Optimising a heat pump

70sbudgie

NedS said:

My general checklist, in no particular order:

1. Reduce unnecessary complexity in the design (buffers, LLH, multiple zones and TRVs all add complexity that potentially reduces performance). Try to keep the ASHP within 10m of the tank/plant to minimise the pipe run.

I already have 3 zones (2x underfloor and all radiators). I have been quoted for a LLH because I have UFH and radiators. What would be the impact of omitting (or even including this)?

The tank and the HP will be within about 3m. But I don't really have much choice in the location.

2. Maximise emitter size (try to increase radiator sizes to the maximum permitted by the available space. Use K2 and consider 700mm high replacements for more standard 600mm radiators etc to maximise the space). Smaller radiators = higher flow temps = higher running costs.

Most of my radiators are already K2, but upgrading those that aren't seems an easy win. I also like the idea of going to 700mm high - are these likely to be ok on the same wall brackets?

I have also been quoted for a K3 in one location is there a reason you didn't mention K3?

3. Maximise the system volume. Maximising radiator sizes will help, as will a volumiser tank (not a buffer) on the return flow and using 28mm/22mm pipework throughout (copper pipes within the heated envelope of the house are also emitters if they are not insulated - see point 2). A large volume, and hence large thermal mass, really helps in winter with defrost cycles and cycling. Aim for a system volume maybe 4 times the recommended minimum (e.g, 200L for a 50L recommended minimum). Run a single zone with no TRVs (or all fully open) to maximise the available volume.

How do I identify the system volume? I think I already have a large volume because of the UFH.

4. Pay attention to flow rates, they need to be significantly higher than for gas boilers (typically 20-30L/min). Large pipe sizes help and also help increase volume (point 3).

Again, how do I actually find out what this is? 

5. Run with weather compensation - automatically adjusts the flow temps to match the outside ambient temperature.

I intend to do this. Am I right to think that balancing the system correctly will make a big difference to how effective this is?

6. Don't undersize the heat pump - thrashing your heat pump to death when it's -5C outside to try to stay warm isn't going to help it's performance.

What do you mean by this? I think this is something I'm instinctively trying to achieve. Design temp for my postcode is -3.8°C (according to Heat Punk). 47°C flow temp seems a bit too close to the max (?) of 50°C, so I would like to bring that down, if I can. 

7. Site the ASHP in a south facing sunny location if you are able as the air may be a degree or two warmer which can make a big difference over the year.

I don't really have much choice of location, there is one obvious place, but it can be a bit of a sun trap in the summer.

8. Switch to a tariff with cheaper rate electricity, especially in winter, as you'll be using a lot of it. Solar panels in summer can offset running costs in winter.

I generally keep an eye on my electricity tariff.

FreeBear

NedS said: 4. Pay attention to flow rates, they need to be significantly higher than for gas boilers (typically 20-30L/min). Large pipe sizes help and also help increase volume (point 3).

Are you sure on the flow rate ?

I've been running my gas boiler at low temperatures (at one point, down to 35°C) and flow rate is just 4l/m.

70sbudgie said: Most of my radiators are already K2, but upgrading those that aren't seems an easy win. I also like the idea of going to 700mm high - are these likely to be ok on the same wall brackets?

I have also been quoted for a K3 in one location is there a reason you didn't mention K3?

Highly unlikely a 700m rad will fit on to brackets intended for a 600mm. Also unlikely a 600mm rad from one manufacturer will fit on brackets from another.

As for a K3, they stick out from the wall by 180mm to 190mm (compared to the 130-140mm for a K2). Big ugly brutes, and also darned expensive should you ever need to replace one.

