Water to Water [W2W]HP vs Air to Water [A2W] HP

Calahonda52

Friend has existing W2W, open loop system in a nursing home.

"Consultant" has suggested ripping it all out, as it is there 10 years and replacing with a A2W HP, "which will be way more efficient."

He has no idea what the energy cost of the existing W2W system is.

My layman's instinct is that since
the thermal conductivity of water is 0.58 vs 0.024 for air
and
the specific heat capacity of water is 4.2 vs 0.7 for air

That the W2W is a "better system"

Would welcome any insights..

The simple math is what energy is being used now to produce X amount of heat with the W2W and if the new system candy it for less, then....

Thanks as always

al2009

Is the existing system performing? If it is, I can't see payback on capital cost of new outdoor, possible modifying of plumbing. Get a meter on the existing unit, check what it is costing to run, "consultant" should be able to provide some figures for running cost of new.

antoinolachtnai

al2009 wrote: »

Is the existing system performing? If it is, I can't see payback on capital cost of new outdoor, possible modifying of plumbing. Get a meter on the existing unit, check what it is costing to run, "consultant" should be able to provide some figures for running cost of new.

As I understand it, the answer is 'it depends'. If the air is warmer than the ground then the air to water is better. If the ground is warmer than the air then the water to water is better. So it depends on the day. This is just in theory of course.

A nursing home is sort of a special case because it requires an elevated temperature all year round. But on the coldest days, the theory is that water to water should be best, because the ground still has some warmth even on a pretty cold day.

If the heat provided is sufficient I can't see the sense in replacing it unless it is woefully inefficient. If you were replacing it I think there are combined systems available that do both depending on temperature.

If it's not adequate you might be able to supplement the heating with an air-air heat pump in a few large rooms?

It might be worth spending a few bob on meters to determine actual consumption.

Condenser

Calahonda52 wrote: »

Friend has existing W2W, open loop system in a nursing home.

"Consultant" has suggested ripping it all out, as it is there 10 years and replacing with a A2W HP, "which will be way more efficient."

He has no idea what the energy cost of the existing W2W system is.

My layman's instinct is that since
the thermal conductivity of water is 0.58 vs 0.024 for air
and
the specific heat capacity of water is 4.2 vs 0.7 for air

That the W2W is a "better system"

Would welcome any insights..

The simple math is what energy is being used now to produce X amount of heat with the W2W and if the new system candy it for less, then....

Thanks as always

The water to water system will be vastly much efficient pretty much year round due to the high source temperature. But neither system will be efficient if the distribution system is badly designed and unfortunately even today this is most often the case.

A ten year old w2w system is not old and could last up to 30yrs but again thats dependent on how the entire system was designed. What your friend needs is to get someone in who truly knows about heat pumps (not your local plumber) and how they should operate to review the system and make suggestions on how to improve it. It is most likely installed incorrectly from day one and therefore never had a chance at a return on investment.

Calahonda52

Condenser wrote: »

The water to water system will be vastly much efficient pretty much year round due to the high source temperature. But neither system will be efficient if the distribution system is badly designed and unfortunately even today this is most often the case.

A ten year old w2w system is not old and could last up to 30yrs but again thats dependent on how the entire system was designed. What your friend needs is to get someone in who truly knows about heat pumps (not your local plumber) and how they should operate to review the system and make suggestions on how to improve it. It is most likely installed incorrectly from day one and therefore never had a chance at a return on investment.

Thanks for two replies to this, forgot to follow thread: Doh:eek: Have now done so

Anyway was at the SEAI show today and learned that having the correct delta T on the flow and return on a W2W HP is critical.
It was suggested that some units work best on a Delta T of 4 degrees C, give a year-round COP of 5.7.

The Delta T on the system I am looking at is currently 7 [12flow 5 return] and the guy I met today says if it goes to 8 it will shut down....

