Going Green isn’t easy

[Sam Vimes]

This debate seems to be going around in circles and getting a little personal.

I'll not lock it yet but my summary is....

Heat pump related equipment is good for some and not others. Just because of one or two of your own experiences, good or bad, does not mean the technology is bad or good in general.

 

[Oscarmax]

We are fortunate our 2 bedroom bungalow was built in 2017, over a 12 hour period in the current cold weather we lose 3 degrees C, this morning to heat up from 19 degrees C back up to 22 degrees C just our 1 hour hour.

When we refurbish the kitchen we will change the Ideal combi to a Bosch and the tumble dryer (which is every other day 365 days a year) to an AEG 9 kg heat pump dyer (as there are no heating element run it off peak rate overnight)

 

[GUMPY]

Would be interesting to know how hard the boiler is running to do that. On our system, gas boiler with weather compensation, it takes about an hour per degree to heat the house from 16 to 18.5* on these cold mornings. However it's a 15kW boiler but apart from when it's heating the hot water when it gets to 11kW (flow temp 80deg) it rarely runs above 5kW due to the lower flow temperature (40-55 Deg C) for the heating.

I'm still experimenting to find the most economical way of running it, so far it seems to be maintaining an even temperature all day, even if we are out, rather than heating and cooling the house.



* Thermostat/timer is in the hall which is a 1-2 degrees colder than the rest of the living area.

 

[otherclive]

We are fortunate our 2 bedroom bungalow was built in 2017, over a 12 hour period in the current cold weather we lose 3 degrees C, this morning to heat up from 19 degrees C back up to 22 degrees C just our 1 hour hour.

When we refurbish the kitchen we will change the Ideal combi to a Bosch and the tumble dryer (which is every other day 365 days a year) to an AEG 9 kg heat pump dyer (as there are no heating element run it off peak rate overnight)

That’s similar to our temperature drop overnight last Tuesday until mid day Wednesday and the overnight outside temperature was minus 2 deg C. Post #194 shows the cooling and heating curve.

 

[Dustydog]

Our four year old Vaillant Eco gas boiler is A rated allegedly reaching 94% efficiency .

Do you think it is cheaper to keep the house at a low of say 15 degs C overnight warming it up to say 19 degs when occupied?
Or let the temperature drop with the weather and then warm up?

 

[otherclive]

Our four year old Vaillant Eco gas boiler is A rated allegedly reaching 94% efficiency .

Do you think it is cheaper to keep the house at a low of say 15 degs C overnight warming it up to say 19 degs when occupied?
Or let the temperature drop with the weather and then warm up?

That’s a difficult one to answer. We tend to just let it drop overnight as the boiler turns off at 2130. It’s scheduled time to come on is 0730 but generally we have it turned off for heating and on for 2 hours for hot water. Putting it on generally around 1300. If the external temperature is forecast to be low we will set it at 17 deg C in order for the inside temperature not to drop further. But last Wednesday it did not come on until mid day as we were going out all morning and had set it at 15 deg c but it never dropped quite that low.

We are away all this coming week and the boiler will be off as there’s no frost forecast. Before we leave to return home I will switch it on using Hive so we will have warmth and hot water for when we arrive home.

 

[GUMPY]

Boiler efficiency figures are almost never achieved in real life. The figures are calculated when the boiler would be at maximum condensing mode. This happens when the flow temperature is below 55degC most boilers are set up by the installers to run at DT50* this means that the flow temperature is above 65degC so no condensing happens at all and the efficiency goes down dramatically.

* DT50 means there is a 50deg difference between flow temperature and the desired room temperature. Radiators are normally sized for this temperature difference, running lower DT means using bigger radiators.


The system here is set to vary the flow temperature depending on the outside temperature, the colder it is outside the higher the flow temp and radiator temperature.

Even at 0deg outside the house here only loses about 3 Deg overnight. I'm looking at setting the overnight temperature up a degree or two (it's 16deg at present) to see how much or little it uses compared to raising the temperature by 3deg every morning.

 

[GUMPY]

It’s scheduled time to come on is 0730 but generally we have it turned off for heating and on for 2 hours for hot water.

Because I use variable flow temperature I have both heating and hot water on first thing. This means flow temp is 80deg whilst the hot water is on dropping to 50deg once the HW stat switches off. This boosts the heating effect of the radiators hopefully causing the house to heat up quicker.
However it's still early days tuning this system.

 

[otherclive]

Boiler efficiency figures are almost never achieved in real life. The figures are calculated when the boiler would be at maximum condensing mode. This happens when the flow temperature is below 55degC most boilers are set up by the installers to run at DT50* this means that the flow temperature is above 65degC so no condensing happens at all and the efficiency goes down dramatically.

* DT50 means there is a 50deg difference between flow temperature and the desired room temperature. Radiators are normally sized for this temperature difference, running lower DT means using bigger radiators.


The system here is set to vary the flow temperature depending on the outside temperature, the colder it is outside the higher the flow temp and radiator temperature.

Even at 0deg outside the house here only loses about 3 Deg overnight. I'm looking at setting the overnight temperature up a degree or two (it's 16deg at present) to see how much or little it uses compared to raising the temperature by 3deg every morning.

Thanks, I never knew that aspect of a condensing boiler. Bit of a well shrouded secret in the trade. So once our house is up to temperature it might be more economical to wind the boiler heating water temperature lower, so it’s running longer but at lower output and at higher efficiency.

 

[GUMPY]

Thanks, I never knew that aspect of a condensing boiler. Bit of a well shrouded secret in the trade. So once our house is up to temperature it might be more economical to wind the boiler heating water temperature lower, so it’s running longer but at lower output and at higher efficiency.

