Calculation of thermal output???

[joni os]

Joules confuse me, but 66.66 X 14 X 4 X 60 equals 224,000 BTU/hr Approx 67 Kw. This seems high. I would seek confirmation of flow rate or temp. diff.

 

[Nostrum]

Your DT is way to high which suggests a flow rate issue. The reason I asked about which heat pump you have is to understand its capabilities and whether or not it was an inverter driven compressor etc, not because I wanted to slag it of or suggest a different model.

When did you take the DT and under what conditions? You need to allow the heat pump to run for long enough for it to adjust to whatever load you're placing it under.

 

[Dogsbody]

Joules confuse me, but 66.66 X 14 X 4 X 60 equals 224,000 BTU/hr Approx 67 Kw. This seems high. I would seek confirmation of flow rate or temp. diff.

Now I'm worried. The temp readings are on the heat pump control panel, reading off sensors inside. The numbers they give seem to make sense when compared with the operation of the thermostat on the buffer tank, and with the return temperature off the UFH as given by the manifold thermometers. Could be wrong, but they seem to stack up. The flow rate is off the Wilo circulation pump. This has an LCD screen that gives electricity consumption and flow rate amongst other things. I guess I'd assumed this would be reasonable reliable, given Wilo's reputation. Probably naive?

 

[Dogsbody]

Your DT is way to high which suggests a flow rate issue. The reason I asked about which heat pump you have is to understand its capabilities and whether or not it was an inverter driven compressor etc, not because I wanted to slag it of or suggest a different model.

When did you take the DT and under what conditions? You need to allow the heat pump to run for long enough for it to adjust to whatever load you're placing it under.

It's not inverter driven. I wasn't thinking you were about to slag it off, but trust me that if I do mention the brand, there will be a whole chorus of jeering, telling me it's rubbish. I'm basing this on posts in other threads. Don't think it matters for now, I'll share later if people are interested.

The DT was even higher till I cranked up the flow rate!! The Wilo Stratos Pico has adjustable flow rate, and the screen readout tells me I'm at maximum at 4000 litres/hour.

I've been letting the heat pump run and stabilise, although to be honest, it makes little difference. Seems to start out at a DT of 14 and stay there with outlet temperature gradually rising from 43 to 48 when the thermostat cuts out. I mentioned in another post, that I'm running the heat pump into a buffer tank. Would that result in the stable DT? Why do you think it's high?

 

[joni os]

Unfamiliar with pump, but does the 4000 litre/hr. refer to theoretical flow against zero head resistance.

 

[Dogsbody]

Unfamiliar with pump, but does the 4000 litre/hr. refer to theoretical flow against zero head resistance.

Good question. No idea. Would have thought it was a bit odd to display theoretical flow rate on a changing display that can be adjusted by user? Dunno.

 

[joni os]

4000 seemed too round a figure for actual flow rate. Since posting I have goggled pump data and flow drops off considerably with head.

 

[Nostrum]

The heat pump does matter as the controller is what makes a lot of difference with a heat pump.

The basic principles aren't all that different for most heat pumps, maybe different refrigerant etc

How the heat pump output is controlled is another matter. I'm guessing it's a Dream or similar and this is a self fit / non RHI type install?

What size and type of pipework is used from the outdoor unit to the buffer and over what distance?

How is the buffer connected to the heat pump and system, is it in series/parallel, 2 pipe, 4 pipe etc etc

I'm afraid if you want help or answers you'll have to start disclosing more information.

 

[doitmyself]

OK, I'm struggling a bit with the maths.... My figures are as follows:

Flow rate: 66.66 litres per minute

If, as you say in a later post, the flow is 4m³/hr that is 66.66 litres/min. However, my formula needs the flow in litres/sec so the output is 66.66/60 x 14 x 4.18 = 65kW or 65 J/sec. The output over one minute is therefore 3900 Joules. (I know it's the same as you got but, as one of my maths teachers used to say, "right answer, wrong method!")

Your calculations are correct, but a COP of over 16 seems unlikely. You mentioned another post about a buffer tank, but I can't find it. A link would be useful.

 

[Dogsbody]

4000 seemed too round a figure for actual flow rate. Since posting I have goggled pump data and flow drops off considerably with head.

OK, makes sense. I think there is a setting on the pump to tell it what head you are working with. Can't check now as weather is a bit dodgy. Will look tomorrow. I think it was set properly, but as you say, the 4000 sounds a bit nominal....

However, if I can get an accurate figure for flow, I still need confirmation that I am doing the sums right. Any help on that would be much appreciated.

For example, if I halve the flow rate, (seems extreme), I still get a COP of around 8, I think. Something still wrong?