Discussion

One quick thought on the base temperature to use for degree days.

Using the spreadsheet mentioned in 

allows you to estimate the heating and water loads. You can change where you think the transition is from heating and water to just water heating.

In the graph above I guessed about 14 °C. The heating fit (red line) and water fit (blue line) cross at around 14.5 °C. However I noticed that I can change the transition temperature anywhere from 12 °C to 15.5 °C and the cross-over remains at around 14.5°C. This suggests that heating occurs only below 14.5 °C.

So maybe the correct base temperature for me to use is 14.5 °C!

A lot to review/take in @BeePee …

I presume the base temperature might be varied by location (sheltered or not, tree cover etc)? I’ve generally used a figure of 15deg but that is a rough view

I’m not sure. I can see that if my house has a heat transfer coefficient of around 250 W/K that the 100 to 200 W dissipated by various electrical appliances (plus 50 W per person?) then this could account for the 1 °C difference (15.5 to 14.5 °C). I can also believe that location could affect the heat transfer coefficient by reducing losses. Maybe location could improve solar gain.

Sorry don’t know.

Wow @BeePee! Fascinating read, lots to take in. Interesting to see the estimates based upon the amount of global warming.

If the ambient temperature is below 15.5 °C, I wonder what the general consensus would be for optimum temperature. The world health organisation suggests 18 is the ideal temperature for healthy adults. 

Thanks to @PaulWa for providing the link to the nhbc website regarding ventilation rates, which suggests much lower rates than the defaults in the MCS spreadsheet.

See my post Heating your home - (Correct) Size is important | The OVO Forum (ovoenergy.com)

I also managed to find information on the HSE website regarding healthy levels of CO2 in the workplace. It has a handy “rule of thumb” that 10 l/s ventilation equates to around 1000 ppm of CO2 per person (which is the recommended maximum level). 1500 ppm is considered high and can have detrimental health effects. So the levels in the nhbc document are rather low (if considered in isolation) but taken as a “whole house” are probably OK. I’ll have to crunch the numbers for our house.

Sad, but I have just ordered a CO2 monitor so I will be positioning it around the house try to to estimate ventilation rates based on two sedentary people (plus a cat)!

Hi,

I have a one bedroom apartment with electric heaters and the EPC came back as F (but only by 1 point off an E!) It was recommended to change the heaters to high heat retention storage heaters.  I am happy to do that, but feel it would be overkill in the bedroom (9.5 sqm) Do you know if I can still raise the EPC rating by only changing the living room heater (I will need 2 x HHR for the living room) and not the bedroom?

Many thanks for reading and I hope you can help me.

Christine 

I have the same heating arrangement in my flat. It is worth looking at other options beyond storage heaters which perhaps give the combination of stored and instant heat. The German Fisher heaters are worth looking at. Most of these type of heaters run from a standard 13A socket and don’t need tariffs such as Economy 7. While Economy 7 is cheaper during the hours it operates, the day rate is slightly higher than most single tariff options.

Not sure how the various options will effect the EPC rating so looks like you will need to do a bit of research and get some quotations for installing different types of heaters. Bear in mind changing heatings option may alter the load on the wiring and this may need to be updated too. Storage heaters are generally fed on a seperate curcuit to the ring main the feeds the 13A sockets in your home.

Hi @ChrisB1001 and welcome to the community! 

I can already see @DavidWSR has provided some really insightful information so far to consider! 

I just wanted to include a couple of useful links for you. Follow this one for our guide on EPC certificates.

Keep us posted on your journey and any decisions you’re thinking about! 

A problem, as I see it, with using gas boiler data to estimate required size for heat pump is that gas boilers (certainly ours) tend to be used with intermittent heating periods - on 6.30-9am and again 4.30-10.30am. The approach then seems to be to take energy used and spread it over 24 hours for the heat pump. Data for my house for 6 winter months looks like this:

Sorry, not pretty but you get the picture. The X axis shows temperature difference with target inside temperature of 20 degrees.

