@alec-morrow sorry for the slightly delayed response - just been fighting off Covid!
I'm a little unsure about what you mean. Would mixers enable each circuit to have a different flow temperature? We don't have a buffer tank, the flow from the ASHP is routed directly to the UFH and/or radiators.
So, ran our system in Room Temperature mode for a week or so, and monitored flow temperatures. I came to the conclusion that it doesn't work very well with UFH. Confirmed this with a call to Mitsubishi. The response time of UFH is too slow for the Room Adaptation algorithms, so it has a tendency to ramp up the flow temperature if it doesn't see a change in room temperature, which will happen because it takes quite a while for the UFH concrete slab to heat up enough to affect the room. Mitsubishi guy's recommendation was Weather Compensation or fixed flow for UFH, but he did comment that fixed flow was not the most economic!
So, back to weather compensation, although one possibility mentioned by Mitsubishi was to use stats to control weather compensation at this time of year. We get quite a bit of thermal gain during sunny days, so seems very wasteful to leave the ASHP running if the house is being warmed nicely by the sun. After a bit of experimentation I found that our UFH controller will demand heat from the ASHP, seemingly without any necessity for the the FTC to be demanding heat, which seems at odds with the way that it works in Room Temp mode, where both need to be calling for heat! This means that it looks as though I can control the ASHP off the UFH stats, and re-purpose the wireless controller to control the upstairs rads.
I know this is not the best way to run weather comp, ideally shouldn't have any stat control at all, but it seems like a good compromise for spring/autumn, where we can potentially save energy by turning off the ASHP on sunny days. The only potential downside to this is that with the ASHP off, the UFH slab will start to cool down, so possibly need more energy and time to get it back up to temperature when needed.
Interesting; I can see why your ufh behaves slightly differently to radiators.
I'm running mine on weather comp with room stats. Like you I'm not convinced that weather comp alone is best all the time. Periods of solar gain or log/coal burner gain will see the house too hot as you say. But on a dull winter's day I definitely think weather comp is better.
With your ufh controller/weather comp combination, I would have thought if the ufh is calling for heat, the ASHP will heat until the weather comp target is met then stop. The flow will cool a bit then it will kick in again. That's what mine does - at least I think so...
On the graph below, the long stop after 9am is the room stat stopping things. When the stat calls for heat again, the cycling is caused by the weather comp flow temp being reached and then cooling. You can also see the different behaviour/power consumption at different outside temps.
Interesting; I can see why your ufh behaves slightly differently to radiators.
I'm running mine on weather comp with room stats. Like you I'm not convinced that weather comp alone is best all the time. Periods of solar gain or log/coal burner gain will see the house too hot as you say. But on a dull winter's day I definitely think weather comp is better.
With your ufh controller/weather comp combination, I would have thought if the ufh is calling for heat, the ASHP will heat until the weather comp target is met then stop. The flow will cool a bit then it will kick in again. That's what mine does - at least I think so...
On the graph below, the long stop after 9am is the room stat stopping things. When the stat calls for heat again, the cycling is caused by the weather comp flow temp being reached and then cooling. You can also see the different behaviour/power consumption at different outside temps.
We are seeing very similar behaviour with our flow temps.
After a couple of weeks of nice sunny days here it has now become colder and cloudier, so it will be interesting to see how our system copes. If the weather compensation curve is correct it should just keep the house at a comfortable temperature without getting turned off by the stats. I suspect I will have to tweak things a bit though!
@peterr@kev-m interesting conversation re weather comp over this last week or so of coolish nights but lovely sunny days. I currently dont have a room stat installed and just letting the ashp do its weather comp magic. But I did wonder if a stat might have been more energy efficient on those days. ASHP only installed 14 March so nothing to gauge by but wondered if either of you have actual energy usage figures for yours to prove stat + weather comp uses less energy than weather comp on all time on those warmer days? Gotta say that the fan wasnt on and rads were pretty cool daytime so not sure how much energy it was pulling...
It is virtually impossible to carry out accurate back to back tests on a home heating system, because no two days are exactly the same.
One possible way to reduce energy consumption on a system running on weather compensation control, would be to set any thermostats and/or TRV's to a temperature setting 2C above the desired weather compensated setting. Solar gain can then be used to raise the indoor temperature during the warmer part of the day, and hence reducing the heat demand during the colder part.
