General: Gas

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- CommentAuthorWillInAberdeen
- CommentTimeJul 20th 2020
Hi Bhommels, in a previous house the weather compensation didn't work, because the site was very windy and that had big effect on draughty heat losses. Also we wanted powerful heating when we came home and house was cold. Thought about some kind of weather compensation linked to windspeed and calendar...

... but a better option was Load compensation where the controller looks at the flow and return temperature and flow rate and works out how much heat the radiators etc are actually consuming (instead of trying to predict that from the weather). Then reduces flow temperature until the CH flow is just warm enough for the rads to do their task.

We had a heat pump with this as a built in option, but sppose it could be done with a modified weather compensation controller?
- CommentAuthorbhommels
- CommentTimeJul 20th 2020
@WillInAberdeen
The problem is worst in the shoulder months, where night temperatures are already low but the days are still quite warm. Controller wakes up, sees low temperature outside, ups the flow temp. By the time the UFH & screed has warmed up, the outside temperature has shot up and the temperature indoors overshoots some hours later. If the weather controller could look ahead, it would know it should not get too enthousiastic in the first place and avoid wasting energy.
Load compensation sounds interesting, provided the thermal modeling is up to scratch. I'll have a look if it could mitigate this.
BTW it turns out weather comp controllers have lots of patchwork built in (feedback damping etc) to avoid over- and undershoots, most of which could be done away with if the thing had some predictive power.
- CommentAuthorEd Davies
- CommentTimeJul 20th 2020
Posted By: WillInAberdeenThe green line (boiler temperature? can't quite read) …
Sorry, the forum software squished my graph and doesn't let you click for a larger view. I tried posting it as SVG (1/10th the file size) in the hope that that would be scaleable but nope, it wasn't having it.

The green line is pretty close to the boiler temperature; it's the flow temperature to the radiator in my study: DS18B20 tie-wrapped to the radiator pipe then a cloth tie-wrapped over to avoid too much influence from the room temperature.

…the boiler is firing for maybe a quarter of the time at ~55degC when the heating is on,
Yes, 30.6% from 14:00 to 23:00.

* There are actually several Texp(-t/k) terms going on for heat loss from air to fabric and from fabric to ambient and soil, some of those k will be much longer than the experiment, so the apparent steady state temperature will be warmer than ambient.

** We can see Dyce airport, but our temperature is distinctly colder than theirs in winter, they are closer to the sea. The nearby village is in a frost hollow and is colder still.
Indeed, taking the outdoor temperature, particularly one a distance a way, is dubious for all sorts of reasons. However, when I was away for a few days in October 2018 the house [¹] settled around 13.3°C when Wick was showing 11°C. Again, in early November 2019 I was away for a few days and the house settled to 11.2°C when Wick was showing 9°C so I think I can reasonably assume Wick + 2.2°C for the steady state approached in a day or two.

[¹] Actually my study; other rooms were cooler, particularly the living room with its suspended timber floor.
- CommentAuthordjh
- CommentTimeJul 20th 2020
Posted By: Ed DaviesSorry, the forum software squished my graph and doesn't let you click for a larger view. I tried posting it as SVG (1/10th the file size) in the hope that that would be scaleable but nope, it wasn't having it.

You could link to a copy on your site?
- CommentAuthordjh
- CommentTimeJul 20th 2020
A big problem with weather compensation is that the energy balance depends a lot on the solar gain and there's no way to know that from day to day or even hour to hour using historical data. And the data isn't published in free forecasts in any useful way I've found.

Heating and cooling curves also depend on the property's construction. Heating an internally insulated building will cause the air and internal surfaces to heat up quite quickly, but the heat demand will continue at a reduced rate until heat has flowed through the insulation and warmed the fabric somewhat. Decrement delay and all that. Externally insulated buildings behave a bit differently.
- CommentAuthorRobL
- CommentTimeJul 20th 2020
The usual boiler control of air-temperature hysteresis may get very confused when heating a very large thermal mass - say floor screed. There is generally 1 dominant low low frequency timeconstant in a stable heating system - eg conventional boiler feeding rads - these will have a fast timeconstant, and the room air will be slow due to significant coupling to high thermal mass walls. I'd expect though that if you just swap the rads out for UFH, it's a bit trickier - as bhommels says there's likely to be some overshoot in the system, as you now have 2 low frequency time constants inside the feedback loop (air/walls) and floor. By the time the hysteresis clicks out, the floor may be so hot there would be a significant overshoot of temperature.
- CommentAuthorEd Davies
- CommentTimeJul 20th 2020
Posted By: RobLThe usual boiler control of air-temperature hysteresis may get very confused when heating a very large thermal mass - say floor screed. There is generally 1 dominant low low frequency timeconstant in a stable heating system…
There's a chap on another forum who has a high-mass Passivhaus with a cooling time constant of the order of many days; it drops about 1°C per day when the heating is off even in the coldest weather. He heats it with wet UFH which is in turn heated via direct electric heating (Willis heaters) run off overnight tariff electricity.

