All units I've seen are rated at a higher BTU capacity for heating mode as compared with cooling mode. This doesn't make sense to me. It would seem that since the difference in temperature between the water and freon and the air and freon in cooling mode is greater than it is in heating mode so more BTU's would get transferred. Is there something in the system that changes the way it works in each mode?
heat of the compressor motor The motor is going to add some heat in heating mode and its going to distract from the cooling mode.
Correct System power consumption works FOR you in heating mode...Total heating = heat extracted from loop + power used by system. System power consumption works AGAINST you in cooling mode...Total cooling = Heat rejected to loop - power used by system. At a COP of 4, system power roughly equals 1/4 of total heat transferred, so the effect is significant It's a bit like flying - ground speed is much improved flying with a tailwind (heating mode), and is slowed by flying into a headwind (cooling mode)
Dewayne, Check the fine print in the tables you are reading. For the major manufacturers the published ratings are based on testing using ISO 13256 standards substituting water for brine. The ISO calls for different EWT temps in heating as opposed to cooling. In terms of sizing the equipment it makes sense to consider capacity(performance) at the conditions your system will actually operate under. As urthbuoy eluded to, heating and cooling capacity will merge as the operating EWT merges. Give me a call if you want to discuss in depth.
I think that if you account for differences in water vs air temp (The thermodynamic lift the heat pump must provide) you'll see the heat transfer rates equalize, with COP rising by 1.0 for heating vs cooling across similar temp differences You have to look deeper than headline ratings, some of which favor cooling: For instance, the open loop rating for cooling mode has 80*F EAT and 59*F EWT (-21* lift) - that cooling, absent concerns about latent load, could just about be met with just a pump, hydronic coil and blower - no compressor at all. OTOH, the ground loop heating rating incorporates 32*F EWT and 68 EAT - +36*F of lift, net of Delta-Ts across both heat exchangers, which likely combine with average air and water temps within the unit (perhaps 27 and 80) to make the lift 50+ From a lift point of view heating with 32 EWT is like cooling with 116 EWT...look up the hit a unit takes in capacity when cooling with 116 EWT.
Glad to have been useful It's hard enough to get folks on board with Manuals J and D. What we've been discussing is essentially Manual S, so when I'm heard up on that soapbox, this is why. If memory serves Joe A has a rule-of-the-digit-opposing-one's-fingers (since we don't use rules-of-thum*) that figures on just 10kBtuh per nominal ton in heating. When a ton ain't a ton, blame Manual S. To paraphrase, a kBtuh here, A kBtuh there, and pretty soon you come up short...
