ECO4 ASHP, Solar and insulation

NedS

Hi All,

We qualify for an ECO4 grant and have been approved for the following work. I'm in the very early stages of figuring out if this is something we should go for, and need some advice and pointers along the way. At the moment, I don't know what I don't know so please feel free to educate me - I'll tell you what I do know below.

Background

We live in a largely detached property (small adjoining property next door), half of which is pre-1800's solid stone walls and half is brick built cavity extension. It's poorly insulated with an oil fired combi-boiler for heating and hot water (Worcester Bosch, 6 years old, 90% efficient, 12kW)

ASHP system

The scheme is proposing an external 8kW ASHP (Midea) with internal Domestic Hot Water (DHW) cylinder (200L specified for 3-bed property). Replace all radiators (heating guy came to survey this last week) with larger where possible, and installing 2 additional radiators in larger rooms. Pipework to be upgraded - we currently have 22mm loop with micro-bore (9mm?) tee'd off for each radiator. From my research into ASHPs so far, I understand flow rate is key. They will use 28mm for the initial run from ASHP into the existing 22mm loop, and then replace the micro-bore to rads.

I have done some research on Midea ASHPs as I'd not heard of the brand, and gone through their technical specs and they seem OK. From what I've read, an 8kW ASHP seems the correct output for us - I'd calculated between 5.8-7.2kW using heating degree days data for our area.

Solar

Proposing 8 x 434W panels (3.47kW total) and inverter. I do not know the brands (I will be asking). I will be trying to get them to up this to 10 panels if I can, on a 3.6kW inverter I assume? We have a large south facing roof in the south of UK. I do not think there is much scope to add clever/extra stuff here - batteries are not an option, so it's likely a very basic solar / inverter installation.

Insulation

Roof space is split between vaulted ceilings and flat ceiling with loft space above. The loft space was previously insulated under a grant with the rolls of itchy fibreglass stuff - they said they may replace this, or top up if required.

The vaulted ceilings are uninsulated - they proposed fitting 4" insulation board over the existing plasterboard, new plasterboard over the top and plastering. We can easily accommodate the loss of ceiling height so I'm happy with this.

Brick cavity walls already have cavity wall insulation (previously filled on a grant ~15 years ago).

The propose to fit internal wall insulation to the externally facing solid stone walls, plastered over.

Costs comparison

I've tried to do some back of an envelope calcs on usage and costs.

We currently use ~1600L of oil per year. This has been reducing from maybe 2000L peak in the past (a cold year?) where we have gradually turned down the thermostat to save money.

So I'll base the calcs on 1600L oil usage. Kerosene has an output of 10.35kWh per litre and my boiler is ~90% efficient, so 1600L * 10.35 * 90% = 14904kW - lets round that up to 15000kW annual energy usage for heating.

Oil boiler: 1600L at an average price of 70p/L = £1120/year

ASHP: 15000kW at 28.71p at 300% average efficiency = £1435/year

So, all things being equal, based on a SCOP of 3.0, the ASHP is a little more expensive to run - BUT this fails to allow for the increased insulation and the solar input we would now have.

What can I reasonably expect a 3.5kW solar array to generate per year usable (not exported back to grid)? We are both home all day, and our current monthly electricity usage is 425-450kWh in summer rising to 600-625kWh in winter (~6000kWh annual usage). A quick bit of googling and back on the envelope maths suggest I might be able to save around £350/year from usage and exporting the excess back to the grid which brings the costs in line with each other.

Is it reasonable to assume my overall costs will be similar once the added insulation and solar are factored in? I guess this really all depends on the SCOP value the system achieves in practice, as with a SCOP of 1 my bills are 4 times higher at £4300/year as electricity is ~4 times the price of oil per kW output. So I recognise the importance of maximising system efficiency.

I've been reading up on COP/SCOP efficiency, and understand flow rates are important, and this varies with outside air temps (over which I have no control) and the leaving water temp which I can control. Reducing the LWT increases COP/efficiency, hence the importance of having more / larger radiators so the temps can be reduced. Also, as someone used to turning back the termostat or turning off overnight, I'm beginning to understand that it is potentially more efficient to run an ASHP 24/7 at a low temp that to turn it off and then have to turn the temps up in the morning to reheat a cold house. I guess the balance point will also depend a lot on heat losses, hence the importance of insulation.

I have tables of data for COP rates at different outside air temps and LWT temps. Can I ask, what LWT do people run their ASHPs at where they have a radiator-based system? How low can you run it? Also, I understand some systems use a temperature adjusted system where the LWT is automatically adjusted based on the outside air temp, which obviously seeks to maximise efficiency (COP) by adjusting down the LWT when it is warmer outside giving a double efficiency gain.

Anyway, that's where I'm at. Please tell me what I don't know and what else I need to consider (point me in the right direction and hopefully I can do my own research).

