Improve efficiency: Was it worth it?

MyForest · 21 February 2020 15:20

Good to know you’ve got your GSHP working well too.

The initial problem with my setup was just as you describe, an aggressive cycling of 5 minutes on then 5 minutes off. Here’s 2019-10-19:

I don’t see how to use the vendor’s controls to put hysteresis into the heat pump. I did widen the band in the room thermostat to 5 Celsius but it is also missing a “delay” setting to slow it down.

My control algorithm has a weather-dependent delay algorithm which seems to be working OK.

I’m holding down the flow temp for efficiency, so my “on” time is longer than your ten minutes, but I am seeing 1 1/2 hours between cycles like you. Just as an example, today some of my cycles were on for 50 mins and some were 15 mins. The long run was the first one of the day and the outside temps were obviously colder early in the morning.

Here’s how things looked this morning (2020-02-21). I’m using the return temperature to tell me when get going again - it’s a proxy for the radiator temps.

stephen · 21 February 2020 15:57

well I guess for my gshp it is a water to water ( I am not sure if yours is air to water or water to water heat pump…) I did not change the room temp differential… I changed the the water tank temp differential when I first got it was 2c differential and just the simple short inrush of cold water into the the tank was enough to trigger another run cycle … changing it to 3c was enough of a change to stop that… do you have full access to your heat pump controls or only basic user setting … my heat pump is fully open I can change any and all run parameters… usually they will not give you full access. and here in north America they rarely give even base user access… as they want “servicemen”. just to change the simplest of settings ie temp differential or even restart your heatpump- which can be triggered by a power outage- and i see people sitting months waiting for someone to reset the heat pump and running on backup booster elements for months – maybe search the web you can figure out the service entry button combo and password then you have all the option I mentioned and more

MyForest · 21 February 2020 16:12

Interesting @stephen - I am tempted to tinker with the system settings. Our installer left them open for me because he gauged that I knew what I was doing. I’ve actually left them alone because I don’t want anyone to question my warranty - I’m on a seven year contract with the installer and the UK government for this system and it would be very expensive for me to fall off that contract. They normally leave the systems with the base user settings open in the UK.

I don’t have a buffer tank like you. My air source heat pump is connected directly to the radiators. As a result, it’s load is very much correlated with the room air temp.

I like the sound of what you’ve done with your system to get it behaving nicely. It’s always nice when a small tweak makes things work properly.

TrystanLea · 22 February 2020 09:55

Great to see the performance improvements @MyForest really quite substantial!

It makes a lot of sense to me that turning your heatpump off and on to create longer cycle lengths is providing better performance, given that the heat pump compressor cant reduce it’s speed enough to run continuously without the excessive cycling you saw.

I have been thinking to do similar for night time set back temperatures. My system is stable at 30C flow temperature but that keeps the house often overly warm at night. If I drop the flow to 28C it starts frequent cycling as the radiators are not able to dissipate enough heat to match the minimum input power of the heatpump, so I could do with this approach at those times. I generally turn it off at night when its a bit warmer as a result rather than cause a lot of cycling.

I undersized my heatpump, partly to better match the minimum compressor speed to the typical heat load of the house and radiator design and its given me COP’s that Im very happy with generally. Average is 3.9. The other reason for initial undersizing is that I plan to do efficiency work on the house.

I do a bit of your ‘holding down the flow temp’ for DHW heatup. The standard control scheme just runs the heat pump flat out to heat the tank and can be a bit noisy especially when still and cold outside. To address this I send a flow temperature ramp control scheme, starting from the tank starting temperature through to ~46C (always heating at least the bottom half of the tank up from cold). I dont react to the heatpump flow temperature or heat output during the heat up period as I’m weary of causing issues for the defrost cycle - and the interaction between the heatpumps own control algorithm and mine just becomes unstable if I try and react too quickly… giving it a stable pre-set temperature ramp seems to avoid that.

