Calculate ROI for a custom Unvented Indirect Heat Pump Cylinder???

[myatix]

Which ever way it makes more sense having a large surface area of pipe in the cylinder to dissipate more heat and the maximum valocity shouldn’t go above 1m/s.

If I understand correctly the flow rate is directly related to the velocity and therefore also determines the pipe size dimensions. Otherwise the HyTool is wrong and as it’s used by plumbers all over the world I don’t think that is the case.

So if dT and energy is used to calculate flow rate this also impacts the velocity of the water in m/s.

 

[MNW67]

If I understand correctly the flow rate is directly related to the velocity and therefore also determines the pipe size dimensions. Otherwise the HyTool is wrong and as it’s used by plumbers all over the world I don’t think that is the case.

Of course the tool is right. It's just that previously you were misunderstanding the tool's results and you were reading velocity as flow rate. I'm glad you get it now. You are welcome, by the way

So if dT and energy is used to calculate flow rate this also impacts the velocity of the water in m/s.

Exactly. They are all linked. The system water passes through the heat pump and energy is added to the system. This raises the temperature by the dT. The size of the dT depends on the flow rate. That flow rate then passes down a pipe, and the velocity it travels at depends on the diameter of the pipe.

 

[MNW67]

@myatix

I am finding this such an interesting topic. I've been finding out all sorts of things about heat pumps and cylinders which I had no idea about.

I don't know whether you are also using other forums. Some have a lot more posters with ASHP experience than this one does. There is a really good thread on this forum which has an ASHP section which may help with some of your questions:

Do I need a HP specific cylinder?

OK, so I've got a boiler coming towards end of life for a small 1 bed flat. I was toying with replacing with a small HP. The problem, as ever, is the hot water. Space is limited. I was thinking of a 90l cylinder to be high mounted out of the way. There are a range of cylinders availible but none ...

forum.buildhub.org.uk

Also, here is another ASHP forum:

Air Source Heat Pumps (ASHPs)

renewableheatinghub.co.uk

 

[MNW67]

But if you want to use an HP to heat a tank of water to 55 degrees, you're going to have to ask it to produce 55 degrees so where are you getting this "must be low flow temp for efficiency" idea from? Doubling your coil area ain't going to make it possible to get a 55 degree tank from a 35 degree flow.

An interesting question which had me scratching my head. I'm new to heat pumps, and it's been a steep learning curve. But I think I finally found the basics of the answer.

When a heat pump heats a cylinder from say 20C to 45C, it doesn't start off with a flow temperature of 50C. Instead it maintains a steady difference between the flow temperature, and the return temperature, of between about 2C and 10C, throughout the cylinder heat period. So, in the example below, this difference is just under 3C. When you have a very big coil, you can use the return temperature as a proxy for the cylinder temperature. The flow starts at about 23C and return/cylinder starts at 20C , and ends with flow 48C and return/cylinder of 45C. The lower you can keep the flow temperature dT, above the cylinder temperature, the greater the efficiency. But you also need to heat the cylinder within a reasonable time frame. I hadn't realised that heat pumps cannot do central heating whilst they are heating the cylinder. That's where a big cylinder coil comes in. In theory, if you increase the size of the coil, you can heat the cylinder with a lower dT, in the same time as the smaller coil achieved with its higher dT. So, you keep the flow temperature lower and that makes it more efficient. And all the time, you need to try to keep the heat pump within its sweet spot for efficiency, which is typically 40% to 60% of maximum compressor speed. So, it's also about the balance between heat pump size and coil size.

 

[ScottishGasMan]

I am considering using a custom built heat pump cylinder with a dual coil setup to maximize the heat transfer from Heat Pump to the DHW Cylinder. Ideally you want to run a heat pump at as low a flow temperature as possible to maximize the SCOP. Therefore having a custom cylinder with 6m2 of coil vs the standard 3m2 for most heat pump cylinders would allow for much lower temperatures and a lower flow rate therefore optimizing's the energy transfer.

I have had a few companies provide a quote for this type of cylinder but I would like to calculate the return of investment to see if it is financially viable over a 5-10 year period.

The tank with the 28mm, 6m2 coil is double the price of the 28mm, 3m2 coil so I am not sure it is worth it...

I look forward to hearing from anyone who might be able to help?

1 question. What output of heat pump would you be using?

 

[myatix]

1 question. What output of heat pump would you be using?

@ScottishGasMan I am planning on installing a Vaillant Aerotherm Plus 7Kw Heat Pump and have just decided to go with the Newark HG 6m2 coil cylinder.

Attached is a copy of my MCS Heat Loss Calculation for anyone who is curious...

Just to point out again I live in Denmark and our outside design temperature here is -12 instead of the approx. -2-3C that you use in the UK.

I have UFH throughout the entire house and have run my gas condensation boiler at a flow temperature of 35C for a year without any problems... My concern as @MNW67 correctly stated is that CH stops when DHW is running and with our cold temperatures it would be nice if that time was limited while trying to maintain a good COP.