My main concern is still the insulation quality of the property - realistically this is never going to be a well insulated property, which is probably reflected in our current energy usage. Are we going to be able to get it well enough insulated for the ASHP to be able to operate within a window of LWTs where it will be highly efficient. For example, looking at the data for 0C outside air temp, we have the following COP values for LWTs: 

LWT  COP

35C   3.88

40C   3.37

45C   3.05

50C   2.79

55C   2.41

If it's 10C outside, and we can turn the LWT down to 40C, the COP is 4.48 which may be achievable in spring/autumn (or 5.58 at 35C)

I'm not sure if this is a useful experiment, but we have tried turning down our oil boiler to 50C rad temp to get a feel for what running at lower temps might be like, and at 50C (it's lowest setting), the system is currently just about keeping the house warm at 20C (with 10C outside temp) but does obviously take longer to heat it up after it's been turned off overnight and the temps have fallen. It would be nice if we could have turned it down to 40C or 45C to see how well that worked! Obviously having more / larger rads (and improved insulation) is designed to offset the lower LWTs.

The worry would be that when it's 0C, if we need to turn it up to 55C LWT to keep the house at 20C, the COP for 0C outside and 55C LWT is only 2.41 meaning it's costing 11.9p per kWh versus 7.5p per kWh for oil, so ~60% more expensive to run during winter under those conditions - ouch!

So the key to an ASHP operating efficiently and potentially saving money (or at least not costing a LOT more to run than oil or gas) is for it to be able to operate in a window of low LWTs where it is most efficient, which relies on warmer outside ambient temps (which we cannot control) and low heat losses which we can control through good insulation.

QrizB

Assuming your roof faices south-ish and you're in England or Wales, you can expect to generate 3000-3500 kWh of electricity a year. You'll probably be able to use 25-35% of that, say 1000-1200kWh a year.

There's a useful tool called PVGIS which will give you a fair estimate if you tell it where you live and give some details of your roof:

https://joint-research-centre.ec.europa.eu/photovoltaic-geographical-information-system-pvgis_en

Any electricity that you don't use, you can sell back to the grid. Depending on which company and tariff you choose, you might get 15p/kWh for it. If you export 2000 kWh a year, that's £300.

Regarding LWT, turning up the temperature won't increase the output of your heat pump. It only changes the output of the radiators. If your radiators are large enough, you won't need a high LWT.

NedS

Thanks - I'd already seem that website which is excellent.

Yes, roof is south facing at , and that site is saying 3500-3600kWh/year for my location depending on data set used for a 3.6kWp array. I selected "free standing" for the array, as it's not built into the roof but installed on top with ventilation underneath. Not sure if that is correct (it seems to add around 100kWh per year to the result), all other settings at default. It seems to broadly agree with the upper end of your 3-3500kWh guidance.

Reed_Richards

NedS said:

So the key to an ASHP operating efficiently and potentially saving money (or at least not costing a LOT more to run than oil or gas) is for it to be able to operate in a window of low LWTs where it is most efficient, which relies on warmer outside ambient temps (which we cannot control) and low heat losses which we can control through good insulation.

This is not entirely correct.  All heating systems will cost you more to run when it's colder outside.  ASHP will operate most efficiently when the outside temperatures are warmer and it will cost you proportionately more than your oil boiler when it gets colder.   But you should be given a figure for the Seasonal Coefficient of Performance (SCOP) which is an average for the entire year.  So if, for example, the SCOP is 3 and your annual heat requirement is 15,000 kWh then that should require 5,000 kWh of electricity.

NedS said:

The worry would be that when it's 0C, if we need to turn it up to 55C LWT to keep the house at 20C, the COP for 0C outside and 55C LWT is only 2.41 meaning it's costing 11.9p per kWh versus 7.5p per kWh for oil, so ~60% more expensive to run during winter under those conditions - ouch!

My heat pump is specified to need 50 C LWT when it's -3.4 C outside to keep the house at 21 C (using radiators).  But your quote should give you a figure for this, based on an outside temperature that is exceeded 99.6% (IIRC)  of the time and the maximum LWT your heat pump is set up for.  Actually my heat pump does better than specified.  Still, when it's really cold out your heat pump will be a lot more expensive to run than an oil boiler; averaged over an entire year it's not nearly so bad. 

Meatballs

With eco4 you aren't tied to one provider. Shop around and check for local reviews. If you can get a target flow temp of 45c or lower as this really helps to bring electric prices inline with gas -although consider octopus tracker or agile with average prices currently about 15pkWh.

Eco4 is a minefield for poor subcontracting and over specified units because the end customer isn't paying. Have seen some very poor outcomes so make sure you find a supplier you trust.

NedS

Thanks, been reading loads and feel I have a lot better understanding now.

The ASHP seems reasonably well specified at 8kW. We will definitely push for 10 rather than 8 solar panels.

I've done some tests with our current oil boiler, and set to it's lowest setting of 50C (it kicks in when the water temp drops to 40C and kicks out again at 50C), it has just about managed to maintain an acceptable temp in the house over the last few days, where the temps have dropped a little down to 2-3C overnight (we are generally mild here and don't tend to see temps below 0C), but it's been marginal at best. I wouldn't want to be running my ASHP at 50C flow temps?

