gaschick

Hello from Wiltshire. I am deep in trying to compare ASHP and insulation! Any help gratefully appreciated!!!!!

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Hi, We live in a 5 bed detached house built in 2004 in the middle of North Wiltshire. We have just put in for planning to demolish our existing double garage as we have clay heave/subsidence in one corner and need extra bedroom space upstairs in our house. The garage would be rebuilt adjoining our house, with extra bedroom space above (to even out the sizes of the bedroom for our three children) with much better foundations! We are also going to remove an extremely draughty oak framed conservatory extension in the existing footprint with proper well insulated cavity walls and roof. Our house is just under 200m2 floor space. The new upstairs space above the garage will add about 40m2 to our foot print. We will put in UFH in the new kitchen diner room where the oak framed extension was.

 

We have been investigating replacing our 16 year old oil fired central heating with an air source heat pump for about two years. We are obviously keen to get the new green homes grant, and are obtaining quotes for this work. Two years ago we were quoted £15,000 for a Viessman system, and we were told that we may need to replace 2 radiators. We weren't in a financial position to do this, and there don't seem to be any Viessmann installers in our area currently.

 

Two quotes have come back for supply and commissioning only to meet the RHI standards, and one is for supply and fit. I am now obtaining quotes for the fitting of the system. I am finding it near IMPOSSIBLE to compare quotes and the efficiency of different systems. We have triple glazing throughout (green building store performance windows (value 0.85), apart from the oak framed conservatory extension. The supply only quotes both used the Mitsubishi Ecodan 11.2 kW system, and the supply and install was for the LG V 16kW system!!!! I can't see much of the maths behind to calms. One quote for the supply only has our floor area at 164m2, which seems wrong, and the other supply only has it at just under 200m2, which is right for the current floor area. Each quote has different storage tanks, and other components. 

 

The supply and fit quote was a shade over £13,000 for the LG V 16kW system. I can't understand how such different values for the best type of heat pump to meet our needs could be obtained.

 

I am not a central heating engineer, but it does seem to be extremely difficult to be able to compare the outputs and efficiencies of the heat pumps specified. Each manufacturer reports their output data in different ways, except the SCOP values, as I understand, and the seasonal space heating energy efficiency. However, I can't find the SSHEE in all the specs! We live in a bit of a frost pocket on damp clay, and I am keen to get the maximum bang for my buck. The other issue is that Mitsubishi have run out of stock, with no prospect of anything happening before Christmas, so need to explore other manufacturers.

 

Any advice greatly welcomed!

 

Zoe 

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Right, first thing is to let us all know how much oil you use, in litres, not cash. And preferably if you can split the usage into heating and non heating times.

This will give an idea if how much is used for hot water (DHW) and house heating (space).

 

If you read up about ASHPs you will notice people saying about over sizing them. This is not a mistake, they perform better when working at about 70% of the maximum rated output.  This is quite different from gas and oil heating which generally performs best when at, or close to, maximum output.

Floor area can be a bit confusing, there is a formula that allows for livable area and one that is for total area. Livable area (excluded hallways, landings and bathrooms I think) will always be smaller.

Also the external wall/door/window area is important, a bungalow, terraced, semi detached and detached will all have different thermal losses.

If you are up for a challenge, you can calculate simple losses if you know the U-Value of all the build components.

Edited by SteamyTea
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Do you happen to know the power of your oil boiler? In cold weather do you notice it burning a lot? Very infrequently? 

 

It's quite hard to find a running cost comparison for different fuels and "boilers". This site (currently down for maintenance) is the only one I have found...

 

https://nottenergy.com/resources/energy-cost-comparison/

 

Scroll down to the table. The figure to compare is the column with a heading something like "cost per kwh after boiler efficiency" as this takes into account the COP of a heat pump etc. Its not perfect. People argue the cost of electricity assumed is too high or the COP too low etc. Last time I looked the efficiency of a heat pump didn't quite make up for the higher cost of electricity needed to run it.

 

https://great-home.co.uk/air-source-heat-pumps-an-alternative-to-gas-boilers/

 



The average energy gains over the year are often described as the Seasonal Performance Factor (SPF). For a well installed ASHP a typical SPF for space heating would be 3.0 with a performance for water heating at 2.3. This is because water heating requires a higher temperature to be achieved, upwards of a 55°C gain, and the ASHP is less efficient when delivering these higher temperatures. In comparison, space heating can be operated with lower temperature gains, typically 35°C. 

 

At March 2019 prices, domestic electricity was circa £0.18/kWh with gas at circa £0.04/kWh, meaning that a heat pump would have to deliver an SPF of above 4.5 to deliver a cost benefit on operating costs. If fossil fuel prices rise faster than electricity in the future (driven by higher gas prices or a carbon tax?)  this would deliver a cost benefit at a lower COP. 

