@editor Thanks for the reply.  I agree .. I’m good at math and reason .. and the mass of the slab greatly exceeds the mass of a buffer.  As long as my pipework runs free I should be good .. and hydraulic separation should not be needed.  The Arctic HP does have ambient temperature compensation curves built in .. so it looks like I can set the HP output temperatures using the curves.  That said, the system is designed to turn on/off based on buffer temperature .. so I’m not sure if the system will be ok with me using the home thermostats to turn it on/off.  I may have to just try it to find out.  It does seem to have inputs that appear to enable external on/off control. 

On a separate note .. my dealer also suggested that a buffer helps for space cooling .. and that in Europe you guys don’t use a/w heat pumps for cooling.  This is news to me if so.  I plan to run cool water into the water to air exchanger on my furnace air handler … I can’t see this being an issue either.  I think the issue is we are just early in the tech here in Canada and maybe many industry people just don’t understand the physics?  I think I’ll study the wiring and controls a bit more and just experiment.  I’m fine trying and failing .. it is how we learn.  I can always add abuffer and rewire it as needed.

Lastly .. my dealer also suggested R290 was not as good at -30C vs R32.  Literally everything I have read says R290 outperforms R32 in the cold .. ie more efficient, accepts a larger temperature delta, and offers higher output temperatures.  That said, I have not seen an R290 HP rated to -30C .. just -25C .. while I have seen R32 units rated to -30C.  Any comments on this point? 

Posted by: @tony-stolz

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On a separate note .. my dealer also suggested that a buffer helps for space cooling .. and that in Europe you guys don’t use a/w heat pumps for cooling.  This is news to me if so.  I plan to run cool water into the water to air exchanger on my furnace air handler … I can’t see this being an issue either.  

On one hand, if the exchanger, ducts and air flow rate are sized for furnace-temperature water, they may be undersized for heat pump temperature water, especially for cooling. On the other hand, considering your climate, it's probably designed for a quite substantial heating power, so it may not be that undersized. You'd have to check.

The system is also probably not built to handle condensation on the exchanger, which will occur when cooling. You'd have to check. If it is built out of copper/plastic/aluminium/stainless why not, but if it has any kind of exposed steel that isn't stainless, then that will rust. Also if you blow very cold air through your ducts you'll get condensation on the outside of the ducts, if they're insulated with fiberglass it'll get soaked. 

In this case you'll have to use water flow temperature above the dew point to make sure condensation does not occur, ie 17-18°C water, and that won't have much cooling power. Still better than nothing though.

That said you can also send water at this temperature into your slab to cool it. If you have a low thermal mass house (ie, wood framing) with all the thermal mass in the slab,  then when you open the windows at night to let cool air in, it'll take a while to cool the slab. Keeping the slab cool using the heat pump means keeping most of your thermal mass cool, so when you open the windows at night, inside temperature drops much faster, which is nice.

Then you add a fan in the room to increase slab/air heat exchange. It's not as powerful as real AC, and it won't dehumdify the air, for this you need air heat exchangers built to handle condensation (ie, fan coils) and properly insulated piping (vapor tight insulation sleeves).

About the buffer tank helping for space cooling, I wonder what that means. When cooling, the flow temperature window is tiny: if you have fan coils and a condensation proof system, you're still using water as heat transport medium, so you're stuck between 5°C and 7°C. Too hot, fan coils cooling power drops ; too cold, your heat pump exchanger may freeze. If you do not have a condensation proof system, then you want water just above the dew point, say 18°C. A few degrees less, it'll drip condensation, a few degrees more and it won't cool. There isn't one degree to waste here, it's even more critical than heating, so you must avoid mixing return water from the installation with flow water from the heat pump and the only way is direct flow without buffer. The system must also be built to handle the full flow rate and the heat pump internal controls and regulation must be smart enough to keep the flow temperature at the set point without oscillations. There's also the question of insulating a buffer tank against condensation.

Posted by: @tony-stolz

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 That said, the system is designed to turn on/off based on buffer temperature .. so I’m not sure if the system will be ok with me using the home thermostats to turn it on/off.  I may have to just try it to find out.  It does seem to have inputs that appear to enable external on/off control. 

I doubt you will want to use (or be successful with using) the home thermostats, it will probably lead to oscillation because the slab will take so long to respond. 

Also remember that turning off the thermostat won't stop the slab emitting stored heat (you essentially have a storage heater but without adjustable output flaps, so you need to get the amount stored right in advance!)

