thanks Dardania. House was built so that thermal mass was maximized - so yes, I am experiencing the effects of this. Because the heat pump is being triggered based on outside temp, it runs. The UFH loops may not have yet caught up to the cut-off temp calculated from the heat curve information and therefore the heat pump continues to run despite the rooms being warm enough.

I was educated somewhat by the installer of the system, and a certain law of thermodynamics quoted that went something like: 'if the heat pump runs even when the room is already warm enough, it doesn't matter. If the temperature of the UFH pipes match the room temp, no heat will be transferred'.
The follow up question I have is why run the heat pump at all in this case. The UFH loop temps are dictating things. To my mind, the most efficient system is the system that only runs when it needs to, and I don't think a weather compensating approach only, gives an accurate picture of the 'need'.

Lastly, on the subject of the TRVs; I was told that I should not modify the loops because this would affect the efficiency of the system. Is this valid in your opinion? I recognize every system is different, and you don't have details of my setup - but generally, should efficiency suffer if loops are closed down?

Dardania

rampantbunny wrote: »

Dardania wrote: »

With proper controls, the internal temp of each room is checked. Not necessarily smart home-esque - could be as simple as a TRV.

So, if you have an additional heat source, the room control (e.g. the TRV) switches off, whilst the rest of the house gets heated.

The problem with this strategy with UFH is the time constant is long - the thermal mass continues giving out it's heat for a long period after the room is satisfied. Which is possibly what you're experiencing?

Nest have this concept of "True Radiant" which I think tries to address this: https://nest.com/uk/support/article/What-is-True-Radiant

Also, note the concept of optimising start: https://www.heatmiser.com/en/2016/02/26/always-feel-comfortable-and-save-energy-with-optimum-start/ - in simple terms it looks at the internal temp, it looks at the setpoint and what time the setpoint should be achieved, and starts the boiler when it figures it needs to in order to meet the setpoint at the right time.

And with the concept of weather compensation, your description / understanding is spot on, but it shouldn't be relied on too much - it serves to adjust the heat input to the house in order to match the outside temperature (relating those two parameters to account for heat lost due to insulation losses) - it still needs decent internal controls

Thanks Dardania. House was built so that thermal mass was maximized - so yes, I am experiencing the effects of this. Because the heat pump is being triggered based on outside temp, it runs. The UFH loops may not have yet caught up to the cut-off temp calculated from the heat curve information and therefore the heat pump continues to run despite the rooms being warm enough.

I was educated somewhat by the installer of the system, and a certain law of thermodynamics quoted that went something like: 'if the heat pump runs even when the room is already warm enough, it doesn't matter. If the temperature of the UFH pipes match the room temp, no heat will be transferred'.
The follow up question I have is why run the heat pump at all in this case. The UFH loop temps are dictating things. To my mind, the most efficient system is the system that only runs when it needs to, and I don't think a weather compensating approach only, gives an accurate picture of the 'need'.

Lastly, on the subject of the TRVs; I was told that I should not modify the loops because this would affect the efficiency of the system. Is this valid in your opinion? I recognize every system is different, and you don't have details of my setup - but generally, should efficiency suffer if loops are closed down?

To answer your first point, the installer's law of thermodynamics question, you're right that the UFH loops will not give out heat if there isn't a difference of temp between the liquid within, and the slab. But the next thing that will happen is that the liquid will be returned to the heat pump at the same temp it leaves, and so the heat pump will stop adding heat (e.g. switch off the generation compressor, while leaving the circulation pump running). You're right that weather compensation doesn't address the full picture - it only addresses part of it (setting the most appropriate flow temp to the UFH loop) not whether there is actual demand or not.

For your second point, this is where it gets tricky. I always feel that one of the biggest problems in engineering is matching variable demand to installed capacity, Machines in general like to run at full capacity - usually that is where they work most efficiently - that is what all of the components are sized for. My knowledge of heat pumps is not as deep as others here, however from working on controls for other more industrial compressors, they generally work better when they're fully loaded. You also have to bear in mind the issue of "short cycling"where the demand is quite small relative to the capacity generated - the only way the compressor can match the small demand to relatively high capacity is by switching the compressor on and off quite frequently (and that's the issue of short cycling)
Techniques to deal with the above situations include: variable output compressors, or introducing a buffer vessel in between the demand and compressor (think of it as a flywheel).
Consequences of short cycling are burning out components, and also reduced overall efficiency. Depends upon the degree of short cycling.
The reason you would have a small demand in your case is if you start shutting off loops when they're satisfied.
In my opinion, it's a valid concern, but you have to look at the big picture - you're overheating. That in itself is wasteful. So wonderful, you're generating heat in a very efficient manner, but it's not needed. To paraphrase what you say, the best way to save energy is not to use it.

In my opinion, I would fit some sort of controls to the affected areas, and experiment to see if there's a negative impact on the heat pump e.g. frequent on/off cycles more than a few times per hour.

rampantbunny

Dardania wrote: »

:
:
In my opinion, I would fit some sort of controls to the affected areas, and experiment to see if there's a negative impact on the heat pump e.g. frequent on/off cycles more than a few times per hour.

Am in the house about a year now but only recently started gathering data on energy consumption. I'm going to see how the summer goes and size up my options after. The waste as you point out does rankle. That and the significant expenditure on a system that I expected to be more intelligent. It's a first world problem. I think my expectations might be a little high for these systems and I def didn't ask enough questions at the outset.

Thanks for the input Dardania...it has been interesting.

antoinolachtnai

Because of the inertia, turning off the heat source when the room has already gotten warm is going to help a bit, but really not that much.

The answer is really to detect the sun or better still, forecast the sun and then heat accordingly. This would be, as you say, very smarthome.

Is there a difference between your south and north facing rooms, or does it even itself out?

rampantbunny

antoinolachtnai wrote: »

Because of the inertia, turning off the heat source when the room has already gotten warm is going to help a bit, but really not that much.

The answer is really to detect the sun or better still, forecast the sun and then heat accordingly. This would be, as you say, very smarthome.

Is there a difference between your south and north facing rooms, or does it even itself out?

It evens itself out after a period presumably due to MHRV. The area subject to the most solar gain is the open plan kitchen/dining area, which has an extract as well as a supply.
I had a plan to shade the area as there is a decent amount of glazing, but we're not long in the house and are in the 'getting used to it' phase.
TBH; the thought of blocking out what little sun we do get is something I wanted to avoid.


Overall, I'm not going to change the system that is installed. I'm looking at some smart-home tech at the moment for occupancy detection, security, humidity (to boost MHRV), and other. I thought I might add temp sensors in certain areas with a view to using them in the future, if the heat pump was amenable to taking extra inputs. I've a conversation going with the installer re. the inputs/outputs on the HP but I don't think I'm going to get any schematic that will allow me to tinker. Might have to interrupt the weather compensation line into the unit to affect any change on the HP triggering.
But, might never happen. I was more wondering if this nut had been cracked before.