So this suggests a heat pump size of 7kwh. - right hand end of implied regression line cuts 7kwh at 23 degrees = minus 3 outside. This confirms the installers calculation based on house fabric/room size etc

But if I look at hourly energy use on a  minus 3 day then we see this:

Note that this shows boiler running through daytime as my wife insisted. Also the bars show energy used per half hour - a quirk of the Hugo app that I use - divide 173 kWh by the 16 hours of data and you see you need to double for kWh.

So my boiler works at around 10kwh during this period replacing heat lost and so suggesting that 7kwh heat pump is too small. But maybe the boiler is not on long enough to heat up the fabric of the house to reach a steady state?

So I will go with the suggested ASHP size of 7kwh and see how we go.

Any thoughts appreciated.

Hi @Martin53 

A couple of thoughts.

Firstly you are probably correct when you suggest that your boiler is spending the first part of the heating cycle replacing heat lost overnight. Although the air heats relatively quickly it takes much longer for the heat to get into the walls and fabric of the building.

Secondly the boiler is not 100% efficient (probably 90% if it's a recent condensing boiler, maybe 70% if it is an older "cast iron job"). The 7kW heat pump rating would be the heat provided under certain operating conditions, it could well be more (or less). So although you see 10 kW of gas consumed, you are probably only getting 9 kW into the house. Also some of the gas demand may be for hot water and cooking.

Modern boilers have quite a good turn-down ratio, so a 30 kW boiler might be able to reduce its output to maybe 4 or even 3 kW. When the demand falls below this low level the boiler will just cycle. Heat pumps are not as good at modulating down (modulation factor 2 or maybe 3), so a 7 kW unit would probably manage a minimum output of around 2.5 kW. This would match demand on a cool day (7 kW for 23°C difference, 2.5 kW for 8°C difference i.e. 12°C ambient). Heat pumps are not so happy cycling so it is best to match the pump to the demand and not over-size.

Although gas boiler are more efficient at lower flow temperatures the improvement in efficiency is relatively small (90% to 95%). With a heat pump the increase in efficiency with lower flow temperatures can be dramatic (200% to 300% or better), so it is important that the system runs at as low a flow temperature as is reasonably possible. This generally means ensuring that radiators are capable of providing enough heat at 50°C or even 40°C flow temperatures. Running at a lower flow temperature will also mean that it may take longer for rooms to heat up.

Hi @BeePee 

Thanks, all useful comments. I've also convinced myself that the tail off in energy use by the gas boiler at around 10pm is a sign the the fabric of the house has reached the required (20 degrees) temperature, so it's reached steady state with (reduced) boiler output matching loss from the house. It's now well insulated so I was hoping for this.

Then the heating goes off at night-time, the air and fabric cool down and so the process starts again the next day.

Of course a better test would have been to run the gas boiler continuously for a few days during a cold spell to see what steady state energy requirement would look like. But it's too late so we'll see how the heat pump performs and whether the insulation to 1905 house - loft, ewi, underfloor, draught proofing, vacuum glazing, chimneys filled - has been worthwhile.

Thanks again.

Hi @Martin53 

Maybe the following graph might help.

It show the temperature in our study (blue line) and the outside temperature (red line). When the heating turns on the air temperature quickly reaches steady state (after a small over-shoot). When the heating turns off you can see the room slowly cooling down to a minimum of around 14°C. The cool-down basically follows the temperature of house fabric, whilst the heat-up is determined by heat from the radiators warming the air.

Newer houses tend to have a smaller “thermal mass” since the internal walls tend to be stud timber and plasterboard.

Older houses will tend to be much more solidly built with internal brick walls. They will tend to hold on to the heat much better, but will also absorb a lot more heat getting up to temperature.

Hope this helps

B

Hi @BeePee That's really useful, seeing real data rather than just theory. We have ewi on half of our house (party wall on another quarter) so I guess the heating up of the walls takes longer (than iwi or cavity) but so does cooling down. I'm going to have fun playing around with temperature settings and optimising tariffs.

From my data shown earlier, it seems that the fabric/walls take around 8-10 hours to heat up and reach steady state which is when the heating can ease off. Of course if the heating is on overnight with the ashp (all be it at a lower temperature to aid sleeping) then then this cycle should largely disappear so heat input will stabilise at a lower level.

I'm looking forward to the extra data available from the smart controls