More likely just use weather compensation.  Something perhaps to experiment with but I would definitely start with weather compensation only and no thermostatic control

@bobflux I’ve been experimenting with the cooling loop for 2 years now .. running 7C well water through it to see if the exchanger could keep up with the AC demand (we have lots of gardens to water anyway .. so no water is wasted).  It works great and no condensation issues on the ducts.  The exchanger is rust proof and I designed in a drain for any condensation there .. so no issues.  Our air is super dry here (think desert) so we all run humidifiers.  BTW .. historically I ran the hydronic slab on timers since using air temp was too unreliable (oscillations) .. a slab temp sensor should eliminate that I think. 

Re thermostat control.  I planned to use a slab temperature sensor wired to a thermostat to control that loop at about 24C in winter (lower floor) and a separate regulator thermostat for the air exchanger loop (both floors).  Both would be wired to signal the HP to run on demand.  Heat rises and my air always circulates .. so the slab would provide most of the houses heat.  I get how the HP should modulate using just the temperature curve alone .. but don’t you still need a way to turn it on/off once you get outside of the demand zone? (Heat gain from windows and “life” often increases winter house temperature above 20C).  The HP manufacturer appears to have designed their system using buffer temperature for this purpose (turning the HP on/off) .. my experiment would use the “manual” on/off terminals on the HP for this purpose and I’d leave the buffer terminals open. 

Great news then! If your heat exchanger handles it, then it's going to work fine.

It needs to handle enough flow and suck enough thermal power from the water to allow the heat pump to run above its minimum power level and not cycle. The fan in the air handler can't be allowed to turn off, if it does the heat pump will cycle like crazy. In other words you can use a thermostat to control both heat pump and heat exchanger fan. If the heat pump has remote control or indoor temperature probe you can set the flow temperature according to your cooling needs.

I got a bunch of fan coils dirt cheap, I'm going to install them. So I'll need motorized valves and a bit of automation to send 18°C water to the slabs, and 7°C water to the fan coils, and set the heat pump flow temperature via Modbus. Not possible to do both at the same time, but slab cooling is good during the night, outside air is not hot so it works at fantastic efficiency, whereas air cooling is good during the day where photovoltaic electricity is free.

Note well water is oxygenated (so it'll corrode steel) and sometimes a bit acidic (so it'll corrode copper) so I hope it doesn't mix in your circuit...

To check the temperature inside the slab, I simply put a temperature probe on the return pipe from the slab. When the circulator is running, it gives good information about how much heat is stored inside the concrete, ie how much heat is going to come out in the next hours, so the control system (currently an ESP32) uses that. Basically it does weather compensation on the slab return temperature.

Thermostat is useful to stop the heat pump and avoid overheating if sunlight enters through the windows and warms the room. It depends on room solar gain so it depends on the building. If you have a lot of solar gain it's useful, otherwise meh. Thermostat as regulation for a slab... in control terms the slab has a ton of phase shift so if there's too much gain it will oscillate. Gain is simply deltaT between flow temperature and air. So if you heat the slab with 23°C water a thermostat can work. With 30-35°C water you get massive overshoot.

Posted by: @tony-stolz

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Re thermostat control.  I planned to use a slab temperature sensor wired to a thermostat to control that loop at about 24C in winter (lower floor) and a separate regulator thermostat for the air exchanger loop (both floors).  Both would be wired to signal the HP to run on demand.  

As @bobflux says this is unlikely to work.

Posted by: @tony-stolz

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 I get how the HP should modulate using just the temperature curve alone .. but don’t you still need a way to turn it on/off once you get outside of the demand zone? (

In principle no because, by adjusting the flow temp according to the outside temperature you are matching the heat input to the heat loss from the house.  I know its a bit counterintuitive, but the physics is sound and it works.  The variation in heat gain from windows and life is generally small compared to the house loss, other than at the ends of the season, so not a problem and more to the point generally much less of a problem that the phase shift in a control loop involving room temperature.

Posted by: @bobflux

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To check the temperature inside the slab, I simply put a temperature probe on the return pipe from the slab.

Surely your heat pump does this anyway.

@jamespa Thanks guys.  Just for fun .. I should also point out that since we are so cold here in the winter (it usually stays above -30C but we do occasionally get down to -40C), it means secondary heat is always needed.  Gas is the best choice in my area for use as a “peaker” under these conditions (since resistance electricity is not economical due to high capacity charges).  Also, many people, including me, also use fireplaces on cold days.  Personally, I burn about 3 cord/year .. mostly at temps below -15C.  

My design vision was to allow the gas furnace to run on its own thermostat set a couple of degrees cooler (ie 18C) than the HP target (20C) and simply allow the furnace to cut in whenever the HP couldn’t keep up.  I also planned to set the HP to cut out below -15 or -20C as COP becomes pretty poor below that anyway.  The fireplace can also heat the whole house and even the slab on its own (it is an interesting set up) .. so my HP design needs to accommodate that as well.  Fun eh 🙂 ?