The control system is some JavaScript running under Node-RED which looks at the air temperature at the end of the day (and, perhaps, the amount the temperature has dropped during the day, I can't remember) and uses that to tweak the amount the heating is run the following night, increasing or decreasing the run-length by a smallish increment (half an hour?) so the overall “time constant” of the heating controls operates over a period of a week or more. That seems to work well in the UK climate.
- CommentAuthorrevor
- CommentTimeJul 20th 2020
Interesting what one pick up on this forum. Not heard of Willis heaters so googled it so now even more intrigued that it is for heating small quantities of DHW. So how do you use it to heat a house UFH even a passive house. My build is not PH but used the principles of one and last year the heating requirement was about that for a PH. I am on bottled gas thermal store and solar thermal. The gas not very green but make up for it elsewhere. Heating the house with electric hot water at night tariff appeals to me but using the immersion in the TS would not be enough so are these Willis heaters ganged in multiples?.
- CommentAuthordjh
- CommentTimeJul 20th 2020
Posted By: Ed DaviesThe control system is some JavaScript running under Node-RED which looks at the air temperature at the end of the day (and, perhaps, the amount the temperature has dropped during the day, I can't remember) and uses that to tweak the amount the heating is run the following night, increasing or decreasing the run-length by a smallish increment (half an hour?) so the overall “time constant” of the heating controls operates over a period of a week or more. That seems to work well in the UK climate.

That sounds a bit like us, except that I do the prediction manually because a major influence on how long the heater has to run is the amount of sunshine the following day.
- CommentAuthordjh
- CommentTimeJul 20th 2020
Posted By: revorMy build is not PH but used the principles of one and last year the heating requirement was about that for a PH. I am on bottled gas thermal store and solar thermal. The gas not very green but make up for it elsewhere. Heating the house with electric hot water at night tariff appeals to me but using the immersion in the TS would not be enough

How big is the house? We have a 1.8 kW heater that we almost always only use during E7 hours and that's enough.
- CommentAuthorEd Davies
- CommentTimeJul 20th 2020
Posted By: revorSo how do you use it to heat a house UFH even a passive house. … so are these Willis heaters ganged in multiples?.
Yes, IIRC he has two so that'd be 6 kW total output I think.
Going on the PH limit of 10 W/m² max heating with, say, a 120 m² house that'd be 28.8 kWh per day which would be output at 6 kW in 4.8 hours so for all but a few weeks of the year you ought to be able to do it on just one heater in the E7 period.

Using direct electric heating is less efficient than using an ASHP but for the amount of electricity he uses they payback on even a self-installed non-MCS ASHP (say £2000) would be approaching the expected life of the HP.
- CommentAuthorrevor
- CommentTimeJul 21st 2020
<blockquote><cite>Posted By: djh</cite>How big is the house? We have a 1.8 kW heater that we almost always only use during E7 hours and that's enough</blockquote>

Some 340sq M. Would have built smaller but were not allowed to knock down the original old stone building so built around it. Thought PH limit was 15 kw/sqM. My logic was in gas, was given the amount of insulation I have, the thermal mass and solar gain gas would be the most cost effective option. We top up with a wood burner in the lounge in winter so consumption is a bit more Kwh than a PH but of the gas we did use, a lot was consumed commissioning the UFH and conditioning the screed before tiling. So this coming winter will be the first proper test. Hot water has been on solar thermal since end of February apart from 1 hr boiler use In March to top up the hot water. Bathing is via electric shower at moment as bathrooms not complete. Typically consume 6 to 7 kw per day for everything electric and a proposed solar pv and batteries should mean that grid usage will be much reduced, but this project is not very cost effective unless electricity prices rise to compensate but seems the right thing to do.
- CommentAuthorWillInAberdeen
- CommentTimeJul 21st 2020 edited
>>> "proposed solar pv and batteries ... is not very cost effective unless electricity prices rise to compensate but seems the right thing to do."

I'm struggling too, to make sums work for retrofitting PV. PV is more cost effective in large scale solar farms, they can install in bulk with ideal orientation and slope and no scaffolding or shading. I sometimes wonder if domestic PV is going the same way as domestic wind turbines (anyone installed one of those recently?)

If it feels the right thing to do, people can use their PV kitty to buy a share of a solar farm company. Just for an example, Next Energy has market capitalisation of £634m and has 744MW of PV so I can buy in at 634/744 = £850/kW, which is cheaper than I could install on my roof. At current dividend rates it pays back straight away, rather than waiting 20years for rooftop PV to finish paying off its cost.

(Disclaimer: I have no idea if this is a good investment compared to alternatives, have no expertise in investment, own no shares yet, and advise everyone not to take advice from me...)
- CommentAuthordjh
- CommentTimeJul 21st 2020
Posted By: WillInAberdeenI'm struggling too, to make sums work for retrofitting PV. PV is more cost effective in large scale solar farms, they can install in bulk with ideal orientation and slope and no scaffolding or shading. I sometimes wonder if domestic PV is going the same way as domestic wind turbines (anyone installed one of those recently?)

