@trebor12345 Your heat pump is a 7kW unit, not 3kW as per your description. The 3HP in the model number is the horse power rating of the compressor. To provide 7kW with 5C DT, the flow will be 20 l/min. I presume that this will be the 100% pump speed setting on your unit.
I done some checking with chatgpt and yes its 7 KW output with an input of approx 2KW, thank you for correcting this.
So at 20W/m2 and 200sq/m that's 4KW. So it still looks like I am oversized.
2024 build bungalow
Southern england 179 m2 High level of insulation Underfloor heating All 12 circuits are fully open all the time 1 thermostat in family room 7KW heat pump 50 litre buffer tank (4 port)
3.6KW solar panels Energy used by heating 2527 KWh - 7527 KWh (SCOP 3.5 approx)
@trebor12345 What are the u values of your walls and ceiling?
From the Full SAP Report:
Space heating requirements: 8314 Kwh/year
I can see the U values for all of the structure, but don’t understand the final result of this. Is there anything there that helps?
I also found a result from a form of Energy Performance Certificate which states the primary energy for the property is 14 kilowatt hours (kWh) per square meter.
This post was modified 4 days ago 2 times by trebor12345
2024 build bungalow
Southern england 179 m2 High level of insulation Underfloor heating All 12 circuits are fully open all the time 1 thermostat in family room 7KW heat pump 50 litre buffer tank (4 port)
3.6KW solar panels Energy used by heating 2527 KWh - 7527 KWh (SCOP 3.5 approx)
2024 build bungalow
Southern england 179 m2 High level of insulation Underfloor heating All 12 circuits are fully open all the time 1 thermostat in family room 7KW heat pump 50 litre buffer tank (4 port)
3.6KW solar panels Energy used by heating 2527 KWh - 7527 KWh (SCOP 3.5 approx)
@trebor12345@jamespa From the calcs you sent, your house conforms with modern insulation standards e.g. roof u-value 0.11. Your overall heat loss is 112.5 W/K or W/C. If your design temperature is -2C and target room temperature 21C, then your maximum room DT is 23C. Multiply this by 112.5 and you get 2.5kW unless I am bonkers! Your heat requirement is 13W per sqm which is not surprising for a current insulated new home. You have a nominal 7kW heat pump (max. 11kW) which is way off beam. How did the installer justify this? You will cycle like mad.
Your installation looks very neat. However, you have a 4 port buffer tank and a secondary pump for the manifold which is really not necessary for open loop working. The heat pump could drive the manifold directly with ease. If I had to guess, I would say that you initially installed a 2 zone system with thermostats? As the HP is fixed flow, a buffer tank was specified.
Unfortunately, you are talking to the wrong person when it comes to operating open loop systems. You need a Vaillant-type person like @jamespa for advice on setup and operation. I have no experience of this as I have UFH with a 4kW Samsung with variable flow control and therefore I do things very differently. Funnily enough, I have a bungalow of same size but with lower insulation standard.
Private individual. No affiliation with commercial "Heat Geeks" of same coincidental name.
@trebor12345@jamespa From the calcs you sent, your house conforms with modern insulation standards e.g. roof u-value 0.11. Your overall heat loss is 112.5 W/K or W/C. If your design temperature is -2C and target room temperature 21C, then your maximum room DT is 23C. Multiply this by 112.5 and you get 2.5kW unless I am bonkers! Your heat requirement is 13W per sqm which is not surprising for a current insulated new home. You have a nominal 7kW heat pump (max. 11kW) which is way off beam. How did the installer justify this? You will cycle like mad.
Your installation looks very neat. However, you have a 4 port buffer tank and a secondary pump for the manifold which is really not necessary for open loop working. The heat pump could drive the manifold directly with ease. If I had to guess, I would say that you initially installed a 2 zone system with thermostats? As the HP is fixed flow, a buffer tank was specified.
Unfortunately, you are talking to the wrong person when it comes to operating open loop systems. You need a Vaillant-type person like @jamespa for advice on setup and operation. I have no experience of this as I have UFH with a 4kW Samsung with variable flow control and therefore I do things very differently. Funnily enough, I have a bungalow of same size but with lower insulation standard.
Your calculation of 13W per sqm fits well with what I found at 14W.
So if I require 14W/m2 and the floor area is 179m. 179x14 = 2505. Does that mean I need a 2.5kw heat pump?
There are 9 wall mounted thermostats they are all off (except 1). So we use the family room to control the heating.
So my heat pump is oversized which now accounts for the Thermal Offs (as Hitachi call it).
I do have the option to change to delta T on the flow rate rather that fixed. I have attached Hitachi explanation. Would this help in my situation?
2024 build bungalow
Southern england 179 m2 High level of insulation Underfloor heating All 12 circuits are fully open all the time 1 thermostat in family room 7KW heat pump 50 litre buffer tank (4 port)
3.6KW solar panels Energy used by heating 2527 KWh - 7527 KWh (SCOP 3.5 approx)
So if I require 14W/m2 and the floor area is 179m. 179x14 = 2505. Does that mean I need a 2.5kw heat pump?