Before I got the new boiler I ran the flow temperature at 55deg in order to get maximum efficiency. The tap water was hot enough even for C.
I reset the cylinder stat to 54deg so it still switched off when hot.
The only downside is that below 50C it is possible for legionella bacteria to grow so it's close to the edge.
This is alleviated by doing, when electricity is cheap or the sun is shining, a weekly heating on the immersion to 60deg.
Most of the year it's not a problem for us as we also have solar thermal water heating.

https://www.hse.gov.uk/healthservices/legionella.htm

 

[Sam Vimes]

While all of this information is of interest, much of it doesn't convey the full picture of each individuals experience.

For example:-

Stating that you have a 3 or 4 bedroom semi or detached, doesn't mean much as its the volume you're trying to heat that's important. Some 4 bedroom houses have less volume than a 2 bed house.

Is the house open plan or a serious of closed of rooms?

When you say the house has heated to a specific temperature - is this in all rooms, which I doubt, because not everyone has a monitor in each room?

Solar gain is important - so are there many large windows and what orientation are they?

Coupled with the above - what latitude are you because daylight hours vary across the UK. Also hills and headlands can make the sun disappear quicker.

Relative Humidity also affects the perception of temperature. Lower humidity - winter days for example - will make it feel colder than it actually is. Having a high humidity house isn't healthy but can make it feel warmer.

Do you have appliances running that dissipate heat into the house. E.g having the electric oven on will provide heat into your house as will your tumble dryer. One particular to note is the switch to LED lighting has removed a source of heat compared with the old halogen bulbs

Outside temperature is one factor but wind chill and wind speed are more important. For example the outside temperature here this morning was 6.5 but the wind chill was shown as 1.9. Houses cool down quicker or lose heat quicker in windy conditions.

There are many other factors when planning a heating system than just boiler/heat pump size.

 

[ProfJohnL]

... I have also been invited to view the construction stages of a small estate of bungalows all designed to be highly insulated and make the best use of air sourced heating.

I am convinced that retro fitting air or ground sourced heating into a my building which was not designed for them would be very inefficient.
...

Why do you think an air or ground sourced heat pump would be "very inefficient"? for any given building it will need the same heat energy input to raise its temperature. The building doesn't care where that heat comes from. so as long as whatever system is installed can deliver the required amount of heat, the heating requirement is satisfied.

Just for clarity when I refer to efficiency I am describing the ratio of usable heat energy output divided by the energy put into the system

Modern condensing gas boilers are pushing 95% to 98% thermal efficiency

Resistive electric heating is always 100% thermally efficient

Heat pump air or ground sourced will have a minimum of 100% but with a more likely 200 to 500% thermal efficiency

Now provided the heating system can provide the total quantity of of heat energy you need, any of the sources will work, but there are usually some compromises which might affect the cost and thus the viability of some schemes, in particular the heat pump.

If you wanted to replace a 40kW combi boiler with a heat pump and still have access to instantly heated hot water, the size and cost of a heat pump to provide that is probably well beyond the budget for most home owners. But if you shift your system back a stored hot water tank, then you don't need such a high powered heat input device as you spread your heat demand over a longer period, and that can be serviced by a much lower heat in put rate.

If you look at the way a larger heat input boiler works, it is only working at full heat for a small fraction of the time that means its capability is unused most of the time. if you work out how much energy you have used over a full day and divide it by 24 hours your likely to find your average hourly over ad is in the order of 2 to5 kw per hour. In theory you could replace your 40kW boiler which runs some of the time with a 5kW Heat pump that runs all the time. and you would receive the same amount of energy.

We have to acknowledge that heat pumps efficiencies do vary because of specific details of an installation, and the concurrent environmental conditions. But unless there is something faulty or the environmental conditions are exceptionally poor, I would expect a heat pump to have at a absolute minimum a CoP of 1 but more typically in the range of 2 to 5. Essentially any CoP of greater than 1 means for every kWh of energy you use from your supplier, you should get more than 1kWh of heat energy into your property. But typically you should see your imported energy needs for heating fall by 50 to 80%

That does not fit your analysis of a heat pump being "very inefficient"

Whether it makes economic sense is very difficult to assess without knowing the tariffs you are on, but there should be no question about the efficiency of a heat pump.

 

[GUMPY]

While all of this information is of interest, much of it doesn't convey the full picture of each individuals experience.

How much detail would you like?
I do have individual read out of temp/RH in the main rooms, plus I've done full heat loss calcs for the house. Also lots of data from other sources.
🫣

 

[Sam Vimes]

I don't need any detail - the point I'm making is that anyone doing a comparison of what they get versus what others get based upon the little information being given here, doesn't really mean much.

FWIW: I've done the same as you - but then numbers on a spreadsheet seldom compare with reality.

 

[Jcloughie]

Why do you think an air or ground sourced heat pump would be "very inefficient"? for any given building it will need the same heat energy input to raise its temperature. The building doesn't care where that heat comes from. so as long as whatever system is installed can deliver the required amount of heat, the heating requirement is satisfied.

I never said ‘for ANY given building’ .

Older buildings were never designed with heat pumps in mind. Therefore retro fitting will have to be a compromise. Also. Most housing built speculatively were built with zero regard quality. Profit being the main criteria. Building regulations, in particular part F, were poor and rarely ever met.

Therefore, retro fitting will always be problematic and a compromise. It may also likely to be designed by someone who has no experience with real life construction standards.

I certainly never suggested that pumps would be inefficient for modern buildings if properly designed. But in poorly built buildings, money would be far wiser spent on draught proofing and insulation.

Pumps work best providing a constant low heat over time. Attempting to use it in a yo-yo fashion might be futile.

John