It would be interesting to hear more about the heat loss factor of your house W/K and kWh/m2.year and heat output rating of your radiators, what’s your total radiator surface area and heat output at their rated values? I see in your other post they are Stelrad K2’s. Is your heatpump a 14kW ecodan?

FredM67 · 23 February 2020 14:57

Hi all,
I’m quite surprised to read that your heat pump was like cycling 80 times a day, that’s a LOT.
I’m heating my house (living surface of 250m2) too with a heat pump. When it’s very cold, around 0°C or less outside, my HP is running about 20mn every 1.5 hours. It’s heating a buffer tank of 400 liters till 35°C. This water circulates in the floor heating… The COP is even higher than the one given by the manufacturer, I’m around 5.5!!
Fred

MyForest · 23 February 2020 15:47

@FredM67 Yep, I was surprised too, but when it was in the “shoulder months” the over-spec of my heat pump becomes a problem with the default control algorithm.

I also wanted a buffer tank but was talked out of it. I accepted that on complexity grounds. I don’t want to create a system that only two engineers who are on-call can fix and then find out they have 10 other customers to fix first.

I have heard a number of stories of heating engineers turning up on-site and finding an unusual system and not knowing how to fix it or set it up properly. I was hoping that after more than 10 years of heat pumps being installed this would be a thing of the past but it’s still happening now.

However, given what I’m hearing about your system and what I’m seeing with mine I could consider a buffer tank. I have space.

You can see in this experiment that I don’t actually need to circulate all the time. My radiators are serving as a buffer tank because they are over-sized. Admittedly I get more fluctuation in my room heating than you would. Being able to turn all the pumps off is good, it makes our house quieter (yes, I know pumps shouldn’t disturb us, but it’s stupidly quiet here)

MyForest · 23 February 2020 16:06

@FredM67 You can see my ASHP is cycling about the same as yours now

Sadly my CoP is only 3.64 though

stephen · 23 February 2020 16:13

sorry – i get a bit of chuckle when I read that. I would consider <-30c very cold and <-40c extremely cold…

not to distract from the topic. But it makes me wonder on your general housing standards over there. as at that temps and other temperatures mentioned I would not even have a heating demand. as I get enough from other heat gains… for me I would be trying to improve the your shell you live in then trying to squeeze more out of the heat pump… as so far all the temps mention you should be at or near zero heating requirements if following similar to Canadian R2000 high efficiency house standards.

FredM67 · 23 February 2020 16:20

Well that with the buffer tank isn’t a tweak from me or any other one. It’s highly recommended by the manufacturer, exactly to avoid that the HP runs to often. The floor heating is making the system very stable… If I want to reduce the temperature in a room, it take like 2-3 days!!
The pump for circulation is so quite, I can only know it’s running by putting my hand on it to sense the very subtle vibration.
By mild weather like today around 10°C the HP is running every 4-5 hours, or even less.

The high quick peaks are when the HP is running.

MyForest · 23 February 2020 16:20

Yep @stephen you’re quite right.

In the UK it’s bad. Most of the old housing stock has poor insulation.

Our house is 200 years old and only one-third of it has wall insulation. I work with a lot of Canadians and they tell me it’s common for houses to only last 30 to 50 years there so when you re-build you bring them to code at the time which means the whole stock is getting better all the time.

We jumped to a heat pump to allow us to get rid of our oil boiler because Greta told us to. The UK government is basically funding the switch to heat pumps at the moment.

We will have to get external wall insulation. The pay-back is about 30 years. Our house is attached to another one and our neighbour isn’t interested in doing it. As a result we’d be doing it to our half of the house which will look weird. It’s not always easy to do the right thing.

MyForest · 23 February 2020 16:27

@FredM67 That looks great.

I like how your system has been designed and how it’s working.

Underfloor heating would be great, but in our 200 year old house it’s a big disruption and it was vetoed.