Reed_Richards

My comments on @NedS s list:

1) Don't sacrifice control for the sake of simplicity for the sake of economy.  Also you may be required to have a buffer, volumiser or LLH to meet the terms of the heat pump warranty.
2) Yes, but make sure your radiators are balanced to the heat requirement of each room or you could end up with a few rooms that never get warm enough (or most of your rads heavily throttled to prevent that).  K3 radiators are fat and look a bit odd if you are used to conventional ones
3) Different heat pumps have different defrost strategies.  Mine has a buffer which it uses to defrost without robbing heat from the radiators or hot water cylinder.
4) Low temperature gas boilers and heat pumps operate at similar water temperatures but heat pumps tend to aim for a 5 C temperature drop between flow and return whilst gas boilers allow this to be larger.  Having a smaller temperature drop squeezes a bit more heat out of each radiator but requires the pump to work harder to give you a higher flow rate.  But I'm not aware of any sensible reason why gas boilers and heat pumps are set up differently.
5) Yes, definitely use Weather Compensation.  If you have your system perfectly balanced and want the same internal temperature 24/7 then you can turn off your internal temperature controls; personally I think that's a bit extreme and I most certainly don't want the same temperature 24/7. 
6) Conventional wisdom is don't oversize your heat pump.  It's rarely - 5 C outside but a lot of the year it could be relatively mild out so your heat pump only needs to operate at low power.  Heat pumps don't have the modulation range of a gas boiler and the modulation range can vary a lot from one model to another.  If your heat pump can't modulate down to the low output required for milder weather if will be forced to make the water hotter and cycle and that will reduce your efficiency.  It could be cycling for months for the sake of not working so hard on a few really cold days so I think Ned is wrong on this point.
7) Site your heat pump where it's convenient.  I'm not convinced this "warmer air" idea is true and in winter when you are working your heat pump the hardest we don't get much sun so there won't be any difference.
8) I now have cheap electricity at night and use that to heat my hot water and get the central heating going.  I haven't worked out how much money this saves me.   

NedS

70sbudgie said:

2. Maximise emitter size (try to increase radiator sizes to the maximum permitted by the available space. Use K2 and consider 700mm high replacements for more standard 600mm radiators etc to maximise the space). Smaller radiators = higher flow temps = higher running costs.

Most of my radiators are already K2, but upgrading those that aren't seems an easy win. I also like the idea of going to 700mm high - are these likely to be ok on the same wall brackets?

I have also been quoted for a K3 in one location is there a reason you didn't mention K3?

No reason. Most installers will try to avoid K3 where possible and only use them where necessary to meet the heat loss of the room where a K2 is unable to do that due to size restraints. This often happens in places like bathrooms where MCS design specs recommend 23C target room temp and the owner wants a towel rail that has relatively low output.

70sbudgie said:

3. Maximise the system volume. Maximising radiator sizes will help, as will a volumiser tank (not a buffer) on the return flow and using 28mm/22mm pipework throughout (copper pipes within the heated envelope of the house are also emitters if they are not insulated - see point 2). A large volume, and hence large thermal mass, really helps in winter with defrost cycles and cycling. Aim for a system volume maybe 4 times the recommended minimum (e.g, 200L for a 50L recommended minimum). Run a single zone with no TRVs (or all fully open) to maximise the available volume.

How do I identify the system volume? I think I already have a large volume because of the UFH.

4. Pay attention to flow rates, they need to be significantly higher than for gas boilers (typically 20-30L/min). Large pipe sizes help and also help increase volume (point 3).

Again, how do I actually find out what this is? 

You can calculate system volume. Most radiator manufacturers will list the volume for their radiator sizes, and if not you can use those from another manufacturer for the same sized radiators (I often use Stelrad as a reference as they stock a wide range of sizes) as that will be close enough. You can estimate or measure your pipe run lengths and calculate the volume knowing the pipe size diameter and then add in anything like a volumiser.

Your system will have a flow meter fitted to measure flow rates. Most pumps are adjustable, either as fixed rate or PWM to adjust the flow rate.

NedS

Reed_Richards said:

My comments on @NedS s list:

6) Conventional wisdom is don't oversize your heat pump.  It's rarely - 5 C outside but a lot of the year it could be relatively mild out so your heat pump only needs to operate at low power.  Heat pumps don't have the modulation range of a gas boiler and the modulation range can vary a lot from one model to another.  If your heat pump can't modulate down to the low output required for milder weather if will be forced to make the water hotter and cycle and that will reduce your efficiency.  It could be cycling for months for the sake of not working so hard on a few really cold days so I think Ned is wrong on this point.

