Increase ΔT to 15K with ASHP

Alex09

 I am seeking your expertise to optimize some heating system’s secondary circuit to achieve a temperature difference (ΔT) of 15K after the buffer tank.

The goal is to maximize energy transport through a DN20 PEX pipe (16mm  internal diameter) for radiators, targeting 5-7 kW heat output with a new air source heat pump. 
Current Configuration:

1. DN20 PEX pipe ( in concrete).
2. Secondary circuit post-buffer tank
3. Buffer tank volume:80L.
4. Radiator type: K22

Key Requirements:

1. Increase ΔT to 15K (e.g., 50°C supply ).
   • Maintain 5-8 kW heat output through the DN20 PEX.
2. Challenges:
   • Ensuring sufficient flow rate and pipe capacity to achieve the target ΔT.
   • Buffer tank temperature settings or control strategies.
     

Your guidance will be invaluable to ensure system efficiency and reliability.

jesmed1

Just for clarification, it seems you are in Europe using degrees Celsius and Kelvin (K) instead of the Farenheit scale we're used to here in the US.

So when you say you want to increase the delta T by 15K, I assume you mean 15 degrees Kelvin. And since a change of one degree K equals a change of one degree Celsius, then here in the US we'd convert one step further to multiplying by 1.8 to get the delta T in Farenheit.

So a delta T of 15K = 15 x 1.8 = 27 degrees F (deltat T)

And your desired supply temp of 50C supply = (50 x 1.8) + 32= 122F

So your system is now supplying 122 - 27 = 95F water?

So you have:

Current supply temp = 95 F

Desired supply temp = 122 F

Desired delta T = 27 F

Does that sound correct? And since you're in a different heat pump market than us here in the US, you might mention what heat pumps you are considering. Obviously one consideration is going to be what water temp the heat pump is able to supply, as you'll need a heat pump supply water temp of more than 122 F in order to reach your desired delta T. I'm not a heating pro, but someone here who is will know what output temperature you'll need for the heat pump in order to get your supply up to 122 F through the appropriate heat exchanger.

Doing the conversion math from kWh to BTU/hr, you say you need to maintain a 5-8 kWh heat output, which converts to a range of roughly 17,000 - 27,000 BTU/hr. For a delta T of 27 F, that converts to a flow rate of 1.3 - 2.0 gallons per minute, or 5 to 7.6 liters per minute.

And your pipe internal diameter is 16 mm, which is 0.63 inches, or just a bit larger than our 1/2" diameter pipe standard.

Sorry that we are metric-challenged here in the US. Someone else should check my math.

Jamie Hall

Most of the math is correct, @jesmed1 .

What doesn't change, on either side of the pond, (and this could be Canadian too — not necessarily EU or UK) is the basic physics. You are looking for a delta T of around 15 C. The delta T will be governed — not by pipe size, but by flow rate (liters per second of gpm) and power extracted from the flow (Watts or BTUh).

Now what the optimum delta T and flow rates are depends on the rest of the configuration. What is the maximum temperature your heat pump can deliver? Is this the most efficient operating point for it? Does that change with heat source (outside air?) temperature? Then what are the characteristics of your internal configuration? Are you using a heat exchanger in there somewhere? If so is it counterflow or parallel flow (with that large a target delta T, I hope it's counterflow!)? Or are you running through a buffer tank (you mention one, I think)? How are the heat emitters in your use space configured (indeed, what kind are they? Baseboards? Radiant floor? hot water to hot air coils?) and piped?

Lots of stuff to unpack here to get to maximum efficiency — but the relationship between flow rater and delta T and power transfer isn't really one of them.

pecmsg

Trying to get anything much over 110°F with ATW heat pumps is pushing the limit!

hot_rod

Tube runs through concrete to K 22 radiators? Are those steel panel radiators?

DCContrarian

On my air source heat pump the delta T is a configuration. You set the target temperature and delta T, the heat pump modulates the compressor speed and circulator speed to try and meet them.

The radiation has to be capable of shedding the heat. If the radiation capacity is too low the heat pump will short cycle, if it's too high the heat pump won't meet the target temperature. Depending on how it's programmed it will either run cool, or it will reduce the flow and hit the temperature target but not the delta target.

Frankly I think you're looking at the wrong thing. You should be looking at what is the lowest water temperature you can have in your radiation in your house and still meet your heating load.

DCContrarian

To elaborate further:

Let's say that to meet your heating load your emitters need to have an average temperature of 110F. You can meet that by sending water at 115F and returning at 105F, or sending at 120F and returning at 100F, or sending at 125F and returning at 95F. It the first case the delta will be 10F, the second 20F, the third 30F. Since the BTU output is the same in every case the flow will be half the first in the second case, and one third in the third case.

While the BTU output is the same, the COP will be lower in the second and third cases because the delivered water temperature will be higher. The total energy usage will be lowest in the first case, with the smallest delta.