ECO4 ASHP, Solar and insulation

NedS

Reed_Richards said:

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

So flipping that around, if I know my room is comfortable at 20C with a flow temp of 50C and return temp of 40C (average 45C), I know my Delta T is 25C. If I can find the rated heat output at Delta T 50C for my current radiators, I can calculate the energy currently used to achieve that comfortable 20C.

Knowing the energy required (Watts), I can then work backwards using the outputs of the new rads to find what flow temps (Delta T) would be required to achieve that same output given larger / more rads that are to be installed.

So if my current radiator has a rated output of 1000W at dT 50C, and I'm running it at dT 25C, it is currently outputting 410W to maintain room temp (820W total as it's an open plan kitchen/living room with two rads). We are having one new radiator installed (3 in total), so the 3 new radiators will need to achieve 820W total, or around 275W each which would require a dT of ~19C (assuming they are still rated 1000W each), so I'd need an average radiator temp of 39C - which may correspond to flow and return temps of 42C - 37C on an ASHP.

Have I got that right - does that make sense?

If that's correct, I may be able to assume (without doing details heat loss calcs, that the additional insulation may reduce my heat input requirements by 15%, so now I only need 820W - 15% = 700W of heat output over my 3 radiators which could be achieved with a dT of around 16C (Flow 38C, Return 34C) which is getting me into a better SCOP range.

Reed_Richards said:

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 water and that tells you how big a heat pump you need.

The surveyor took all the room measurements, details of windows and fabric of the building to do those calcs. A second guy (more a plumber than surveyor) came to check the feasibility of the installation but I get the sense the sizing of rads was more based on "how big a rad can we fit in this space" and "can we put an additional rad in here" rather than sizing each radiator to the heat requirements of the room.

NedS

MultiFuelBurner said:

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.

Thanks for that - it's reassuring to know

Reed_Richards

All that "flipping around" is the correct idea; I haven't checked your figures.  But you said that running your current radiators at 50 C flow and 40 C return was not enough for cold weather and your new system needs to cope with the vast majority of cold weather.  But heat pumps don't have the modulation range of a gas boiler so if you want good economy you should not over-specify for that once-a-decade few days of very cold weather.

NedS

Reed_Richards said:

All that "flipping around" is the correct idea; I haven't checked your figures.  But you said that running your current radiators at 50 C flow and 40 C return was not enough for cold weather and your new system needs to cope with the vast majority of cold weather.  But heat pumps don't have the modulation range of a gas boiler so if you want good economy you should not over-specify for that once-a-decade few days of very cold weather.

Yes, running the current oil boiler at 50/40C flow/return temps, we are just about managing to maintain temps at present, with lows of around 2C. I'm happy to assume that's OK, as things will improve with the added installed insulation and new/larger radiators.

Our oil usage is around 1600L/year as best I can estimate keeping heating at around 19C (really difficult to estimate annual usage as we top up 500L at a time rather than filling the tank, but averaged over last 3 years or so). We have used around 2000L in the past but have reduced our usage). That translates to ~15,000kWh of heat output at 90% efficiency which implies a HTC of around 262kW

Assuming a system design minimum outside temp of -3C and a desired house temp of 19C, we have a differential of 22C x 262 (HTC) = 5.8kW output ASHP required. Rounding up, a 6kW model. They've specified a nominal 8kW model, but from reading the specs it does not produce 8kW at all operating temps with total heating capacity falling at low ambient temps and higher flow temps so I'm happy it is an appropriate match and will be able to provide sufficient heat output in other than the most extreme of cold weather. We may also have estimated our oil usage on the low side, so I'm happy to have a little upside margin here. If our usage were 1750L that would equate to around 6.25kW so I wouldn't want to step down to the nominal 6kW model which would then be under specified.

gm0

Interesting project. 

Vaulted ceilings (especially with historic exposed structure) are quite challenging for this sort of project on the peak heat loss dimension.  We have some.

