HP DT 5-7c - why ?

[omph]

Does anyone know why the desired DT for a heat pump is 5-7c ?
Also, what is a desirable DT for low temp condensing gas systems ? < 35c. A lot of DT talk is about boilers running at 55c+ but a flow return of 20c for a boiler running at 35c can have pretty substantial effects on comfort because the margins are just so much smaller.

 

[ScottishGasMan]

For boilers, the 20DT is a number used to express highest potential efficiency/output when running a higher temperature flow. So you can effectively size it to 70 degrees flow and 50 return so still having at least a little condensing taking place.

In reality you want the flow below 50-55 degrees so condensing is taking place across the whole heat exchanger. As is reflected in the new building regs where new heating systems should be designed to a max of 55 degree flow.

However as you rightly say, 55 degree flow and 35 return is going to suffer on output without having pretty large radiators.

Once the flow temp is low, the difference between flow & return isn't really that important for gas. I run mine at a difference of about 5-6 (flow temp max of around 45) in preparation/testing for suitability of a heat pump. You could happily run 10 degree difference.

The closer the return temp is to flow then the higher mean water temperature of the rad and increased heat output. However it is appliance specific as to what you will get in reality and what is best.



As far as heat pumps go. The refrigerant gas isn't particularly hot in the way a boiler is. for a 50 degree flow on a boiler, burning gas in a boiler will have a flame temp of near 2000 degrees (lower in reality dependent on air mixture) but hot all the same.

Heat pumps depending on refrigerant type, could be closer to 60-80 degrees it has to pass to the central heating water. If the water is flowing out the heat pump at 50, then the refrigerant gas entering the heat exchanger at say 60 can only drop 10 degrees before returning to the refrigerant cycle.

That doesnt mean they cant produce much heat (energy) as the refrigerant will enter the heat exchanger as a hot gas (60degree) and condense where it passes the heat to the water system, this condensing gives up a lot of heat energy (in the same proccess of latent heat that you have on a condensing boiler)

But since there is a narrow temperature between the Refrigerant Gas entering the HP heat exchangers and the condenses liguid refrigerant leaving, it means you need a high flowing supply of slightly cooler water entering the HP to keep stealing the heat.

Add 1kW of constant heat to water flowing at 57 litres per hour, you'll raise the flow temperature by 15 degrees.
add the same 1kW of constant heat to water flowing at 172 litres per hour, you'll raise the flow temperature by 5 degrees.

In both examples youve added 1kW of constant heat to the heating system, but on the heat pump example you need a higher flow rate to maintain the 5 degree lift, in order to prevent the compressor having to work harder to lift the refrigerant gas temperature even higher to compensate for low flow rate/high differential temperature on the water side. Which will significantly impact the efficiency of it and reduce its maximum potential power output.

 

[omph]

For boilers, the 20DT is a number used to express highest potential efficiency/output when running a higher temperature flow. So you can effectively size it to 70 degrees flow and 50 return so still having at least a little condensing taking place.

In reality you want the flow below 50-55 degrees so condensing is taking place across the whole heat exchanger. As is reflected in the new building regs where new heating systems should be designed to a max of 55 degree flow.

However as you rightly say, 55 degree flow and 35 return is going to suffer on output without having pretty large radiators.

Once the flow temp is low, the difference between flow & return isn't really that important for gas. I run mine at a difference of about 5-6 (flow temp max of around 45) in preparation/testing for suitability of a heat pump. You could happily run 10 degree difference.

The closer the return temp is to flow then the higher mean water temperature of the rad and increased heat output. However it is appliance specific as to what you will get in reality and what is best.



As far as heat pumps go. The refrigerant gas isn't particularly hot in the way a boiler is. for a 50 degree flow on a boiler, burning gas in a boiler will have a flame temp of near 2000 degrees (lower in reality dependent on air mixture) but hot all the same.

Heat pumps depending on refrigerant type, could be closer to 60-80 degrees it has to pass to the central heating water. If the water is flowing out the heat pump at 50, then the refrigerant gas entering the heat exchanger at say 60 can only drop 10 degrees before returning to the refrigerant cycle.