@MNW67 FYI, most ASHP installations use CH priority instead of DHW priority where as this tends to be the opposite way around for Gas Boilers.

 

[MNW67]

@myatix

I hadn't realised quite how long these cylinder coils are. As far as I can tell, a 3m² 28mm coil is about 30 metres! That explains why some people struggle with getting enough head from the pump with large surface area coils. I think in the Heat Geek video he says he has got around this problem by running the two coils in parallel.

 

[myatix]

@MNW67 Have a look at the new Newark Heat Geek Cylinder Specs...

In addition to this you can also see Adam's Cylinder performance here on the Open Energy Monitor Community Forum.

HeatpumpMonitor.org

heatpumpmonitor.org

 

[ScottishGasMan]

@ScottishGasMan I am planning on installing a Vaillant Aerotherm Plus 7Kw Heat Pump and have just decided to go with the Newark HG 6m2 coil cylinder.

Attached is a copy of my MCS Heat Loss Calculation for anyone who is curious...

Just to point out again I live in Denmark and our outside design temperature here is -12 instead of the approx. -2-3C that you use in the UK.

I have UFH throughout the entire house and have run my gas condensation boiler at a flow temperature of 35C for a year without any problems... My concern as @MNW67 correctly stated is that CH stops when DHW is running and with our cold temperatures it would be nice if that time was limited while trying to maintain a good COP.

@MNW67 FYI, most ASHP installations use CH priority instead of DHW priority where as this tends to be the opposite way around for Gas Boilers.

Bigger surface area and bore coil the better, but I'd not be paying significant amounts extra for it.

Im not sure in denmark if the aroTHERM plus is badged the same as here i.e. 7kW @ -7degrees outside 35 flow, or whether they are badged like the European Norm of 7kW at +7degrees outside and 35 flow.

but either way if your running it on ECO DHW which holds the compressor power back to try raise the Flow & return temps in line with the cylinder thus keeping the heat pump running cooler for longer, then it restricts the compressor speed, which means your probably putting out 3-5kw into the coil, so even a 3m coil would be pretty efficient.

Bear in mind in the heatgeek videos, theyre heating the cylinder from nearly cold every time using the ECO setting. Its quite a difference in efficiency to leaving it on constant top up 24/7 and running NORMAL mode (full compressor power to cylinder).

To add, for that little extra effieicny if one or two cylinders a day is enough then only heat it during the day when air temps are warmer (added benifit you loose less heat from the property then aswell) and HeatGeek generally dont do weekly legionella runs either so save again on that.



Whats typical in denmark for heating? i.e big buffer/pump group/assume it will have internal unit with backup heater? Or do they try to keep them simple?

 

[Johntheo5]

These 6M2 coils are very tall (by necessity) so IMO unless the full contents are used up and then reheated then its hard to see any huge ROI over a more conventional cylinder, it does state that the "coil has reverse return configuration", wonder does this mean that the hot inlet is into the bottom coil? or is it just counter flow like most are configured, with the hot inlet at the top.

 

[old duffer]

Obviously, but it's irrelevant. Mass flow rate is a specific and well understood technical term and its SI unit is kilograms per second (kg/s). The volume of my kettle is 1.7 litres. The mass of that water is 1.7kg at 4C at sea level. But the volume of my kettle will never be 1.7kg.

Anyway, the clue is sort of in the title

If people want to use scientific terminology, then use it properly. Which was why I suggested above that simply using "flow rate" would be much less confusing. Then it can mean whichever you want.

What is the mass? well what is the volume! interchangeable....So a volume (which = a given weight) flow at a speed = mass flow rate...the mass(volume) how do you measure the flow rate...or is this just another guess?

 

[ScottishGasMan]

What is the mass? well what is the volume! interchangeable....So a volume (which = a given weight) flow at a speed = mass flow rate...the mass(volume) how do you measure the flow rate...or is this just another guess?

But the mass of water in a given volume changes dependant on temeprature. Which is why l/s is technically wrong for mass flow rate.

Although for the purpose of domestic heating calcs I'm also very guilty of interchanging them

 

[MNW67]

These 6M2 coils are very tall (by necessity) so IMO unless the full contents are used up and then reheated then its hard to see any huge ROI over a more conventional cylinder, it does state that the "coil has reverse return configuration", wonder does this mean that the hot inlet is into the bottom coil? or is it just counter flow like most are configured, with the hot inlet at the top.

I don't understand what reverse return means but this is what is looks like on his diagram.

 

[Johntheo5]

100 kgs of water at 4.35C is almost exactly 100 liters.
100 kgs of water at 60C is 101.7 liters or 1.7% greater so reasonably acceptable to use liters for these type calculations.

 

[Johntheo5]

I don't understand what reverse return means but this is what is looks like on his diagram.

View attachment 332489

That just looks like two coils connected in parallel? but doesn't show the flow/return.