Presumably things will improve with the added insulation and the larger rads, but by how much is a guessing game. I guess insulation (heat loss) really is the key to getting the operating temps right down. We will pay particular attention to what they are saying on the insulation front. We have a vaulted ceiling in the main living area of the house that has no insulation and everyone gets a bit vague around that - the quote says loft insulation, but there is no loft there. In my mind it will need at least 4" of insulation board (Kingspan TP10 100mm) applying with plasterboard over which should make a huge difference to our heat loss.

NedS

NedS said:

Thanks - I'd already seem that website which is excellent.

Yes, roof is south facing at , and that site is saying 3500-3600kWh/year for my location depending on data set used for a 3.6kWp array. I selected "free standing" for the array, as it's not built into the roof but installed on top with ventilation underneath. Not sure if that is correct (it seems to add around 100kWh per year to the result), all other settings at default. It seems to broadly agree with the upper end of your 3-3500kWh guidance.

After accurately measuring our angles, I've further adjusted for our roof slop and azimuth, and adjusted down for 8 x 434W panels giving 3350kWh/year.

We already use ~6000kWh/year, and the ASHP will increase that further, so I'm confident we will be able to maximise our usage fairly well negating the need for a battery (especially with Octopus 15p SEG rates).

NedS

Meatballs said:

With eco4 you aren't tied to one provider. Shop around and check for local reviews. If you can get a target flow temp of 45c or lower as this really helps to bring electric prices inline with gas -although consider octopus tracker or agile with average prices currently about 15pkWh.

Yes, I've been looking at Octopus Agile and like the look of that - it seems far more suited to our needs than the Cosy tariff. I think I'm going to switch to Octopus now to get that in place ASAP.

We don't really have that much electricity usage that we can shift - during 4-7pm we will still need to cook with the electric cooker at peak rates and will have to suck that up, but we can heat water (ASHP) and use washing machine at cheaper times.

When prices fall away overnight, I'm thinking it would be beneficial to still run the heating overnight rather than allowing the house to cool too much, as it will just cost more during the day to heat the house back up. We would then hope to turn the heating off completely 4-7pm and hope temps do not fall too much (heat from cooking during this time will help) allowing us to minimise our peak 4-7pm usage. As we are in all day, keeping the house at a petty constant 19C would work well for us, if it didn't drop more than 1-1.5C during 4-7pm

MultiFuelBurner

If it in any way helps we moved to Agile on 15th Feb this year and set our ASHP from 24 hours a day constant flow of 35oC to off during 4-7pm and on the other 21 hours. We don't notice the temp drops and we have continued to use the oven, hob and microwave etc during the 4-7pm period.

Since the 15th Feb we have averaged 13p kWh on Agile but we can take advantage of cheap slots with programming hot water, washing machine and dishwasher loads to make the most of them and are around as we work from home to put the tumble dryer on in cheap slots during the day etc.

Sadly we will probably move before it's worth getting solar and batteries as we do tend to move every 5 years on average. But I could see us making our way easily to zero energy bills if Mrs MFB would just stay away from Rightmove and our favourite local builders webpage.

Reed_Richards

NedS said:

Presumably things will improve with the added insulation and the larger rads, but by how much is a guessing game. 

I disagree completely; it's anything but guesswork. 

You can look up tables of how radiator output varies with the average water temperature inside the radiator and the desired room temperature.  So when you had 50 C in, say 40 C out then your average radiator temperature was 45 C, if your room temperature was 21 C then you were operating at a Delta T of (45 -21) = 24 C.  So then you can consult a table like the one here https://www.clyderadiators.co.uk/delta-t-conversion and use that to work out what size radiator you need for a given delta T to achieve a specified heat output

Likewise if you know how the fabric of your house is constructed you (or your installer) should be able to make or reasonable estimate of how rapidly each room will lose heat for a given outdoor and indoor temperature.  So you match the rate of heat loss to an outside temperature that is rarely encountered and size each radiator so it is big enough to cope.  Add all the radiators heat outputs together, allow for heating the hot watetr and that tells you how big a heat pump you need.

matelodave

I agree - you can do some fairly detailed calculations which make the determination of heat requirements versus radiator sizes, flow temps and other parameters much more accurate than a random guess, which is how it used to be done.

There are plenty of calculators on t'interweb to help you input all the details together with insulation values for various building materials which can get you pretty close to the optimum.

Doing the sums yourself and working out your requirements can help to evaluate quotations and  knowing how the system works should help you to operate the system a bit more effectively in the future.

Most of the problems that people encounter are for poorly specified systems and a lack of understanding how the system works and how to operate it effectively. Hopefully there are far fewer poorly spec's systems nowadays as MCS specs have been tightened and there is a lot more knowledge about (although you should still beware of cowboys - grants and incentives etc seem to  bring a lot of them out of the woodwork).

However even a well spec'd system operated badly will give you a poor experience. They aren't like gas or oil boilers and so need a bit of understanding to get them to operate both effectively and economically.

I've had mine for fourteen years and it did take a bit of perseverance and tweaking especially in the first winter (which was exceedingly cold) to get it working well and economically. It now chunters away 24/7 doing its thing, we are warm, cozy and its still pretty economical to run even with todays stupidly high electricity prices