I have a Grant Aerona 290 15.5 kW with the Smart Controller.  I have radiators throughout the house and no underfloor heating.  I am running on Shift = 1 and Heating Curve = 0.8.  Obviously, the right numbers depend on how well the house is insulated.  

Our system was installed in August and I soon discovered that the water circulation pump was on all the time.  Since the heating was hardly on at all, this seemed to impose unnecessary wear on the pump and waste electricity.  In the summer, one solution is to reduce the temperature at which Winter Mode activates.  However, that is not appropriate in winter.

Grant recommends that Pump Blockade should be set to No, if you are using weather compensation.  In the colder months, I think this makes sense.  Ideally, the heat pump compressor should be running 24 hours per day, so the water pump must be designed to handle that level of work.  When the compressor turns off, the water is still hot, so it is good to keep it circulating to impart the maximum energy to the house.  It also enables the heat pump to measure the temperature of the water and thus to decide when to turn back on again.  

We are not using a set back temperature at night for a couple of reasons:

1. We are on the Intelligent Octopus Go tariff, which costs only 7p at night, compared to almost 30p during the day.  Therefore, it makes sense to make the house as warm as possible while the cost is a quarter of what it is during the day.
2. By running at the same temperature all day, we minimise the required flow temperature, which again keeps our costs down.

If you prefer your bedrooms to be colder, maybe you could use flow regulating valves or lock shield valves to reduce the water flow in those rooms?

I ran it last winter 24/7 with no stat. Feeling smug, I told Grant that for over 14 weeks my heat pump only cycled once. That was met with Whoaa! that could wear out the compressor, please let it cycle a few times per day. So I started to question compressors and pumps running 24/7 and the overall feedback I received was that its not a good idea. There should be a balance between efficiency and longevity.

I put the stat back because the WC still requires constant attention. This is despite using a combination of curves, circuit temp correction, and set point correction to fine tune it. The stat acts like Grant intended, a safety net if the internal temp climbs too high. For example, yesterday was a steady 5 degrees outside, and 21 inside. When the outside temp dropped to 3 degrees, my internal temp shot up to 23. If I fine tuned that out, then it would cause issues with the start of the day. Even Grant having looked at my Data concede that it will be an ongoing issue for me so the stat was put back.

If only I could find a way of having pump blockade on without it shutting down the 2-port valve.

@unsure you make an interesting point about longevity.  Most of what I have read and heard says that compressors are worn out most by starting up and not by continuous running.  Apparently, they are designed to handle so many thousand on/off cycles.  If you run a compressor for 24 hours per day and it stops once for 10 minutes per hour, then that reduces its run time by 16%, which is pretty marginal.  On the other hand, it adds 24 start stop cycles.   

Do you have any concrete evidence for the view that continuous compressor run time is a problem?

I don't quite follow your comment about weather compensation.  You said that you managed to run your compressor continuously for 14 weeks, which is very impressive.  You also said that you can't adjust weather compensation to keep a constant indoor temperature.  Does that mean that you spent 14 weeks continually tweaking it? 

My experience is that weather compensation cannot hold to an exact temperature, because of various factors that WC cannot detect - e.g. solar gain on a sunny day, closing the curtains at night, a gaggle of relatives arriving and leaving the front door wide open while they bring in all their luggage...

Having said that, I have found that weather compensation alone can probably keep the temperature within 1C of the target. It is harder when the outside temperature varies a lot, but I have adjusted the Shift and Heating Curve and I think I am fairly close now.  The key is to get the slope right and not just set the flow temperature at a specific outdoor temperature using Shift.

@grahamF

Posted by: @grahamf

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Do you have any concrete evidence for the view that continuous compressor run time is a problem?

Only the comments I've received from old school HVAC engineers in the states and nordic countries. There is a logic to what they're saying, but I'd be interested if any scientific evidence exists.

Posted by: @grahamf

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 Does that mean that you spent 14 weeks continually tweaking it? 

Yes ! My first heat pump experience during the winter. A kind of obsession creeps in with graph paper, calculations, and reaching heat pump nirvana. Then reality sets in. No work gets done around the house, waking in the night to tweak the EcoNET app whilst in bed annoys the partner, and you find a new mental health medical condition starts to emerge.....Heat Pump Mania.

Posted by: @grahamf

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My experience is that weather compensation cannot hold to an exact temperature, because of various factors that WC cannot detect - e.g. solar gain on a sunny day, closing the curtains at night, a gaggle of relatives arriving and leaving the front door wide open while they bring in all their luggage...

Having said that, I have found that weather compensation alone can probably keep the temperature within 1C of the target. It is harder when the outside temperature varies a lot, but I have adjusted the Shift and Heating Curve and I think I am fairly close now.  The key is to get the slope right and not just set the flow temperature at a specific outdoor temperature using Shift.

I came to the same conclusion as you, hence why I reinstated the stat to prevent temp runaway. You can fine tune the HC with 'Decreasing Fixed Water Temperature',  'Circuit Temp. Correction', and 'Set Point temp. Correction' in Service settings > 0000 > Main Heat Source and Circuit 1. It takes a lot of time and experimenting, but I did achieve some good results during Jan/ Feb this year. But once the temp swings during the day and the sun is strong it all goes to pot.

I'm confident that in the near future with AI and a few more sensors, we will be able to enter the room temperature and it will remain constant without have to touch the controls for at least a..................day 🙂

Posted by: @unsure

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the sun is strong it all goes to pot.