Most of the systems in the UK don’t have the buffer tank. The installers don’t seem keen on those models. It’s the same with plumbing in the existing hot water system behind my log fire…no-one wants to do it.

I’m willing to live with the limitations as long as things are working OK. I was seeing CoPs of 2.5 at 5 Celsius so was motivated to at least see if I could improve that.

BTW our pump is less than 30dB at one meter - I’m just fussy.

stephen · 23 February 2020 16:47

if a 200 year old house - then it a mostly a high mass house… and your biggest issue would be draft… for me I would insulate from the interior and put in a vapour barrier if you can not do the outside wall… and leave the exterior alone to breath … with modern insulation that might mean loosing about 2 in around the perimeter on the inside to get your self up r20 wall as currently from your estimated heatloss it seams you are around R10 wall. and then blow in R50 in to attic space. and try to insulate to 20R in the floor space… Canadian house today have a lifespan of about 25 years due to the use of engineered joist and the over use of OSB… after that major overhaul will be required at some point soon after. if the house is not well maintained … if you want a to see where you can get the best bang for you buck - download and install remrate it a 14 day trial but that enough time to figure out what you could do to your house to improve it

MyForest · 23 February 2020 16:56

Thanks @stephen I’ll look into remrate.

We’re relatively draft free for such an old house because we blocked up a lot of leaks ten years ago to stop the mice getting in. We had a pressure test done back then and the chap doing it was aghast that a house like this was so leak free. This thermograph shows a lot of heat leaking out of the older part of the house through the walls to the left of the picture (although it did help us find a few more leaks under the windows).

I’d like to keep the walls inside the thermal envelope to keep the thermal mass bigger. This is partly due to my concern about damp between a cold outside wall and a layer of internal insulation.

The Canadian use of timber makes some sense, you have quite a lot more of it than we do.

stephen · 23 February 2020 17:02

that’s why you have a vapour barrier on the inside and you allow to breath outwards… you have more of an issue from the warm damp air migrating through the cool wall. if you prevent it from migrating then there is no issue… currently my outside temps are quite warm ( about 0c ) tonight I will try to take a thermal image

MyForest · 23 February 2020 22:43

@FredM67 This’ll “give you kittens” as we say in the UK.

This is 2019-10-20 when the system was running with it’s default setup. As you can imagine, I didn’t want to leave the system behaving this way.

A similarly warm period of time now looks like this with the new control algorithm. Clearly there are more distinct cycles where it does work and then goes back to sleep. Hmm, it does appear to have cost twice as much. Off to ponder that.

MyForest · 23 February 2020 23:57

Hi Trystan,

I also plan to wrap the house in a nice big blanket but I know I’m not going to get round to it soon so I was OK with being over-specced a bit. Our only backup is the log fire so erred on the side of caution, at the expense (literally) of doing badly when it’s warmer.

On the bright side, it was able to hoover up my solar PV today and quickly heat the tank to kill legionella before it got cloudy again

Before I got Emoncms set up I wouldn’t have been able to easily see the solar versus heat pump curves because they were in two different systems. So, yet again, thank you for Emoncms!

MyForest · 24 February 2020 01:38

@TrystanLea My EPC rather pessimistically thinks I’ll be using 20,468kWh for heat and 2,975kWh for water heating.

That’s 140kWh/m2 a year for space heating.

At a rough guess, based on what we’ve used since October, it’ll actually be about 4,500kWh consumed and 14,000kWh generated for space heating which is 30kWh/m2 consumed.

Hot water should be about 2000kWh produced. Power consumption is a bit hard to gauge until I’ve been through a summer with the solar + heat pump + solar diverter. I’m expecting it to be about 1000kWh for a year. There are four of us.

Oil with our very old boiler was 2,000 litres a year so that’s 2000l * 10kWh * 70% = 14,000kWh so it’s about on-par with that.

