Heat pump supply and return temperatures

[TSN79]

A building is to be heated by radiators using an air-water heat pump. While radiators are not the ideal choice, it's beyond my decision making. Naturally I want the supply temp. to be as high as possible, hoping for 55°C in order to keep the physical size of the radiators to a minimum.

As low return temperature supposedly is what every heat pump wants, the return water temperature can be reduced even more by pre-heating tap water through a heat-exchanger before returning to the heat pump. What I wonder is, will this additional reduction in temperature spell doom for my 55°C supply? Is there a maximum temperature difference between the supply and return?

 

[Travis_King]

You want the return water to be as low a temperature as possible because you want: a) your radiators to be functioning efficiently and heating your building, and b) you want your heat pump to be functioning efficiently and not pumping in unnecessary heat to the system.

By that I mean, if you put X amount of energy into the system and the radiators dissipate Y (a number significantly less than X) then your system is not operating efficiently and you are wasting money by having heated that water up in the first place.

Then there's the issue that if you return water is too hot, two things may happen: a) your heat pump will not have the delta-T required in order to heat the water to the proper temperature, and if it does, it will be operating extremely inefficiently, or b) the heat pump will do it's job, but intstead of having, say, 60 C water to the radiators, you get 80 C water, and so on.

The heat pump acts by adding energy to the system, it has a limit on how much it can add, so yes there is a maximum delta-T which will change based on system conditions.

 

[TSN79]

You want the return water to be as low a temperature as possible because you want: a) your radiators to be functioning efficiently and heating your building

But isn't a high supply temperature and a low delta T to radiators a good thing? For instance, if the supply temp. is 80C and the return is 70C, the radiator surface would be 75C. If the supply and return temperatures were to be 55/35, a given radiators' Watts would drop significantly, and would have to be physically larger to make up for it - which is of course bad.

 

[russ_watters]

I haven't done much with waterside heat pumps, but typically there are flow and delta-T limitations on refrigerant systems. Minimum flow rates are common, then a maximum delta-T follows based on the capacity. In addition, there is often a low temperature limit on the incoming water. And, of course, you'll need to account for the extra load of the domestic water heating in the total capacity of the heat pump.

All of this will be discussed in the vendor's literature (engineering literature, not sales literature) and/or can be answered by a good sales engineer. That's what sales engineers are for.

 

[russ_watters]

But isn't a high supply temperature and a low delta T to radiators a good thing? For instance, if the supply temp. is 80C and the return is 70C, the radiator surface would be 75C. If the supply and return temperatures were to be 55/35, a given radiators' Watts would drop significantly, and would have to be physically larger to make up for it - which is of course bad.

Define "good". Physically larger, with higher delta-t means more efficient. And if you have control valves to vary output, it will vary anyway, providing better efficiency at part load.

 

[philrainey]

Unless you do the final part of the water heating by desuperheating the refrigerant in the heat pump I would think there would be limited efficiency gain making the return water temperature as low as possible as the lowest condensing temperature still has to be above that of the supply water to get the heat to flow from the condensing refrigerant to the water otherwise the refrigerant won't condense ( it will the condensing pressure/temperature will go up till the system balances) There is only so much high temp discharge temperature (from the compressor) superheat that the refrigerant will have. You would have to plot the refrigerant cycle on a (sorry can't spell PF won't like me) pressure/enalthpy chart to see how much.

 

[philrainey]

What I'm saying is it is the supply temp, and the air temp. where it is getting its heat from, which governs the limit on the max possible efficiency of the heatpump. However how the way heat pump is engineered may place additional limits . Real outputs can be gained from sales technical information. A heat pump is able to heat from a low return to relatively high supply, one just reduces the water flow to get it at the expense of reduced total output from the heatpump and reduced efficiency of the heat pump plus there is a limit on the temp. the heat pump can to heat for most standard heat pumps where the life of the heatpump is not reduced.
New inverter heatpumps where the compressor is slowed down at low loads means that there is lower temp. differences between the air and refrigerant, and the refrigerant and water creating a lowest temp. distance that the heat pump has to pump the heat making it more efficient.

 

[philrainey]

Maybe we should make systems where refrigerant condenses in a radiator as most split heat pumps (a high wall) have a air off temp. at full throttle mostly at 50 degrees C (assuming a temp. in the room of 23 degrees C)so the refrigerant is condensing hotter about 60 degrees C and you could have the whole radiator at that temp. But one would have to pipe refrigerant all around the place with potential leaks developing. However they pipe refrigerant all around now in complicated variable refrigerant flow air conditioning systems. Be a nicer heat than the pure convective heat from a high wall.