No significant difference. COP is largely determined by the source temperature (i.e. the outside air). There is some influence on the size of the condensers and evaporators and the type of compressor used but for current technology there should be little difference between the two. Improvements in COP over the years have come from this area but these improvements apply to both types. The main advantage of air-to-air is that it is more straightforward to also provide air conditioning (this can be done with air-to-water and water-to-water but requires additional hardware (fan coil units). If you need dual functionality, consider the highly efficient "mini-split" units - these have a single outdoor unit and multiple indoor units and currently have the highest performance of any air-sourced heatpumps (mainly due to the type of compressor used).

Paul in Montreal.

Posted By: MarkHPerhaps the volumes of air and location of vents to rooms are very different between MHRV and warm air heating system?

That's exactly it. We have one house with both HRV and a water-to-air heatpump: the latter requires 1600 cubic feet per minute, the former is only 200 CFM on "high speed". [ 755 l/s versus 94 l/s ] The main ducts for the heatpump are 24x10 inches and the HRV ducting is merely 6".

Paul in Montreal.

Could it be that air-> water requires a higher output temperature then air ->air. So a bigger compressor.
Frank

MarkH,

I'm afraid your conclusions don't make any sense.

HRV and air-to-air heat pumps can coexist quite happily and, in fact, are the norm over here in North America where air-based heat distribution is used in >95% of cases due to the ability to supply cooling/dehumidification as well as heating. The misconception you're having is that you have to use the same duct work for heat supply and ventilation; this is not the case.

The usual method of combining the two is to have dedicated duct work properly sized for the heating/cooling system (i.e. to be able to supply the 700l/s or whatever is needed) and to have a separate system for the HRV extraction side - usually 6" ductwork or the like. The supply side of the HRV then uses the return air path of the main heating system's duct work. See pages 4 and 7 of http://www.vanee.ca/literature/install/bronze_Install.pdf for example.

Paul in Montreal.

Paul,

Thanks - I'd only been thing about "mini split" ASHP in conjunction with MHRV But since I'd thought of a small ASHP with MHRV I don't know why I did't ask if larger units work oops...

From your link it suggests that registers (vents?) are best in ceilings or high up on walls - I hadn't expected that since I though the heated air would need to enter rooms at or near floor level. This could potentially simplify fitting in a renovation project, when MHRV is already being consdiered (what I'm thinking of doing). Then again I could have missed something else....

Posted By: DantenzGiven the drop off in ground temperatures during the first 3 or 4 years of running a gshp (already discussed on this forum), IMHO you will achieve a very comparitive COP from a ashp.

Since when was this a given? Care to cite any studies that back up this claim? The biggest killer of the COP of an ASHP is the need to defrost the outdoor unit when the temperature is below about 7C as the coils freeze up. If your outdoor temperature never goes below this then you are better off with an ASHP as the COP is about the same as a GSHP for heating, though, of course, the ASHP has a far worse COP for cooling than a GSHP.

As for register/vent locations, in systems that are used for both heating and cooling (as is the case over here in North America), either location works well. For cooling, ceiling mounted vents are better since cool air falls but a well designed system should work just as well either way. We have floor mounted registers downstairs and ceiling mounted upstairs in the 110 year old house - works well for both heating and cooling. In the new house, they're all floor mounted (though the HRV exhausts in the new house are all in the ceilings).

Paul in Montreal.

Is this for horizontal or vertical closed loop systems? I know of some commercial systems in the southern US where the ground has heated up over time because the ground loops were undersized but I don't know of any cases where the ground has cooled over time (not saying there aren't any as it's perfectly possible to incorrectly size the exchanger such that the rate of extraction is greater than the ground can support - but it will come to equilibrium at some point even in those cases).

Paul.

Posted By: Dantenz. In many cases the source temperature from a vertical loop, midway through winter here in the Uk is approximately 2-3'C falling to around O'C at the end of the winter season. Again, I know these temperatures can be considered normal for a gshp however, in view of this, I refer back to my comment that an ashp for the Uk (perhaps not Scotland) returns a similar or even better (summer hot water heating) COP to that of a gshp.

