@soniks Firstly, the engineers at Grant need to buy a new calculator or go back to school. 50 L/min will transmit 28Kw. You only need to transmit about 14Kw.
you have the wrong valves, the valves you have block flow.
22mm plastic is not the same as 22mm copper, it has similar flow characteristics as 15mm copper again blocks flow.
22mm copper is too small for a 17 Kw unit, it is boarder line velocity but will require 1 m head for every meter of pipe run.
A bigger pump will do nothing except raise the heating bill.
Air valves will not make a difference, the air will accumulate in the radiators.
Solution
replace as much of the 22mm as possible with 28mm or 35 if it's going to be replaced, to where the system starts to branch to the radiators.
replace the valves with high flow, full bore units
remove the majority of the TRV valves on the system, only control the bedrooms.
set the weather compensation to control the house temperature.
A heat pump should never have been connected to this system, it never stood a chance of working. The grant engineer, if they were worth his salt should have told you this and recommended solutions to get a suitable outcome.
Firstly, the engineers at Grant need to buy a new calculator or go back to school.
It did seem a bit odd that the rated flow rate was temperature dependent; 50.88 l/min at 35 degrees C flow temperature and 27.90 l/min at 55 degrees C flow. Power = heat capacity * flow * deltaT and all of that.
Ok thanks for this I think it's becoming clearer to me what needs to be done from all of your suggestions. Maybe this is going to be a bigger job than expected and I agree the ex Grant engineer who did the install quite frankly has done an appalling job.
Downstairs we are looking to install underfloor heating we will
install floor insulation lowering heat loss further
16mm UFH pipe with 100mm pipe centres
have no zoning or thermostats. Is there anyway to monitor the temperature of the rooms so we can see how well things are working in each room?
This should cover any piping issues downstairs although we will exclude kitchen dining for now which will still have radiators due to an extension we plan to do in the future at which point UFH will be installed
The pipework from heat pump to UFH manifold will need to be 28/35mm
As we are having to lift the flooring downstairs we will route the primary piping under the flooring instead of going into the loft.
Pipework for upstairs heating circuit will need to be upgraded to 28/35mm as I believe it's only 22mm at the moment
The zone valves removed with diverter installed instead with hot water priority in the garage
Other zone valve replaced with full bore valve
Should the volumiser be kept in the design? If we do, then connect the immersion to the heat pump so it can control turning it on during defrost cycles
Would I need both pumps ie i the ASHP and second one in sequence as we have at the moment with the changes in pipe sizing?
Use the Grant controller to control the heat pump fully and move it from the garage into the lounge. We have a 2 channel programmer from before and passive system for the hot water and metering and monitoring
It did seem a bit odd that the rated flow rate was temperature dependent; 50.88 l/min at 35 degrees C flow temperature and 27.90 l/min at 55 degrees C flow. Power = heat capacity * flow * deltaT and all of that.
Grant make the assumption that higher leaving water temperatures have a higher delta T between flow and return, compared to lower temperatures. For that reason, to achieve the nameplate rating, the mass flow is higher at lower leaving water temperatures compared to higher LWTs. It's not an unreasonable assumption. Wet rads may have a delta T 6-8degK, whereas UFH might be 4-5degK. Every installation is different and the flow rate should be set to suit the heating capacity required and the delta T between flow and return. As is always, the easier the flow, the more efficient the design. Retrofits can be a compromise, but 28mm flow and return plus hot water cylinder, with 22mm copper for heating circuits is normally OK.
My experience with Grant ASHP is, because they have fixed flow circulating pumps, and usually incorporate low loss headers, the delta T tends to be on the low side. Even when I'm generating hot water from a cold tank, the delta T on my 10kW Aerona3 is never above 6degK. It's transferring about 9-10kWh at that delta T based on the indicated flow rate. My heating circuit quite often has a delta T of 2 or 3degK when it's in fairly low space heating demand. The Grant ASHP seems fine with this, it modulates down and doesn't start to cycle.
@heacol thanks for all your responses. The problem I have is when the Grant engineer came he disconnected the HP from the system and connected his flow monitoring rig he was able to see flow rates up 30lpm. Once he connected it back to the system it dropped to 17lpm at 50c for heating and 15.5 lpm at 55c for DHW. He suggested there is an issue in the system and not the heat pump itself.
Looking at the Grant install manual I can see this. At 7c/35c water 50.88 lpm is required by the pump. If that is correct how could we achieve this given we are hovering around the 17lpm mark.
From what I can understand this could be because :
Pipe sizing issues - the house was extended by the previous owners and we don't know how the piping has been done in the areas that were extended
Incorrect piping somewhere in the property. Part of the reason we are looking to UFH as then we know there can't be any piping issues.
Blockage in the system - the system was flushed before the heat pump was installed but maybe this is required again
Currently the piping goes up from the garage into the loft across a long run to the middle of the house where the old cylinder was and connects back to the original piping - I think this is was the distribution point previously. The piping is 22mm copper as it comes into the loft and then 22mm plastic for the loft run then back to copper. I'm not sure whether going into the loft affects the pump performance given it's effectively 2 stories
Stronger pump required maybe?
I noticed there were no air vents installed in the garage and I can hear what sounds like trickling water in the volumiser when the HP is in operation (the volumiser is installed on the return). Is all of this normal? Perhaps there is air trapped and has no where to escape out from the volumiser - could this cause issues. There are air vents installed in the loft however.
Could there be any other reason for a degradation in flow rate?