This is a really difficult one for me, but I'll try to explain my thinking. By way of a bit of background - we had a heat loss survey of 7.4kWh which I thought was on the high side (they often are). The installer specified a 12kW Samsung unit (actually a 16kW unit limited to 12kW output in software). I knew this would be massively oversized for spring/autumn and would cycle so I pushed hard for the 8kW unit instead. Trouble is, the 8kW unit can only deliver less than 7kW output at -2C (design temp) so didn't meet the predicted heat loss. They ignored me and installed the 12kW unit.

Sure enough, in autumn we were unable to run constantly on a low and slow setting (32-33C flow temps) as the unit simply put out way too much heat, so we ended up running more like a conventional boiler with the heating running as low as possible for a few hours until the house was warm enough and then turning off. A smaller unit would have allowed us to do longer runs at maybe 30C which may have been marginally more efficient, but we were still getting COPs of over 5 and not using that much electricity as it's still mild outside.

Then into winter, where we are able to run constantly, but only once the temp gets down to 5-10C and below. We are still running low and slow with flow temps of 32-35C depending on the weather, and the heat pump is running well within itself and performing well.

Heat pumps are most efficient when the compressor is running in the 30-70% range. Below 25% performance falls off a cliff, hence why they will only modulate down to around a quarter of their rated output (this is software limited by the manufacturer to prevent their performance figures from looking awful), and above 70% performance again tails off. It's a bit like driving a car - fuel efficiency is going to be best driving at 50mph rather than at 10mph or 100mph - you probably don't want a car that will only do 70mph flat out if you do most of your mileage driving on a motorway. So if you have a heat pump that can only just meet your heat loss on the coldest days, you are going to be thrashing it and getting poorer performance just at the very time you are using the most energy and want it to be as efficient as possible. Hence I'd rather sacrifice a little efficiency during spring/autumn when I'm using very little energy and already achieving very high COPs to gain a little when COPs are poor and my usage is through the roof. And it's really not that inefficient to run for a couple hours in spring/autumn and then turn off for a couple hours.

Another consideration is defrost cycles. These generally happen when ambient temps are between 2C and -3C, and there is moisture in the air (i.e, a normal British winter). The system frosts up and needs to take energy from the system to defrost itself. The harder a system is working, the quicker it is likely to frost up and the harder it then has to work to recover the lost heat. It is unfortunately not uncommon for systems that have been very tightly specified to frost up more regularly and then have to take heat out of the system to defrost and end up in a cycle where they are then unable to provide sufficient heat to the house (we've all read the headlines - I have a heat pump but my house is always cold in winter). Similar can happen when the system diverts from space heating to DHW for maybe 60-90mins per day, and is having to work flat out to catch up that lost heating time. If there is no headroom in the system then things can spiral quickly.

Finding the right sized heat pump for your property is a near impossible task (because, as you say, ASHPs simply don't have the modulation range a boiler has). There is often a sweet spot or goldilocks zone for your property which may not be met by the manufacturer. As in my case, the 8kW unit was just too small, and the 12kW (actually a 16kW) is too big, and a 9-10kW unit would have been better.

Anyway, knowing what I know now, having lived with my system through both mild and colder weather, I am now very glad I have an oversized unit verses an undersized unit, and am glad the installer insisted on the 12kW unit for me (I now accept and understand why the 8kW unit would have been a mistake). I'm just trying to highlight some of the potential issues and considerations to help prevent folks making the same mistake I almost made, as I too was influenced by that conventional wisdom to not oversize the heat pump*. Of course a correctly sized heat pump is always best, but where that is not possible or does not exist, I'd rather go too big than too small.

* Which comes from the practice of commonly installing massively oversized gas/oil boilers - our last oil boiler was 18kW and the boiler before that was a 24kW unit.

Edited to add: This year the highest generation we have seen in a day is 95kW, giving a heat loss of ~4kWh so that 8kW unit would have been fine so far this winter, but we do keep the house at 19-20C rather than the 21C design temp.

70sbudgie

I am a little confused by the comment

Heat pumps don't have the modulation range of a gas boiler

I thought that was the whole point of the weather compensation - to modulate the flow temperature. And I thought gas boilers rarely modulate, they switch on and off. 