 

 

Edited by Temp

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This may be of some help.  I think it is the latest version.  Been a decade since I was doing them, things were more basic then.

 

https://mcscertified.com/wp-content/uploads/2019/08/MIS-3005.pdf

 

And the MCS estimator

 

https://mcscertified.com/wp-content/uploads/2019/09/MCS_031_MCS_Heat_Pump_System_Performance_Estimate_Issue-2.1.xlsm

Edited by SteamyTea

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On 18/10/2020 at 18:11, SteamyTea said:

Right, first thing is to let us all know how much oil you use, in litres, not cash.

Thank you for your replies.

 

We use 1200-1300l of oil per year. I have no idea really how to split this into heating and non-heating times! 

 

We currently use 2900 kWh per year on our house (we have a biodigester plant which seems to need electricity all the time).

On 18/10/2020 at 18:11, SteamyTea said:

And preferably if you can split the usage into heating and non heating times.

This will give an idea if how much is used for hot water (DHW) and house heating (space).

 

If you read up about ASHPs you will notice people saying about over sizing them. This is not a mistake, they perform better when working at about 70% of the maximum rated output.  This is quite different from gas and oil heating which generally performs best when at, or close to, maximum output.

Floor area can be a bit confusing, there is a formula that allows for livable area and one that is for total area. Livable area (excluded hallways, landings and bathrooms I think) will always be smaller.

Also the external wall/door/window area is important, a bungalow, terraced, semi detached and detached will all have different thermal losses.

If you are up for a challenge, you can calculate simple losses if you know the U-Value of all the build components.

We only specifically know the U-value of our windows as we installed them!

My husband has spent most of this week off work putting in extra loft insulation to get it to >300mm as we only have between 150-200mm at the present time. I don't want any arguments when it comes to the assessment of any RHI based on a new EPC.

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On 18/10/2020 at 20:58, SteamyTea said:

This may be of some help.  I think it is the latest version.  Been a decade since I was doing them, things were more basic then.

 

https://mcscertified.com/wp-content/uploads/2019/08/MIS-3005.pdf

 

And the MCS estimator

 

https://mcscertified.com/wp-content/uploads/2019/09/MCS_031_MCS_Heat_Pump_System_Performance_Estimate_Issue-2.1.xlsm

Thank you very much. It's been very difficult to get any heating engineers interested in coming to look at the system with a view to installing an air source heat pump - so I think that the supply and commission only is not viable for us currently.

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28 minutes ago, gaschick said:

Thank you very much. It's been very difficult to get any heating engineers interested in coming to look at the system with a view to installing an air source heat pump - so I think that the supply and commission only is not viable for us currently.


Definitely worth giving SWAT Engineering In Bournemouth a call 

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3 minutes ago, PeterW said:


Definitely worth giving SWAT Engineering In Bournemouth a call 

Is this supply only? We are in between Royal Wootton Bassett and Malmesbury - quite a distance away from Bournemouth.

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13 minutes ago, PeterW said:


Definitely worth giving SWAT Engineering In Bournemouth a call 

Thanks - but they don't appear to be MCS accredited. This means we wouldn't qualify for the RHI after installation.

 

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4 hours ago, gaschick said:

We use 1200-1300l of oil per year. I have no idea really how to split this into heating and non-heating times! 

That will be about 10.5 MWh (assuming boiler is 80% efficient).

How water usage will be about 2.5 MWh a year.  7 kWh/day (for a bath and a shower a day).  Ideally you want to heat water fairly quickly, say 3 hours.  So 2.5 kW.

So that leaves 8 MWh/year for space heating.

Assuming you heat the house for 8 months of the year, that is 1MWh/month.

As a rough average, that is 33 kWh/day.  This is a power delivery of 1.4 kW.

 

Now you hardly need the heating on between May and November, so that is probably really only serious heating for 6 months, with the greater heating needed in December, January and February.  A bit of spare is needed for extreme conditions, so let us tripple the power delivery to 3.5 kW for space heating, plus the DHW of 2.5 kW.  This gives you a rough size of 6 kW for a heat pump.

Now it is normal to oversize a bit, somewhere between 15 and 30%.  So somewhere around 8 kW is, from these rough estimates, is the size of heat pump needed.

 

It is very tempting to install an ASHP that is only just large enough, as this is the way that traditional boilers a sized.  Traditional boilers tend to work best when they are at maximum power, this is because they have less losses per power delivered.

Heat Pumps are not like that, they tend to loose efficiency the closer they get to maximum power, and can even rely on an inbuilt electrical resistance heater.