Anyone got any " Fundamentals of W2W HP design" on the e-shelf:D
Thanks as always

Condenser

Calahonda52 wrote: »

Thanks for two replies to this, forgot to follow thread: Doh:eek: Have now done so

Anyway was at the SEAI show today and learned that having the correct delta T on the flow and return on a W2W HP is critical.
It was suggested that some units work best on a Delta T of 4 degrees C, give a year-round COP of 5.7.

The Delta T on the system I am looking at is currently 7 [12flow 5 return] and the guy I met today says if it goes to 8 it will shut down....

Anyone got any " Fundamentals of W2W HP design" on the e-shelf:D
Thanks as always

Whoever you were talking to was talking through his hoop. Yes ideally you delta t would be 3 to 4k but to claim a year round COP of 5.7 is arbitrary to say the least.

COP will change from hour to hour. As I said above it is dependent not only on your collector but also what temperature you are rejecting the energy at. This is true of all heat pumps. The less work that needs to be done to bring the energy from source temp to distributed temp the more efficient the heat pump. A heatpump collecting energy at 10c and rejecting at 27c could have a COP of 8 or above.
Same heat pump gathering energy at 4c and rejecting at 55c might only have a COP of 3 or less.

Same rules apply to all heat pumps despite what some manufacturers would try to have you believe. Some manufacturers are better at optimising machines than others but mainly most machines fall down on installation.

You need someone who knows these principles inside out to review the system. You won't get that from a plumber or energy consultant.

Calahonda52

Thanks for this.
I understand the principles of the heat pump, I was just wondering was there anything different about W2W and this narrow delta T band as opposed to the wider ones generally seen in the A2W snake oil dept! SIZE="1"]In at minus 10 , out at 65 and so efficient that it puts lecy back to the grid![/SIZE

Condenser

Calahonda52 wrote: »

Thanks for this.
I understand the principles of the heat pump, I was just wondering was there anything different about W2W and this narrow delta T band as opposed to the wider ones generally seen in the A2W snake oil dept! SIZE="1"]In at minus 10 , out at 65 and so efficient that it puts lecy back to the grid![/SIZE

No all units work pretty much on the same principle. The only difference is how the energy is collected and the requirement of an a2w to defrost which in itself makes it less efficient before you go near COP reduction in line with air temp.

3 to 4k is the optimum but your bills won't double if it goes to 8k. The main problem is that the incoming water is unlikely to be above 11c and so is running the risk of freezing.

Calahonda52

Condenser wrote: »

No all units work pretty much on the same principle. The only difference is how the energy is collected and the requirement of an a2w to defrost which in itself makes it less efficient before you go near COP reduction in line with air temp.

3 to 4k is the optimum but your bills won't double if it goes to 8k. The main problem is that the incoming water is unlikely to be above 11c and so is running the risk of freezing.

Thanks, just to make sure I follow:
if the input/flow temp is too low and the W2W HP creates a Delta T that is too high, the return could freeze...
Having said that would not expect the flow temperature in an open loop ground source well to change much...am I wrong?

Condenser

Calahonda52 wrote: »

Thanks, just to make sure I follow:
if the input/flow temp is too low and the W2W HP creates a Delta T that is too high, the return could freeze...
Having said that would not expect the flow temperature in an open loop ground source well to change much...am I wrong?

Just like the reason the warning light in your car comes on for icy roads at 4c, water can begin to crystalize below this temp. So if your water temp was 11k and your delta t 9k your outlet water will be 2c and therefore the heat exchanger could ice and block.

Water temp in a well wont change much provided there isnt a large ingress of surface water near by. Its rare but can happen.
Also, sometimes drilling companies drill the dump well upstream of the extraction well meaning the cold dump water short cycles back down to the extract cooling the water being extracted.

Calahonda52

Just to wrap this up: got the full manual from the manufacturer and it covers a lot of the design issues kicked around here, especially by Evaporator, sorry Condenser, getting mixed up now:D
Thanks for your insights