Doing it "properly" with a historic roof and keeping interior heritage aesthetics would involve raising it.  Doing a proper one above exposed features. And dropping the renewed historic appearance one back on top. 
Perhaps with a load of in roof solar blended in.  First action is to go an buy a lottery ticket

Or

Strip back - use high tech u value stuff - and keep exposed features.  Doesn't deliver the full insulation.  Labour costs spiral for infilling.

Strip back - then cover up with standard insulation - risks disturbance but you can spot and renew anything before covering it.

Or just run in and cover it up with sheet material.  This suits the installer but I don't know.  I don't like it. 

BR on all these methods in a renovation scenario - is another mystery to explore.  

But the last one is for sure the project that it will be easier (or even possible) to find an installer to quote for and do. 

I am in the foothills of our journey.  And would not claim significant expertise.

But I would be careful with flow as a parameter. And review how the installer assumptions about the selected ASHP interact with this vs your current overall system behaviour with boiler pump flow and higher operating temperature.  Your existing plumbing.  Then bigger rads and any extras.   And how well insulated transit pipework is

If an oil boiler has any end of system is cooler issues with peak demand conditions.  So insufficient flow and higher temperature drop.  Then an ASHP isn't going to magically solve that.  Quite the opposite.  Which is not a problem with the heat pump.  It's an overall system design upgrade "need" with your plumbing - which may suit your renovation plans or not.  Or just end in a compromise about flow rate combined with (higher) operating temperature to achieve a workable result without the renovation project scope spiraling out of control

Good luck

NedS

gm0 said:

Interesting project. 

Vaulted ceilings (especially with historic exposed structure) are quite challenging for this sort of project on the peak heat loss dimension.  We have some.

Doing it "properly" with a historic roof and keeping interior heritage aesthetics would involve raising it.  Doing a proper one above exposed features. And dropping the renewed historic appearance one back on top. 
Perhaps with a load of in roof solar blended in.  First action is to go an buy a lottery ticket

Or

Strip back - use high tech u value stuff - and keep exposed features.  Doesn't deliver the full insulation.  Labour costs spiral for infilling.

Strip back - then cover up with standard insulation - risks disturbance but you can spot and renew anything before covering it.

Or just run in and cover it up with sheet material.  This suits the installer but I don't know.  I don't like it. 

BR on all these methods in a renovation scenario - is another mystery to explore.  

But the last one is for sure the project that it will be easier (or even possible) to find an installer to quote for and do. 

I spoke with the company today to try to get more details and they use PIR insulated plasterboard (with 50mm PIR insulation), fitted to solid external walls (spot and dab) and screwed to our vaulted ceilings over the existing plasterboard (no infill insulation currently fitted as per survey - guy drilled a hole and put a small camera through - no wonder house gets cold!). 

There are no exposed features to worry about - there are new exposed structural beams, but they can easily fit panels between and beams will remain exposed given the 65mm overall depth. I'm happy with this solution, other than I would have preferred 100mm insulation rather than 50mm on ceilings where there currently is none. 50mm in external wall insulation seems a reasonable compromise over lost room space. Other conventional roof areas have accessible loft space which they will top up as required with rockwool type product. All is better than we currently have and it's at no charge.

NedS

I've also performed calculations today for my radiator outputs (thanks to @Reed_Richards for the pointer)

I started by recording the size and types of my current radiator, and looked up their output at Delta 50 at a typical radiator manufacturer website (Stelrad). I entered the outputs in a spreadsheet, and corrected for a 50C flow temp (Delta 30C) giving a de-rated power output (I should probably re-do this based on 45C flow temp, as the boiler kicks in at 40C and cuts out at 50C, so my average mid-point temp is 45C ?? This gives me a starting point of comparison for my current oil system.

Next I tabulated the new (proposed) radiator sizes and their outputs at Delta 50, and applied correction factors to obtain the de-rated outputs at 45C (Delta 25C) and 40C (Delta 20C). I would need a temp of ~42C to match the heat output of old/current system at 50C

I think I will re-do the calcs for a mid-point flow temp of 45C to better reflect my current system.