What words do I look for to find out the modulation range of my proposed HP? It is an Ecodan 11.2kW. 

NedS

70sbudgie said:

I am a little confused by the comment

Heat pumps don't have the modulation range of a gas boiler

I thought that was the whole point of the weather compensation - to modulate the flow temperature. And I thought gas boilers rarely modulate, they switch on and off. 

What words do I look for to find out the modulation range of my proposed HP? It is an Ecodan 11.2kW. 

Adjusting flow temperatures is not the same as modulating.

Modulating refers to reducing the energy output (or rather input) of a heat pump. The compressor speed will reduce as the heat pump modulates down and reduce it's output as a result. As an example, my heat pump will draw ~800W as a minimum, and how much heat it outputs for that 800W input will depend on the COP at the time. With a COP of 3 in very cold weather, it will be generating 2.4kW heat (0.8kW x 3). In milder times, with a COP of 4.5, it will be generating 3.6kW of heat. In the mildest of conditions when we need the least amount of heat, it will perversely be generating the most (as a minimum amount) with a COP of maybe 5 for example, giving 4kW when the house may only need 1.5kW.

This is different to setting the flow temperature, which you can set to anything you like, or use weather compensation to try to adjust it automatically depending on ambient weather conditions.

Where they are related is if the system cannot dissipate the heat being generated. Say on a mild spring day you have a COP of 5, and the heat pump's minimum output is 4kW, and you've set your flow temps low to 32C but at that flow temp your emitters cannot emit the 4kW of heat being generated, then the heat pump will cycle (turn off) if it cannot modulate it's output any lower. Or you may have to turn the flow temps up to 40C or 45C to be able to dissipate all of the 4kW of heat that is being generated, and then the house gets too hot very quickly and again you are turning off the heat pump (or opening the windows).

NedS

70sbudgie said:

What words do I look for to find out the modulation range of my proposed HP? It is an Ecodan 11.2kW. 

Not many/all manufacturers publish that information. As a general rule of thumb, most will modulate down to around 25% so you might reasonably expect it's minimum output to be in the region of around 3kW.

Some manufactures may publish tables of minimum input power and corresponding output at a range of temps, but by no means do all publish this data. I've not looked at the Ecodan's to know.

Whilst it is an issue, for the reasons I discussed above I think it's actually more important to ensure it is sufficiently capable to provide the required output at the design temperature. It's easier to turn a heat pump off in spring than it is to find an alternative additional source of heating in winter if the heat pump is constantly frosting up and can't cope when it's -5C outside and we are in the middle of the latest cold snap.

FreeBear

70sbudgie said:

I am a little confused by the comment

Heat pumps don't have the modulation range of a gas boiler

I thought that was the whole point of the weather compensation - to modulate the flow temperature. And I thought gas boilers rarely modulate, they switch on and off.

Modern gas boilers do modulate as it is far more efficient than on/off control. But gas boilers are often hugely oversized for central heating (mine is rated at 30kW), so a 10:1 modulation range is not that uncommon. However, you are unlikely to find a boiler that will modulate below 3kW.

If you are interested, a graph showing heat output of my boiler (a Viessmann 050 30kW combi).

After 09:10, the thermostat is calling for a lower flow temperature as the house heats up.

Reed_Richards

70sbudgie said:

I am a little confused by the comment

Heat pumps don't have the modulation range of a gas boiler

I thought that was the whole point of the weather compensation - to modulate the flow temperature. And I thought gas boilers rarely modulate, they switch on and off. 

What words do I look for to find out the modulation range of my proposed HP? It is an Ecodan 11.2kW. 

You're confusing gas boilers, which typically have a wide modulation range, with oil boilers, which are rarely capable of modulation and do indeed just switch on and off.  But the modulation I am talking about is the power that they draw.  The lower limit for any heat pump can be hard to discover.  In the spec sheet of your proposed Ecodan 11.2 kW it mentions a power draw of 1.88 kW giving an output of 6 kW under certain operating conditions but there is no indication of whether that is a minimum.