So our pumps are in sequence with the second pump installed in the middle of the property maybe 10m away. Based on your previous response you don't think this would cause an issue with distortion? Is your suggestion to keep both pumps in that case?
Many thanks again
Whilst the information in the table may be factually correct, I wonder what size the heat emitters would have to be to dissipate even 14kW of heat energy at a water temperature of 35C.
If the present heat demand of a home is 14kW, then the LWT to meet this heat demand will be far higher than 35C, probably more in the region of 50C to 55C. The data in the table is therefore rather meaningless.
Probably the best overall efficiency would be achieved by operating the system at the lowest LWT, that supplies sufficient heat energy into the home, at a water flow rate that produces a DeltaT of around 5C. None of the above parameters need to be set in tablets of stone, and may indeed need to be varied for different weather conditions.
@william1066 with your setup are you finding your system cycling? What is the heat loss for your property out of interest.
I have no cycling when I use sensible settings. I had been playing around with lowering the flow temp at night, but I have too much thermal mass in the floor to do this and get cycling when I do. But no cycling outside of my experiments.
My heat load is around 14kW. - this is of course only for some days in the year ( -2 degC design temp).
Using kW = l/s ⋅ 4.18 ⋅ ΔT for dT of 5 degC gives me 40 litres per minute for part of the year (when it is -2). This flow rate through a 28mm pipe, gives 1.24 m/s flow velocity. The best practice target is 0.8 m/s, the max should be 1.2 m/s. I am fine with this being slightly over the max, as it is only for a small number of days in the year and I have options with tuning dT. If I did the install again, I would go for 35mm pipes to the first manifold but will live with what I have as the pipes are already installed.
Pipework for upstairs heating circuit will need to be upgraded to 28/35mm as I believe it's only 22mm at the moment
You need to look at the flow rates in each circuit or pay someone to do the design for you.... AND then explain it to you. What is your overall heat load in the upstairs circuit? what flow temp and dT are you targeting in the upstairs circuit?
If you want to do this yourself, the math is all available on the internet, you just need to draw out your circuit, using the heat loads from the survey of the property and work through each circuit one by one. Taking into account what your emitters can actually emit, of course.
This may save you replacing pipes that you might not need to.
As @heacol said, be rigorous about anything in the circuit which looks like it could restrict flow rate and make sure it is suitable for heat pumps. I am removing all flow control and filter equipment that does not have flow/pressure data available OR does but the specs show it does not meet my heat pumps needs.
Finally, I need about 40 litres per minute for the whole house, but after reaching the first manifold, I only need about 30 liters per minute for the next three manifolds. The last manifold only needs about 10 litres per minute (rough estimates, but you get the picture). I am not planning on zoning the house at all, so can rely on all circuits being open all the time (perhaps with the exception of the bedrooms)
Pipework for upstairs heating circuit will need to be upgraded to 28/35mm as I believe it's only 22mm at the moment
You need to look at the flow rates in each circuit or pay someone to do the design for you.... AND then explain it to you. What is your overall heat load in the upstairs circuit? what flow temp and dT are you targeting in the upstairs circuit?
If you want to do this yourself, the math is all available on the internet, you just need to draw out your circuit, using the heat loads from the survey of the property and work through each circuit one by one. Taking into account what your emitters can actually emit, of course.
This may save you replacing pipes that you might not need to.
As @heacol said, be rigorous about anything in the circuit which looks like it could restrict flow rate and make sure it is suitable for heat pumps. I am removing all flow control and filter equipment that does not have flow/pressure data available OR does but the specs show it does not meet my heat pumps needs.
Finally, I need about 40 litres per minute for the whole house, but after reaching the first manifold, I only need about 30 liters per minute for the next three manifolds. The last manifold only needs about 10 litres per minute (rough estimates, but you get the picture). I am not planning on zoning the house at all, so can rely on all circuits being open all the time (perhaps with the exception of the bedrooms)
From the heat loss survey it looks like upstairs has a heat loss of 5.69KW. I've also been told that some heat loss calculations are overly conservative it depends on the software used so it might be the actual heat loss could be less than this. We are certainly going to be putting in insulation under the floors and will be installing more insulation in the loft which should bring down the heat loss further.
The problem I've found is finding someone I trust to do a good job. After being burnt with the installer first time round I'm being a lot more cautious about who takes on the work and is able to provide the right information required like a proper design based on the heat loss of the property.
I'm happy to try giving it a shot myself if you could share the resources that you found useful for the calculations.
Have you installed flow meters in your install so you can see flow rates through your system?
@william1066 Grant engineer said the flow rate would be lower with DHW because it's heating to a higher temperature unlike the heating circuit
Oh dear, that's a worry. The Grant engineer doesn't know how the Chofu/Grant ASHP works. It has a fixed speed/head curve pump, flow rate is constant regardless of leaving and return water temperatures. Flow rate is manually set, by one of 3 fixed speed/head curves via dip switches at the ASHP control board, and fine tuned with a flow setting valve. If anything the flow rate during DHW generation is higher than for CH, because the heating coil in the cylinder has lower pressure loss than the CH circuit. Lower pressure = higher flow with a fixed speed pump and vice versa.
Doesn't matter anyway, but it's another example highlighting poor training of the industry installers/engineers and lack of product understanding by some of the bigger names.
Thinking about installing a heat pump but unsure where to start? Already have one but it’s not performing as expected? Or are you locked in a frustrating dispute with an installer or manufacturer? We’re here to help.