They also rely on the outside air being dry.  This leads to a quirk where the CoP can be lower at a higher air temperature.  This is because air above 0° C can be more humid, and at 4°C can have the largest amount of water in it (water has greatest density at 4°C).  By over sizing an ASHP, there is a larger area of the heat exchanger (just a car like cooling radiator), This allows for a bit of frosting up while still allowing energy to be transferred (the energy is gained by a cold liquid expanding into the radiator and turning to a gas, and then being warmed by the outside air, some designs are different, the the theory is the same).

 

If you look at your boiler, it should state how large it is.  This may be in kW (kilowatts, not KW, killa wot or the big sin, killer Watts per hour) or BTUs, British Thermal Units.

A BTU is an odd unit, but 1 kW is equal to 3412 BTU/hr (this is where the confusion of with power and energy come from I think).

So if you can have a peak at it sometime and see if you can find a number, that would be useful.

Edited by SteamyTea

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15 minutes ago, tonyshouse said:

Or add insulation 

Yup. Our house was built in 2004 with cavity wall insulation. We are in the (slow) process of adding extra insulation in the loft and some more loft boards. 

 

I forgot to add that the spanner in the works for ALL of this wrt the air source heat pump is the fact that we have some microbore heating pipes (10mm) to each radiator. The world of ASHP seems pretty conflicted as to whether this can really be overcome (as will need a higher pressure to feed the radiators) with a low temp ASHP. Some thoughts seem to be that 'go higher or go home'. But, I have found others who have added some extra pumps, +/- a low loss header/extra buffer storage tank. We are planning to have a 50L buffer if we can anyway.

 

Thanks very much for all of your thoughts so far.

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10 hours ago, SteamyTea said:

That will be about 10.5 MWh (assuming boiler is 80% efficient).

How water usage will be about 2.5 MWh a year.  7 kWh/day (for a bath and a shower a day).  Ideally you want to heat water fairly quickly, say 3 hours.  So 2.5 kW.

So that leaves 8 MWh/year for space heating.

Assuming you heat the house for 8 months of the year, that is 1MWh/month.

As a rough average, that is 33 kWh/day.  This is a power delivery of 1.4 kW.

 

Now you hardly need the heating on between May and November, so that is probably really only serious heating for 6 months, with the greater heating needed in December, January and February.  A bit of spare is needed for extreme conditions, so let us tripple the power delivery to 3.5 kW for space heating, plus the DHW of 2.5 kW.  This gives you a rough size of 6 kW for a heat pump.

Now it is normal to oversize a bit, somewhere between 15 and 30%.  So somewhere around 8 kW is, from these rough estimates, is the size of heat pump needed.

 

It is very tempting to install an ASHP that is only just large enough, as this is the way that traditional boilers a sized.  Traditional boilers tend to work best when they are at maximum power, this is because they have less losses per power delivered.

Heat Pumps are not like that, they tend to loose efficiency the closer they get to maximum power, and can even rely on an inbuilt electrical resistance heater.

They also rely on the outside air being dry.  This leads to a quirk where the CoP can be lower at a higher air temperature.  This is because air above 0° C can be more humid, and at 4°C can have the largest amount of water in it (water has greatest density at 4°C).  By over sizing an ASHP, there is a larger area of the heat exchanger (just a car like cooling radiator), This allows for a bit of frosting up while still allowing energy to be transferred (the energy is gained by a cold liquid expanding into the radiator and turning to a gas, and then being warmed by the outside air, some designs are different, the the theory is the same).

 

If you look at your boiler, it should state how large it is.  This may be in kW (kilowatts, not KW, killa wot or the big sin, killer Watts per hour) or BTUs, British Thermal Units.

A BTU is an odd unit, but 1 kW is equal to 3412 BTU/hr (this is where the confusion of with power and energy come from I think).

So if you can have a peak at it sometime and see if you can find a number, that would be useful.

It's 70,000 BTUs/hr (a Firebird 70). So, equivalent to 20.5 kW from your calculations.

 

Thanks you very much for doing some helpful calculations. We tend to turn the heating on at the end of September, and only in the evening and morning for a few hours. We have several thermostats dotted around the house, and each radiator has a thermostatic valve as well. The temperature of the boiler can be adjusted to between 65-85Oc.

 

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13 minutes ago, gaschick said:

It's 70,000 BTUs/hr (a Firebird 70). So, equivalent to 20.5 kW from your calculations.

 

Thanks you very much for doing some helpful calculations. We tend to turn the heating on at the end of September, and only in the evening and morning for a few hours. We have several thermostats dotted around the house, and each radiator has a thermostatic valve as well. The temperature of the boiler can be adjusted to between 65-85Oc.

 

We have just checked, and our boiler is set to just under 65oC. This is plenty warm enough for us!

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32 minutes ago, gaschick said:

The temperature of the boiler can be adjusted to between 65-85Oc.

Temperature is not heat (heat is the old word for energy).