That doesnt mean they cant produce much heat (energy) as the refrigerant will enter the heat exchanger as a hot gas (60degree) and condense where it passes the heat to the water system, this condensing gives up a lot of heat energy (in the same proccess of latent heat that you have on a condensing boiler)

But since there is a narrow temperature between the Refrigerant Gas entering the HP heat exchangers and the condenses liguid refrigerant leaving, it means you need a high flowing supply of slightly cooler water entering the HP to keep stealing the heat.

Add 1kW of constant heat to water flowing at 57 litres per hour, you'll raise the flow temperature by 15 degrees.
add the same 1kW of constant heat to water flowing at 172 litres per hour, you'll raise the flow temperature by 5 degrees.

In both examples youve added 1kW of constant heat to the heating system, but on the heat pump example you need a higher flow rate to maintain the 5 degree lift, in order to prevent the compressor having to work harder to lift the refrigerant gas temperature even higher to compensate for low flow rate/high differential temperature on the water side. Which will significantly impact the efficiency of it and reduce its maximum potential power output.

Excellent post.

Add 1kW of constant heat to water flowing at 57 litres per hour, you'll raise the flow temperature by 15 degrees.
add the same 1kW of constant heat to water flowing at 172 litres per hour, you'll raise the flow temperature by 5 degrees.

What formulas do you use to make such calculations ?

 

[JonathanM]

What formulas do you use to make such calculations ?

This is how I would do it, so probably a bit clunky.

1 watt = 1 joule per second

1 KW = 1000 joules per second

3600 seconds in an hour, so

1 KWh = 3,600,000 joules

The specific heat capacity of water is 4184 J per Kg per C

Divide 3,600,000 by 4184 = 860.5

Divide this by the temperature rise (say 15C) to get the number of Kg

860/15 = 57.3 Kg

Density of water is 1

So 57.3 litres per hour at 15C rise

 

[JonathanM]

What formulas do you use to make such calculations ?

Or if you just want a quick formula for the flow rate in litres per second.

Flow (l/s) = Heat (KW)/4.2/temperature rise

 

[omph]

Or if you just want a quick formula for the flow rate in litres per second.

Flow (l/s) = Heat (KW)/4.2/temperature rise

Thanks Jonathan, as always.
I am trying to understanding how the formula translates in the real world. 1KWh @ 57L p/h = a 15c rise. When you say the water will rise 15c, does that mean, at 1PM the water is 15c, 2PM it is 30c, 3PM it is 45c and so on ? or does it mean that a constant temp of 15c will be maintained across the 3 hours assuming the starting point is 0c ?

 

[JonathanM]

I am trying to understanding how the formula translates in the real world. 1KWh @ 57L p/h = a 15c rise. When you say the water will rise 15c, does that mean, at 1PM the water is 15c, 2PM it is 30c, 3PM it is 45c and so on ? or does it mean that a constant temp of 15c will be maintained across the 3 hours assuming the starting point is 0c ?

You may remember I mentioned before that I’m better at the science than the practical side of things, but here goes.

I was thinking about it in the context of a closed heating system with a heat input (e.g. a boiler) and a heat output (e.g. radiators)

If you imagine an idealised system where, once the system gets up to the set flow temperature, the boiler modulates down to exactly 1KW. And imagine you have just one radiator, which at that combination of flow temperature and room temperature, emits exactly 1KW. The system is in perfect balance. All the heat being put into the system by the boiler is being released again by the radiator and so the flow temperature from the boiler stays constant.

If the pump is running at 57 l/h, the water that has flowed out of the boiler will decrease by 15C every time it passes through the radiator. It will then increase again by 15C every time it passes through the boiler. And so the flow temperature coming out of the boiler will always be the same.

If you run the pump at 172 l/h instead, the water in the system will gain and lose 5C on every circuit. But again the flow temperature coming out of the boiler will remain the same.

So, by changing the pump speed, you are changing the dT, across both the radiator and the heat exchanger.