We have an A2A heat pump, which at this time of year we power during the day from our solar PV system. This raises the indoor temperature by approximately 2C, and in doing so shuts down our gas boiler. The gas boiler only fires up again when the indoor temperature falls to the desired setting. Today the gas boiler was off for only 5 hours, but yesterday the off period was 14.5 hours.
If you have a system that uses modulating room temperature control, a similar saving could possibly be made by raising the indoor temperature by increasing the desired setting during the warmer part of the day, and then lowering it again during the evening and nighttime.
@peterr@kev-m interesting conversation re weather comp over this last week or so of coolish nights but lovely sunny days. I currently dont have a room stat installed and just letting the ashp do its weather comp magic. But I did wonder if a stat might have been more energy efficient on those days. ASHP only installed 14 March so nothing to gauge by but wondered if either of you have actual energy usage figures for yours to prove stat + weather comp uses less energy than weather comp on all time on those warmer days? Gotta say that the fan wasnt on and rads were pretty cool daytime so not sure how much energy it was pulling...
Very timely question. I've already 'proved' (subject to the caveats @derek-m mentions) that weather compensation only uses less energy than a fixed flow (in my case 45 deg) and thermostats, to keep the house at the same temperature. I used outside temp vs energy scatter diagrams to compare. I'm now doing something like what Derek is suggesting. I'm keeping my thermostats at the temperature I want the house to be but I've increased the weather compensation curve a little so that, at least at higher outside temps and when it's sunny, it causes the thermostat to stop the ASHP. I've only been doing this for a few days. It looks like this may use a bit less energy than weather compensation only but it's early days. I'll leave it like this over the nest few days as it gets a bit colder then post the results.
@derek-m “It is virtually impossible to carry out accurate back to back tests on a home heating system, because no two days are exactly the same.“
This is why knowledge and understanding as to what, why and how controls relate to the thermal performance of a building should be valued. Instead it is trivialised. I speak first hand as a campaigning heating engineer who has wasted 15yrs of his life trying to get the powers that be to understand the controls.
Hence the resource wasteful mismatching of on-off controls with compensations controls.
I am an optimist by nature and when I found this forum I was upbeat. Then Graham Hendra, a manufacturers employee, spoke about weather compensation and trivialised it. My heart sunk.
Heat pump technology is world class. so is boiler technology. That means the grief people suffer in the U.K. is totally unnecessary.
Just one more thing along with Smart motorways, aggressive fining policy of car owners and brexit, that make this country “crap”.
Raising a heat curve will always use more energy. But if you turn off with on off controls you will need more energy at the start if a phase change.
Mixing and matching on off controls with compensation controls is fraught with frustration.
Using a weather station which looks at wind and solar gain from a sensor, that’s the way they do it in more advanced countries..that uses a feed back signal to vary flow temps!
Raising a heat curve will always use more energy. But if you turn off with on off controls you will need more energy at the start if a phase change.
Mixing and matching on off controls with compensation controls is fraught with frustration.
Using a weather station which looks at wind and solar gain from a sensor, that’s the way they do it in more advanced countries..that uses a feed back signal to vary flow temps!
Alec,
I know; I have monitoring and can see exactly how much energy the ASHP uses minute by minute. I'm just doing some experiments where I'm trying to show, for my house, which approach uses least energy under different conditions. I can see the conditions where weather compensation and the room stats don't work well together but there are other conditions where they are OKish.
Yes, a higher heat curve uses more energy while it's running. On the other hand, it may be running for less time than a lower one. And, although the COP will be reduced by higher flow temperatures, this may be partly counteracted by the fact that when the ASHP is running at low output, the it still has a background consumption (pumps, controller, etc.) that won't change. I can see this effect in my numbers; low outputs don't have the best COP.
Of course,it would be better if the system/software/sensors controlled all of this and I'm sure they could do it a lot better that I can. My next experiment will be to try and get the Mitsubishi room temperature control working and see what that looks like.
Imagine if heating systems had the same sensors, controls, automation and efficiency of modern cars. Things would be very different. If cars were like heating we'd still be driving a 1970 Ford Escort with points and a manual choke.