The reason you can adjust the temperature is that there is sweet spot for effective heating, or reheating, the cooler returning water after it has been though the radiators.

This sweet spot is generally after half the energy has been removed.  So say your water leaves the boiler at 80°C and returns at 60°C, then the radiators, on average, should have a temperature of 70°C.

[to get the ° sign, hold the Alt key down and then put in the number 248]

 

So my estimate of 8kW sizing was a bit low, this is the problem of working with averages, rather than actual data.  I also probably under estimated the DHW needed, I forgot you had a large 5 bed house.

When using a heat pump, it is more normal to have them running constantly, or as close to that as possible.  This is generally not how a thermal boiler is set up.  They tend to put is a lump of energy, then none, then another lump.  This is partly why they are not that efficient at converting chemical energy to thermal energy.  The stated efficiency of a boiler is the maximum it can achieve in ideal laboratory settings, real life knocks a few percent off that figure.

18 minutes ago, gaschick said:

We have just checked, and our boiler is set to just under 65oC. This is plenty warm enough for us!

So it seems your boiler is a bit oversized as it it set to the lowest temperature.  I don't know if it is possible to downgrade your boiler to a lower output, some can be changed with smaller jets. That would reduce the fuel used.

 

I do not have much against oil boilers, they are pretty good and the emissions are not as bad as people think.  There is price volatility in the fuel costs which can be an issue as we have got used to relatively low heating oil prices over the last decade.  Generally electricity prices are less volatile and new generation capacity from renewables are now the cheapest method to increase capacity.  So that should suppress prices long term.

Edited by SteamyTea

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1 hour ago, gaschick said:

Yup. Our house was built in 2004 with cavity wall insulation. We are in the (slow) process of adding extra insulation in the loft and some more loft boards. 

 

I forgot to add that the spanner in the works for ALL of this wrt the air source heat pump is the fact that we have some microbore heating pipes (10mm) to each radiator. The world of ASHP seems pretty conflicted as to whether this can really be overcome (as will need a higher pressure to feed the radiators) with a low temp ASHP. Some thoughts seem to be that 'go higher or go home'. But, I have found others who have added some extra pumps, +/- a low loss header/extra buffer storage tank. We are planning to have a 50L buffer if we can anyway.

 

Thanks very much for all of your thoughts so far.

FWIW I run our flow at ~45C as if we had ASHP, and some of our rad pipework is microbore, eg to the rad next to me, and everything works.  (We don't have heating on yet, note.)

 

Rgds

 

Damon

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9 minutes ago, DamonHD said:

We don't have heating on yet, note.

Nor me, got another month to go at least, maybe 6 weeks.

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1 hour ago, SteamyTea said:

Nor me, got another month to go at least, maybe 6 weeks.

We'll probably only manage a couple of weeks before the heat goes on here: we'll see.

Rgds

Damon

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4 hours ago, gaschick said:

I forgot to add that the spanner in the works for ALL of this wrt the air source heat pump is the fact that we have some microbore heating pipes (10mm) to each radiator. The world of ASHP seems pretty conflicted as to whether this can really be overcome (as will need a higher pressure to feed the radiators) with a low temp ASHP. Some thoughts seem to be that 'go higher or go home'. But, I have found others who have added some extra pumps, +/- a low loss header/extra buffer storage tank. We are planning to have a 50L buffer if we can anyway.

Makes no odds if you employ a buffer as you'll have a 6m head pump which will whizz around the microbore with ease, as it does right now. Change all the radiator valves if they're 15+ years old and that will help a lot. A good chemical treatment and a power-flush before doing  the conversion is a no-brainer. I would go bigger with the buffer if you're not up-sizing ( replacing ) the existing radiators. 

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Heat pumps loose efficiency (CoP) as delta T rises and low temperature rads work more economically

 

insulate and if you deduce overall heat demand then existing rads will do the job of a low temp rad 

 

for heat pump lower the flow temperature the better and cheaper it will be to run 

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All comes down to the nature of heating and cooling.

 

IMG_20201026_110603.jpg

 

With a cooling curve.

IMG_20201026_111126.jpg

Edited by SteamyTea

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IMVHO, it's all very enticing to look deeply into the new tech for heating the house, but for something like an ASHP, you really need to assess the fabric of your building and how well it is insulated over its entirety. It is usually much easier to passively reduce consumption compared to utilising higher capital intensive technologies to reduce consumption - particularly when the real world benefits of ASHP are still debatable.

 

If you are able to do the calculations, or find someone who can help, you might find investing in wrapping the house in a better duvet could serve you better over the long term and worth investigating fully.

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You only buy insulation once fir the life of the building, fuel you buy forever and it’s very unlikely to go down in price. Unlike most on this forum I didn’t do any maths, but followed others who had as a general rule of thumb, yes a gamble but one that’s paid off IMO.

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