I'm not surprised. When we installed our panels, I was strongly influenced by the FIT scheme and I don't know whether the export price mechanism now provides any kind of positive return instead. We also already had scaffolding in place, so avoided that cost.

I think domestic wind turbines are a different case, though, because they suffer from all sorts of problems that PV doesn't. Noise and vibration, wind shadows and turbulence, risk of damage in storms etc.

If it feels the right thing to do, people can use their PV kitty to buy a share of a solar farm company.

That's always been my view. In a neutral financial environment it feels like that's the rational choice.
- CommentAuthordjh
- CommentTimeJul 21st 2020
Posted By: revorSome 340sq M. Would have built smaller but were not allowed to knock down the original old stone building so built around it. Thought PH limit was 15 kw/sqM.

The PH limit is 15 kWh/m² per year and/or an instantaneous maximum of 10 W/m². Our house is just under 150 m² and we do meet both limits so a 1.5 kW heater operated continuously would be enough, but in practice a 1.8 kW heater operated for seven hours a day is enough almost all the time.

My logic was in gas, was given the amount of insulation I have, the thermal mass and solar gain gas would be the most cost effective option. We top up with a wood burner in the lounge in winter so consumption is a bit more Kwh than a PH but of the gas we did use, a lot was consumed commissioning the UFH and conditioning the screed before tiling. So this coming winter will be the first proper test. Hot water has been on solar thermal since end of February apart from 1 hr boiler use In March to top up the hot water. Bathing is via electric shower at moment as bathrooms not complete. Typically consume 6 to 7 kw per day for everything electric and a proposed solar pv and batteries should mean that grid usage will be much reduced, but this project is not very cost effective unless electricity prices rise to compensate but seems the right thing to do.

Sounds pretty sensible - I've not yet been convinced that domestic batteries are worthwhile but they do seem to be sensible at a larger scale.
- CommentAuthorEd Davies
- CommentTimeJul 21st 2020
Posted By: revorTypically consume 6 to 7 kw per day
kWh per day.

Posted By: WillInAberdeenAt current dividend rates it pays back straight away, rather than waiting 20years for rooftop PV to finish paying off its cost.
Err, don't they both pay back as you go along, one as a dividend and one as reduced electricity bills/some sort of feed-in compensation? The difference is you can sell shares easier so your money isn't tied up in the same way.
- CommentAuthorWillInAberdeen
- CommentTimeJul 21st 2020
Should have been clearer: I can borrow money at a lower % mortgage rate than the dividend % rate, so it pays back straight away to do so. I'd have the shares and probably can sell them when I need to repay my additional borrowing. Not so for home PV panels.

(Same disclaimer: I can't offer investment advice, ask a grown-up to help you, property is theft, etc etc)
- CommentAuthorrevor
- CommentTimeJul 21st 2020
<blockquote><cite>Posted By: djh</cite>Sounds pretty sensible - I've not yet been convinced that domestic batteries are worthwhile but they do seem to be sensible at a larger scale.</blockquote>

The logic of batteries is that you can opt for a cheaper tariff by not using electric at peak times 4pm to 7pm. You use the battery that has been charged up during the day (hopefully). Then at night 1am to 4am you can top up your battery with off peak electric about 5p kwh. And your EV battery could also be for the house I am led to believe. It is a fair investment though, but some batteries are now warranted for 20 years so can make it cost effective, whether on the green front it is eco enough I do not know someone somewhere should be able to do the sums. There are some interesting you tube videos on Fully Charged covering the latest in EV and battery technology including electric bikes. Here is an example of a Tesla kit.

https://www.youtube.com/watch?v=9xuleBA4Zcg&list=PLzD0K2OhbVfGiPXOWRLJLaxx4aLHGX0nu&index=16
- CommentAuthordjh
- CommentTimeJul 21st 2020
Posted By: revorThe logic of batteries is that you can opt for a cheaper tariff by not using electric at peak times 4pm to 7pm. You use the battery that has been charged up during the day (hopefully). Then at night 1am to 4am you can top up your battery with off peak electric about 5p kwh.

Yes, I understand the idea. But the sums don't add up. Maybe they do if you consume large amounts of electricity but that's not very green for an individual house.
- CommentAuthorgravelld
- CommentTimeJul 22nd 2020 edited
I'm wondering if I should heat more continuously. Part of the motivation for intermittent heating was that six years ago I invested in a zoned heating system which turns heating on and off to 15min increments. However I think this just measures air temperature; as a result we can still feel cold even if the the temp is reported as being the same as it is in summer.

Our house has a lot of masonry. We tend to just heat during the periods we're in the room. The room will get cosy, but that's probably actually overheating, to an extent.

Heating continuously would be appropriate for us as the house is constantly occupied (but large).

Do people use IR thermometers on all walls, ceilings, floors etc to judge whether to heat more continuously?
- CommentAuthorgoodevans
- CommentTimeJul 27th 2020 edited
For those following this thread I have made an edit to an earlier post (post 6 in this thread) - please ignore some of my over ambitious claims on the performance of my heat pump earlier for DHW.

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