@newhouse87

Posted by: @newhouse87

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Essentially its the delta t causing my pump cycling, so i need to alter curve so rwt isnt getting so close to lwt. That indicates to me that lowering my weather curve has caused this. as i dont recall it happening before. The pump does slow flow rate which should increase delta t but when it doesn't get to the 5deg it shuts off for a while. As you said the milder weather today doesn't help but it seems trying to achieve the lowest lwt is also causing cycling. I will bring back up the curve and see how it acts as i presume its not possible to lower curve anymore as it would only increase cycling

Yes and no (but mostly no).  There is a point where cycling is inevitable because the heat pump simply cant turn down low enough to match the demand from the house.  Yesterday it was fairly mild so its quite likely that point has been reached, now you have warmed the slab up.  There is no point in adjusting the WC curve to minimise cycling, you need to adjust the WC curve to get the house at the right temperature.  However as @derek-m says this is a slow process, thing weeks (in reality a whole 'season') to complete not days, and remember that the weather keeps changing so its best to try to do it when the weather is fairly stable.  

Once the WC curve is adjusted to get the house at the right temperature cycling, will or will not occur depending on the outdoor temperature and your heat pump's modulation capability.  As long as its not short cycling (10mins or less) don't worry about it, because you cant really do anything about it.  The house demand is what it is and the pumps minimum output is what it is and if the second is higher than the first it will cycle. This will inevitably occur in some circumstances, unless your pump has been chosen so that it barely meets peek demand,

In short, chill a bit, you tell us your house is warm, that's a good start.  As long as it isn't using enormous amounts of energy (which you really cant judge until it gets colder) then that's actually good enough.  Tweaking for maximum efficiency and comfort can come with time.

Posted by: @newhouse87

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@derek-m Essentially its the delta t causing my pump cycling, so i need to alter curve so rwt isnt getting so close to lwt. That indicates to me that lowering my weather curve has caused this. as i dont recall it happening before. The pump does slow flow rate which should increase delta t but when it doesn't get to the 5deg it shuts off for a while. As you said the milder weather today doesn't help but it seems trying to achieve the lowest lwt is also causing cycling. I will bring back up the curve and see how it acts as i presume its not possible to lower curve anymore as it would only increase cycling.

As James has correctly pointed out, the WC curve should be adjusted to try to keep the IAT reasonably constant at the desired temperature. In milder weather conditions cycling may occur, which actually could be a good thing, since it will mean that you are probably using less energy.

Cycling is pretty much always going to happen during mild weather.  Heatpumps tend to only modulate down to 30% or so of their peak capability - and the peak might not even be available when it's cold, so it might be in practice not as good as that if it's oversized.  So if your heatpump is intended to raise your indoor temperature by 18C (over background levels due to people, elec stuff, solar etc), then when it's asked to give a 6C or lower difference, it just has to cycle.  Sure you can make it cycle quicker or slower with controls and a water buffer - and it's usually considered that so long as this cycling is <6 per hour, it's generally considered ok.  If it cycles more frequently than this, then copper brazed pipes near the compressor might fatigue fail unduly quickly.  This advice is old mind - it may not hold so true with modern inverter stuff which might start up less abruptly.

@jamespa yeah man, house warm which is main thing, again quite mild today so sooner colder weather here the better haha. Its just something i haven't seen before but was obviously happening some bit. As you can tell i query alot of things. But al info here much appreciated. Its amazing how little installers tell you.

Checked my cop for yesterday, was pleasantly surprised at 5.05. Way above last winter, wasn't coldest day yesterday but things def seem to have improved for cop aspect anyway.

We're at over 6 some days, with low flow temps between 23 and 25 - happy days!

@rod yeah thats top class man, if you read back here, im having issues getting my flow temps much lower due to cycling/heat demand but i hope when weather gets colder i can experiment bit more and get them lower.