Interesting, I'm also going to combine a hydronic fireplace and a heat pump. Currently the heat pump is installed, but the fireplace is still boxed in storage while I finish that damn drywall...

Some heat pumps have a "bivalent" dry contact output that means "It's too cold, I give up, turn on the boiler." You're supposed to wire that to the boiler, probably to its thermostat input. 

Note if it's too cold for the heat pump to run, something has to be done about the water freezing in the pipes.

@bobflux … yeah .. we have to run glycol here. The HP I’m looking at can signal a secondary system … so I may experiment with that as a control for sure.  The fireplace is trickier in a way since it is obviously manual on/off and heat output is variable .. I think a house thermostat with a high temperature shut off for the HP will pretty much be needed from that perspective alone.  I could use a sensor on the chimney to simply cut off the HP when a fire is burning I suppose.  I’m pretty sure I’m going to need some on/off control for the HP due to these secondary heating systems plus my heat gain in the winter really can be substantial.  Keep in mind our houses here are often really well insulated and airtight .. my attic has 18-24” of insulation! On a -15C day with full sun our house can easily reach 24C with no heat running! Its tricky! 

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Posted by: @tony-stolz

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My design vision was to allow the gas furnace to run on its own thermostat set a couple of degrees cooler (ie 18C) than the HP target (20C) and simply allow the furnace to cut in whenever the HP couldn’t keep up.  I also planned to set the HP to cut out below -15 or -20C as COP becomes pretty poor below that anyway.  The fireplace can also heat the whole house and even the slab on its own (it is an interesting set up) .. so my HP design needs to accommodate that as well.  Fun eh ?

Definitely.

If I were you I would read up a bit on the engineering topic of 'control theory'.  This deals with how you control real world systems which have inbuilt delays (in the case if a house these are enormous), noisy interference, nonlinearities, the possibility to control only in one direction (in this case up only) and other embuggerations.  The maths is horrendous and well beyond me even when I was at uni, but some understanding of the basic principles, even if you don't go beyond the effects of delay (what @bobflux calls phase shift) and the basic principles of pid, will definitely help. 

I think you may have a simplistic view that measuring and controlling based on the desired output (in this case room temperature) is guaranteed to work.  Unfortunately, even though intuitively this the true, it is not true in fact.

Perfect is the enemy of good.  I can’t perfectly control some of the “overshoot” in house temperature .. ie due to sunshine and / or burning too much firewood / and/or rapid ambient temperature changes from chinooks … and I’m not worried about that either.  A few tics in house temp higher or lower isn’t a big deal .. a first world problem really. 

The tricky one is the hydronic slab due to its much slower reaction time as you point out.  My strategy with it is to run it on the cool side during the heating season (about 21C) as at that temp it would act as a base load heat source for the house without any overshoot possible.  I would then rely on the fireplace and air handler loop (gas or HP) to raise the last few degrees to keep the space comfortable.  Sweaters also work great to even out some lumps in heating 😉 Our house is actually really stable regardless of ambient temperature due to how well insulated and tight it is .. where some houses drift materially within an hour, ours drifts over about 4 hrs if that makes sense .. even at -30C!

Posted by: @tony-stolz

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Perfect is the enemy of good.  I can’t perfectly control some of the “overshoot” in house temperature .. ie due to sunshine and / or burning too much firewood / and/or rapid ambient temperature changes from chinooks … and I’m not worried about that either.  A few tics in house temp higher or lower isn’t a big deal .. a first world problem really. 

How true, and an argument I am having elsewhere with those who want every ASHP installation to be totally bespoke, even at the end of the market where you are fitting a 5kW come what may because its the lowest rated one most manufacturers do.

Posted by: @tony-stolz

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Posted by: @tony-stolz

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The tricky one is the hydronic slab due to its much slower reaction time as you point out.  My strategy with it is to run it on the cool side during the heating season (about 21C) as at that temp it would act as a base load heat source for the house without any overshoot possible.  

That should work, although there is still the possibility of overshoot depending on the reaction time of the other components

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Our house is actually really stable regardless of ambient temperature due to how well insulated and tight it is .. where some houses drift materially within an hour, ours drifts over about 4 hrs if that makes sense .. even at -30C!

Ours is like that as well, despite being a 1930s originally solid wall (albeit partly upgraded) house.  The excursions due to delay will likely be slow, but nonetheless annoying.  As I say its house dependent.  Ours now runs (except at the very end of the season) on pure weather compensation with no thermostats or other room influence, much better than it ever ran with loads of TRVs etc.  Basically the only variable taken into account by any part of the control system is OAT.  Its not absolutely perfect, but its a whole lot better than it was previously