EDIT: Just re-done the calcs for the old/current system based on a midpoint flow temp of 45C, and it corresponds to a corrected midpoint flow temp of 38C once the new larger/extra radiators are taken into consideration. So the new ASHP should give the same heat output at a leaving temp of ~40C and return temp of ~35C (mid point ~38C), and that's before considering the extra insulation being installed. That should mean I'm able to operate in a COP range of 3-4 for the vast majority of the winter months.

shinytop

NedS said:

I've also performed calculations today for my radiator outputs (thanks to @Reed_Richards for the pointer)

I started by recording the size and types of my current radiator, and looked up their output at Delta 50 at a typical radiator manufacturer website (Stelrad). I entered the outputs in a spreadsheet, and corrected for a 50C flow temp (Delta 30C) giving a de-rated power output (I should probably re-do this based on 45C flow temp, as the boiler kicks in at 40C and cuts out at 50C, so my average mid-point temp is 45C ?? This gives me a starting point of comparison for my current oil system.

Next I tabulated the new (proposed) radiator sizes and their outputs at Delta 50, and applied correction factors to obtain the de-rated outputs at 45C (Delta 25C) and 40C (Delta 20C). I would need a temp of ~42C to match the heat output of old/current system at 50C

I think I will re-do the calcs for a mid-point flow temp of 45C to better reflect my current system.

EDIT: Just re-done the calcs for the old/current system based on a midpoint flow temp of 45C, and it corresponds to a corrected midpoint flow temp of 38C once the new larger/extra radiators are taken into consideration. So the new ASHP should give the same heat output at a leaving temp of ~40C and return temp of ~35C (mid point ~38C), and that's before considering the extra insulation being installed. That should mean I'm able to operate in a COP range of 3-4 for the vast majority of the winter months.

A couple or three points.  

You should check with your installer what the output of the 8kW ASHP is at your design temperature, which will be something like -2 deg C.  Some ASHPs (and Midea is one such) can only achieve their badged output at warmer temperatures. 

At any temperature below about 4 deg C, your ASHP will be defrosting regularly. This can reduce the effective heat output by quite a lot, maybe 15%. 

If your designer specifies some sort of buffer or hydraulic separation (e.g. a low loss header) this can affect efficiency and therefore maximum output (and of course running costs). 

Having said that, an 8kW Midea has a bit of headroom for you so may well be fine. But it's worth asking the questions. It's important your ASHP is the right size, not to big or too small. 

I have radiators and the flow temp is around 38-45.  My overall COP is 3.2 for last year including HW.  I think your calculations look good. 

NedS

Luckily the weather is quite mild here so we do not often see temps below freezing, and anything below -2 or -3 would be quite unusual. I get they normally perform the calcs to a design temp of -3 for MSC certification purposes.

The installer is recommending a 50L volumiser. I've done some reading around this, not ideal from my understanding, but I don't know if there is sufficient volume in the system and they are insisting on it to ensure the installation is within the manufacturers specs and therefore within warranty. They also mentioned a plate to plate heat exchanger but I have no clue what that is. I have asked them to send me a schematic of their installations. As this is on an ECO4 scheme grant, it is what it is and there is not much scope for changes other than dumping them and going with an alternative installer.

They are doing the detailed heat loss calcs this week and should send them to me once completed.

Reed_Richards

From the guide: https://mcscertified.com/wp-content/uploads/2020/07/Heat-Pump-Guide.pdf

External design temperatures should reflect typical low temperatures experienced in the winter heating season and as a guide should be exceeded for 99.6% of the year.

So the MCS external design temperature is a temperature that should be exceeded 99.6% of the year, using local weather station data to determine/estimate what this is.

I imagine the plate-to-plate heat exchanger is the interface between the refrigerant in your heat pump and the water it is heating.