Posted by: @unsure

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I'm confident that in the near future with AI and a few more sensors, we will be able to enter the room temperature and it will remain constant without have to touch the controls for at least a..................day 🙂

 

Not unless either you allow the AI to turn on cooling when the sun is strong, or you give it control of the windows and curtains also!

No amount of fiddling with heating controls alone will deal with sunny days at the ends of the season when the potential solar gain well exceeds the loss yet there is still a big diurnal swing.  Perversely this problem gets worse for lower loss houses because there isnt the constant drag of heat escaping.

I have a Grant Aerona r290 9kw installed in a house with 10.03kW heat loss. It was designed with a target of 45C flow temp at -2.4 (capacity 10.47kW). When installed it had the weather compensation enabled, and a room thermostat set at 17C and setback of 15C 10pm-5am. 

Once the thermostat reaches 17C it turns the heat pump off because the blockade is on. The temperature then drops to 16.2 - 16.4C with an outside temp of 8C, before the room starts reheating, even with hysteresis set at 0.4C. Making the house too cold to be comfortable. I am concerned about reducing the hysteresis further in case of short cycling.

This is what lead me to look at potentially turning the the blockade off. I have only run it for 2 days currently. The house is pre 1900, single brick, four storeys (converted loft pre 1980, unconverted cellar), semi, mostly double glazed with minimal loft insulation. In essence a very leaky house. Temperature has been holding well, but the pump is running constantly.

Is the general consensus that the blockade on strategy is still better despite temperature swings?

@rhiannon Welcome to the forums and feel free to ask any questions.

The best way to run almost any heat pump is 24x7 with any thermostat set to say 25-30C so it has no effect, then adjust the weather compensation (generally down) to achieve the right house temperature.  This will generally give lowest operating cost and greatest comfort with minimal temperature swings.  All TRVs (or certainly most) should be fully open so they also have no effect.

You need to unlearn almost everything you learned about running a boiler (much of which was actually wrong for boilers as well).  

Here is a brief introduction to heat pumps and weather compensation.  Can I suggest you read this and then ask for some more clarification.

It will take you a couple of weeks to get the WC curve right (if necessary 'balancing' radiators as you go along).  However once done its pretty much fit and forget.  Mine is now so good I almost never touch the controls other than to find out what the outside temperature is (because I can no longer tell from the house temperature).  For almost the whole season I run on pure WC with no working TRVs/Thermostats, only at the extreme ends of the season do I switch to the nearest Vaillant equivalent of blockade mode, this is to deal with 'excessive' solar gain at a time when the OAT is also quite high so the loss relatively low.  It is of course a bit house dependent but for sure pure WC correctly adjusted is the place to start.

Loft insulation btw is the quickest payback home improvement you can do.

@jamespa Thanks for your help.

With your current set up does the compressor still cycle on and off or run continuously? Before I removed the blockade the pump cycled 5-10 times a day. Since turning it off, there has not been any cycling. Elsewhere in the thread there is mention that it would be working the compressor too hard if no cycling at all. That is where my confusion comes from on best way to run the heat pump.

Posted by: @rhiannon

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With your current set up does the compressor still cycle on and off or run continuously?

Depends on OAT.  Mine cycles at OAT>10 and of course has periodic defrosts at OAT<4.  Steady operation typically occurs between 4 and 10.  This is the expected behaviour for a heat pump the capacity of which is well matched to the house.  Above ~10 the heat pump must cycle because it cant modulate its output down enough to match the relatively low heat loss from the house.   For an oversized heat pump the no -cycling 'window' will be smaller.

Posted by: @rhiannon

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Elsewhere in the thread there is mention that it would be working the compressor too hard if no cycling at all.

SOFAIK there is no evidence at all to support this and nor is there any logical argument.  If its cycling at a demand when it need not cycle (ie a demand that is within its modulation capabilities) then you are working the compressor harder than you need to albeit for a shorter time.  No reason to suppose this would be less wear on the compressor, if anything more likely the opposite.  Heat pump compressors are designed for continuous operation because thats the best way to operate them.

Posted by: @rhiannon

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That is where my confusion comes from on best way to run the heat pump.

Low and slow.  Operate them at as lower flow temperature as you can consistent with heating the house.  That gives greatest efficiency (=lowest cost) and greatest comfort.  At more elevated OATs this will involve cycling to match (down to) the load, if the heat pump is well sized there will be a range such as that I experience where it will operate continuously.  Thats what the designers intended! Of course if you have a ToU tarrif you might want to tweak a bit, but thats only after you first get it working on WC and bedded in. 

Also control by thermostat is for many, perhaps most, houses fundamentally flawed.  The problem is that (as you have discovered), by the time it reacts, its too late to stop temperature swings.  Control by weather compensation effectively anticipates whats going to happen to the house due to changes in OAT, and adapts the settings accordingly and in good time.  There is both science and sound engineering behind this, which I will happily recite if you want me to, but in essence you can just trust it.

My strong advice is let the heat pump do its job by turning up your thermostats and TRVs so they have no effect, and turning down the WC until the house is comfortable (balancing the rads if necessary).  Done properly you may be surprised how much more comfortable your house is.  That said with poor loft insulation you are always going to get cold spots and be spending a lot more on heating than you need to.  I would fix that as soon as you can.  300mm is a good thickness to aim for.

PS Oddly enough boilers are also best operated the same way, but they are generally grossly oversized so its not possible and, for some almost inexplicable reason, we don't generally enable weather compensation on boilers in the UK.