 

[ScottishGasMan]

Thanks Jonathan, as always.
I am trying to understanding how the formula translates in the real world. 1KWh @ 57L p/h = a 15c rise. When you say the water will rise 15c, does that mean, at 1PM the water is 15c, 2PM it is 30c, 3PM it is 45c and so on ? or does it mean that a constant temp of 15c will be maintained across the 3 hours assuming the starting point is 0c ?

It's an instantaneous thing, so Imagine a boiler heat exchanger. water is pumped into one side which pushed through the heat exchanger and comes out the flow towards the system.

3 things will determine the temperature of the water coming out the heat exchanger (HEX) to the flow.

1. The initial temperature of the water coming into the hex
2. The amount of heat being applied to the hex
3. The volume of water (flow rate) of the water being pumped through it per second

If the incoming return temperature was fixed, say 10°C. And the heat applied was fixed, say 20kW. And we pumped 5 litres per minute through. The temperature that should be coming out of the hex flowing towards the system would be:

Temp Rise (°C) = "Heat (kW) x 60" divided by "4.2xFlow rate (litres/min)"

Temp Rise (°C) = "20kW x 60" divided by "4.2x5 l/min"

Temp Rise (°C) = "1,200" divided by "21"

Temp Rise (°C) = 57°C

The temperature flowing into the hex was 10°C, the temperature rise is 57°C so the temperature flowing out the hex is now 67°C




Lets do the same again, but increase the flow rate of water, still 10°C water entering, and 20kW being applied but this time we pump 20 litres per minute through

Temp Rise (°C) = "Heat (kW) x 60" divided by "4.2xFlow rate (litres/min)"

Temp Rise (°C) = "20kW x 60" divided by "4.2x20 l/min"

Temp Rise (°C) = "1,200" divided by "84"

Temp Rise (°C) = 14°C

The temperature flowing into the hex was 10°C, the temperature rise is 14°C so the temperature flowing out the hex is now 24°C

Were still adding 20kW of heat energy per second to the system water, the same amount of heat is flowing in the pipes and radiators, only now its at a lower temperature.


The formula works the same for heat given off. If I had a huge radiator, I could give the same flow water at 20 litres per minute at 24°C into it, with 10°C coming out the return or the radiator, that radiator would have to be loosing (Giving up) 20kW of heat to the room.

Of course the room would be very cold if the return was 10°C and the radiator huge if it was giving out 20kW, but the calculation is the same both ways.

So although a heat pump may not be lifting the water temperature as high as a boiler, the same energy can be put into the water at a lower temperature, and the same energy released from the water at a lower temperature to the room.

To heat a room with a lower temperature water flow, means a bigger radiator, as the higher surface area with the air lets the same heat out using a lower size of panel. And the higher flow rate means bigger pipes typically, as to flow between 2 and 4x as much water through the pipework that would have been used for a boiler, would create too much resistance in the pipe, resulting in very big pumps being needed, making the flow of water audible, and potentially even erroding the pipework away with the friction.

This is why generally, most houses will need bigger radiators and pipework to suit a heat pump.

This also assumes the pipework and radiators that are currently in place with a gas boiler have been sized up as part of a design. (more often than not they have not been "designed" and just fitted as thats what fits in the space given, so often some existing radiators and pipework may well be suitable for low temperature/high flow from a heat pump. But there will usually be at least some of the system needs altering to accomodate.

The efficiency of a heat pump also dramatically increases the lower the flow temperature it runs at. So where a boiler might be 10% better off running 50°C flow temp vs 75°C, a heat pump could be 250% better running 35°C vs 55°C cost wise.

However 35°C flow with 30°C return is very unrealistic for heating by radiators in most houses due to the size they would be.

45°C flow with 40°C is pretty achievable in most houses, sometimes with Triple panel radiators (ugly in my opinion) or by fitting 2 normal sized radiators in a room that only had 1.

55°C flow temp is the highest a new system is allowed to be designed to, whether its a gas boiler or heat pump (Thats a new heating system not a new house, new building regs for the replacement of any heating system)