Yes in principle but you wont find one and anyway you would want larger for DHW. A genuine (not a software limited) 5kW is probably the sweet spot.
8 room thermostats are all in Frost Protection mode. The one in the family room controls the heating. All the manifold actuators have been removed. Does that explain?
It's very much an approx of the SCOP as reported by the Hitachi system, which it states is not that accurate.
These thermal offs are where the heat pump turns off for 3 minutes and then 8 minutes latter does a de-ice. This repeats itself under all conditions (inside/outside themos, different flows, etc). See attached (note I used short cycle in my graph).
2024 build bungalow
Southern england 179 m2 High level of insulation Underfloor heating All 12 circuits are fully open all the time 1 thermostat in family room 7KW heat pump 50 litre buffer tank (4 port)
3.6KW solar panels Energy used by heating 2527 KWh - 7527 KWh (SCOP 3.5 approx)
8 room thermostats are all in Frost Protection mode. The one in the family room controls the heating. All the manifold actuators have been removed. Does that explain?
yes thanks!
Im not sure I can really comment further as your key concern (unless I misunderstand you) seems to be the thermal offs which appear to be simply a feature of how Hitachi do defrost. Unfortunately I don't have a Hitachi to compare it with!
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
8 room thermostats are all in Frost Protection mode. The one in the family room controls the heating. All the manifold actuators have been removed. Does that explain?
yes thanks!
Im not sure I can really comment further as your key concern (unless I misunderstand you) seems to be the thermal offs which appear to be simply a feature of how Hitachi do defrost. Unfortunately I don't have a Hitachi to compare it with!
I am drawing the same conclusion that this system for some reason switches off for 3 minutes then 8 minutes latter does it's de-ice cycle. It does this even when the outside air temperature is 20 centigrade. Seems like it creates a lot of cycling unnecessarily.
2024 build bungalow
Southern england 179 m2 High level of insulation Underfloor heating All 12 circuits are fully open all the time 1 thermostat in family room 7KW heat pump 50 litre buffer tank (4 port)
3.6KW solar panels Energy used by heating 2527 KWh - 7527 KWh (SCOP 3.5 approx)
Thank you all for your help, this has been a learning experience. So in summary. My heat pump is oversized, this causes Thermal Off’s or short cycles where is switches off for 3 minutes then 8 minutes later does it's de-ice cycle. It does this every time it sees a short cycle and even at 20C outside and above. It’s appears to be a function of the Hitachi system. So how do I manage this going forward. Longer term change the heat pump to a smaller one. Short term:
Switch to a Delta T Flow rather than Fixed flow?
Turn the heat pump off and on for a hour at a time (for example)? So I will only see 12 offs in a 24 hour period. * See note below.
Heatgeek had this explanation of heat transfer across the water heat exchanger:
"This gets carried away by the water through the combination of DT and flow. Whether it is high flow/low DT or low flow/high DT, the heat carried out is the same. Heat is power. As an analogy, the power of electricity is given by the voltage multiplied by the current. Therefore 2W = 1V x 2A or 2V x 1A. So, slowing flow will not carry away more heat. It will widen the DT as the power output is constant".
So is there any benefit in me having all of my flow values on the UFH manifold fully open or throttled back, to reduce cycling? I think from the explanation above it’s going to make no difference.
*Just as a note, I am unable to run the system 24/7 as the house gets to hot. In winter with the heating off overnight the bedrooms only drop by 1C.
This post was modified 3 days ago 2 times by trebor12345
2024 build bungalow
Southern england 179 m2 High level of insulation Underfloor heating All 12 circuits are fully open all the time 1 thermostat in family room 7KW heat pump 50 litre buffer tank (4 port)
3.6KW solar panels Energy used by heating 2527 KWh - 7527 KWh (SCOP 3.5 approx)
"This gets carried away by the water through the combination of DT and flow. Whether it is high flow/low DT or low flow/high DT, the heat carried out is the same. Heat is power. As an analogy, the power of electricity is given by the voltage multiplied by the current. Therefore 2W = 1V x 2A or 2V x 1A. So, slowing flow will not carry away more heat. It will widen the DT as the power output is constant".
I agree.
The fundamental is that the house determines the heat loss and the compressor determines the minimum output. Changing the transport mechanism between the two may make a small difference, but is unlikely to make a major difference assuming that its in a sensible ballpark (and sufficient of course).
Turn the heat pump off and on for a hour at a time (for example)? So I will only see 12 offs in a 24 hour period. * See note below.
I guess the question to ask yourself is - (why) do you think you are better at designing a cycling algorithm than the manufacturers? Fridges cycle all the time but last for decades, so whilst wear and tear is potentially a worry it must be questionable how significant it is. Of course the manufacturers probably desing on the assumption that the heat pump is right-sized, which yours appears not to be.
However as you cant heat 24*7 (Im not sure how that can be the case if you set your flow temp low enough - but will take the comment at face value) then forcing on/off may make some sense, particularly if you can coincide it to take advantage of a ToU tarrif. It rather depends on how quickly your house cools. If its got a big slab of concrete batch heating is very likely your best bet.
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
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