Heat loss is pretty bad because we have uninsulated brick walls in two-thirds of the house. We have about 150m2 of wall and 100m2 of roof - there’s also a lot of quite big windows from the 1980s. I haven’t got a W/K figure yet.

Here’s the relevant radiator info and room area in m2. The one wrinkle is that there’s a room with two radiators in it.

As you can see we now have about 6 times the radiator surface area that we had before.

As you guessed, we have a 14kW Ecodan so the power output can take the radiators to ΔT 25. If we imagine the rooms are at 21 °C then that puts the flow temp at 46 °C. With my compensation curve we’d have a flow that high when it is -5 °C outside.

compCurveValue = int(30 - effectiveOutdoorTemperature / 3)

Of course that’s all theoretical because it’s not 21 °C in the house. Some of the TRVs are at 15 °C and some are at 25 °C. It’s pretty hard to describe a house with occupants in random flux, especially in half-term week.

Clearly we need to stop the walls leaking heat so we can bring the power usage down. We’re in the pre 1900 bit of this…

However, if we look at a “cold” day (by UK standards) the we can see we’re consuming 1.26kW for space heating. When I think of something like my fan heater I used to use in my office which consumes 1.7kW I’m amazed that the heat pump is keeping the entire house warm whilst using less power.

I know the numbers still look bad to everyone here.

TrystanLea · 24 February 2020 09:29

Thanks @MyForest Looking at the minimum output at 35C for the 14 kW ecodan it looks like ~6kW at 7C outside temperature. With ~1.5kW elec input.

If all your TRV’s were open it looks like you could absorb that heat at 35C flow temp and keep the heatpump running continuously? But maybe the house would get too warm?

Thanks for all the detail!

MyForest · 24 February 2020 11:15

That page of their manual is my bible. It’s what I’m using to draw the “nominal” parts of the efficiency graph.

There’s no time like the present…

As you can see, it’s trundling along happily. It’s using 1.6kW though which is more than we’d typically be using. Here’s a recent comparison from a slightly colder day where the average is just under 1kW.

I’ve never seen it running stably like this for a long period of time, so now I’m seeing a thing happen that you all talk about that we should expect. The outdoor temperature went from 7 to 8 Celsius at 10:48. About 3 minutes later the return temperature also went up. That might be because the house is warming after an hour running the pump or it might be correlated with the external change. However the same thing just happened when outdoors went from 8 to 9…

So I think the changing external temperature is going to knock it out of steady-state because it’s getting above the target flow. I’m going to keep watching to see.

Of course if I raise the target flow to match the increased outside temp then the radiators will get even hotter and the house will get hotter and at some point it’s just going to be too much. Of course my efficiency will also fall - although it’s current 381% is pretty impressive. At the moment it’s modulated down to it’s minimum of 26Wh per minute (1.56kW) so we’re stuck with that well known problem of a device that’s rated for cold weather struggling to modulate down when it’s warmer. It’s a bit sad that the target cold temp is -2 and the current temp is 8 and it is struggling to manage comfortably across even that small range.

Here’s the buckets of power usage since 2019-10-01:

So it can go down to 1.08kW but it doesn’t seem to want to.

Clearly “off” is it’s most common state by a long way.

MyForest · 24 February 2020 13:06

As expected we’ve started cycling.

I raised the flow temp to 37 and it’s modulated down so it’s using 1.6kW. The flow-return delta is 3 Celsius, that’s quite common here because the house is warm.

As we can see, the flow is still edging up above the target. I’m nudging the flow up to 39.

Unfortunately the sun is breaking through now and the outside temperature is rising. The heat pump just can’t cope running constantly when it’s this warm. Arguably it shouldn’t be needed, but of course our old house is leaking heat through the walls. So we need some heat, but not as much as we get from the pump running all the time.

Bumping it up does seem to have stopped it cycling, but the house is certainly getting too warm in a number of places now. I’m going to have to bring the experiment to an end.