Those temperatures seem very low for a climate where the deep ground temperature should be at 11C or thereabouts. It sounds to me like the systems are seriously underlooped (i.e. too short). I agree that there is a cost involved in putting in the correct sized ground heat exchanger but there's a bigger cost of underlooping and having to use backup resistance heating to make up the supply in the depths of winter. An ASHP should work well in most of the UK, though it's the falling output with temperature that's the biggest problem in winter, not so much the drop-off in COP. How are vertical loops designed in the UK? Over here, there are various programs in use to calculate the size of ground exchanger needed, one of them being gs2000, others being supplied by the manufacturers themselves (such as the ClimateMaster GeoDesigner software - http://www.climatemaster.com/index/geodesigner )
A typical "rule of thumb" is to use 13m of vertical borehole per kW of required output. Of course, like all rules of thumb, it's no substitute for a proper calculation but it does give a first order approximation.

Paul in Montreal.

p.s. I don't have a flow/return temperature sensor on my own system but a friend does - even by the end of winter his flow temperature is around 4C (this is with 6.5C annual average deep ground temperature).

Posted By: DantenzAs you are probably aware, heat pumps tend to be sized to meet 75-85% of the peak heating load at design conditions which, invariably means that when temperatures are low the heat pump will endure long running times. It can be a result of these extended running periods that the ground does not get "sufficient rest" for temperature recovery and, there are arguments to suggest that the heat pump should be upsized so that running periods are shorter.

Over here the same rules are normally used, but that's more because the heat pumps are also used for cooling where sizing to meet 100% of the heating load will result in gross oversizing for cooling and the problems that causes (poor humidity removal due to short cycling). In the depths of winter my system will run 24 hours a day constantly but that doesn't seem to affect performance too much. When temperatures go below design conditions (which they do of course) the auxiliary backup resistance heat is engaged. I'm interested to hear what's used in the UK for these cases since, as far as I understand, virtually no-one uses reversible pumps that provide cooling (so there is no danger of poor cooling performance on oversizing). Upsizing equipment is a delicate balancing act at the best of times between increased capital cost versus lower overall running cost. Oversizing ground loops does increase the COP a bit but the payback is very long due to the cost of drilling.

Can I inquire as to what the typical drilling cost is per metre for a vertical system in the UK? I know this is not necessarily an easy question to answer as there are many variables. Also, what is the typical output capacity required? I would have thought it would be a lot less than over here, but GSHPs here are not typically used to provide domestic hot water (just to preheat) so there may be some interesting nuances between the UK and Canada. Just for my point of reference one system we installed was Can$45 per metre and the other was Can$55 (more remote location). Output of the 1st system is about 11kW with 128m vertical bore and the other is about 14kW with 152m bore (both less than the rule of thumb incidentally - but rock conditions said this was OK).

Paul in Montreal.

Paul, to reply to your questions:
Most heat pumps here, that's both gshp & ashp utilise a built in electric heater for any "boost" in output when the balance point of the unit is exceeded and, as you suggest, in the main, very few heat pumps in the Uk are used for cooling.
Drilling costs still remain high although, as more drilling contractors move towards the market growth of geothermal drilling over water well drilling the prices are falling fast. Typically, the price range is from £50 to £75 per metre. Drilling into a dense hard rock such as granite will yield about 70W/m i.e. 100m borehole = 7kW however, a soft dry chalk may only provide just over half this amount of energy. These output's are based on a single loop probe and where double loop probes are used (4 pipes down one borehole) these can provide approx 15% more energy. My experiences suggest that geothermal borehole drilling in the UK. is not at all straightforward and can be fraught with problems. Regardless of whatever sizing program is used to determine the depth of the vertical loop, I would seriously recommend that a full geological prognosis report be undertaken to determine exactly what the strata is. From this the average Lambda value (thermal conductivity) can be calculated.