Following some of John Cantor's videos 

explains why cycling gently (2-3 times/hr) is actually just creating an average lower flow temp. Our pump cuts out when flow reaches 5c above the set temp and the flow then falls back again, particularly when the circulation pumps keep running when the HP is off. It's worth setting the flow temp lower, each degree C lower just extends out the pause between the cycles as the flow temp slowly drops back. On ours we can go as low as 23C against full system volume and still get a couple of cycles an hour, but at 22C it takes forever for the flow temp to get back down from 5C above - 27C - to 22C, because the floors inside are around 21C anyway.

With flow temp at 23 the average temp of the water through the cycles is probably about 25, house if really warm and COP can be 6+ so it's cheap to do.

@rod think lowest i got to was 26@10deg outside. In colder weather will try bit lower, still a lot lower then installer had it originally set to. Approx 12deg lower difference in my curve now. My issue was delta t was being reduced too much between lwt and rwt so slightly different to your scenario i think.

When rwt is same or slightly higher then lwt, does that mean floor/house has taken as much energy out of water as possible? Iv set curve lowest  so far 30@-5/25@20, house is nice at 21/22 in living areas and i dont seem to be using much electricity and its coldish today, approx degreee now10.Pump is obviously not on continuously due to delta t so is this continuous really necessary to achieve.

Posted by: @newhouse87

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When rwt is same or slightly higher then lwt, does that mean floor/house has taken as much energy out of water as possible? Iv set curve lowest  so far 30@-5/25@20, house is nice at 21/22 in living areas and i dont seem to be using much electricity and its coldish today, approx degreee now10.Pump is obviously not on continuously due to delta t so is this continuous really necessary to achieve.

If the heat pump has stopped, but the water pump is still running, then the LWT and RWT should be approximately the same. Some thermal energy will still be flowing from the UFH into the slab and hence your home whilst there is a temperature difference, but of course the water will cool since the heat pump is no longer providing thermal energy to keep the LWT at the required level.

The heat pump will only need to run continuously, if the thermal energy demand is greater than that provided by the heat pump when operating at minimum continuous output. If the heat pump therefore provides more thermal energy than that being absorbed by the UFH, the RWT will start to increase which in turn will push up the LWT, until the LWT reaches its upper limit (normally 5C higher than the required LWT) at which point the heat pump will stop until the LWT falls to the lower limit.

Posted by: @derek-m

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Posted by: @newhouse87

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When rwt is same or slightly higher then lwt, does that mean floor/house has taken as much energy out of water as possible? Iv set curve lowest  so far 30@-5/25@20, house is nice at 21/22 in living areas and i dont seem to be using much electricity and its coldish today, approx degreee now10.Pump is obviously not on continuously due to delta t so is this continuous really necessary to achieve.

If the heat pump has stopped, but the water pump is still running, then the LWT and RWT should be approximately the same. Some thermal energy will still be flowing from the UFH into the slab and hence your home whilst there is a temperature difference, but of course the water will cool since the heat pump is no longer providing thermal energy to keep the LWT at the required level.

The heat pump will only need to run continuously, if the thermal energy demand is greater than that provided by the heat pump when operating at minimum continuous output. If the heat pump therefore provides more thermal energy than that being absorbed by the UFH, the RWT will start to increase which in turn will push up the LWT, until the LWT reaches its upper limit (normally 5C higher than the required LWT) at which point the heat pump will stop until the LWT falls to the lower limit.

 

So i think only way i could get it to continuously run would be by having a higher lwt and higher heat demand which i dont need as house is well insulated. Another way i could get it to run continuous would be to leave house get cold over few days or longer which would mean slab my rwt would be lot lower then it is now. RWT seems to be at 23-25 always. 

The only worry is how do i know if its cycling too much, i dont have monitor for that and im not going to stand at unit for an hour to see. When ye say cycling, ye mean heat pump stopping flow and starting up again essentially?

Just there, flow stopped for 5 minutes and now back on, the compressor however is off, is it only cycling if compressor on and off?