General: Gas

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- CommentAuthorTwiglet
- CommentTimeJul 10th 2020
Is gas out-moded? And if so, what is a viable alternative for hot water - not for central heating, but bath etc?
- CommentAuthorMike1
- CommentTimeJul 11th 2020 edited
Yes. Electricity is the obvious alternative.

New gas boilers must achieve a minimum ErP (Energy related Products) rating of 92%; I'm not sure what that corresponds to in terms of gCO2/kWh (grams of carbon dioxide equivalent per kilowatt-hour), but the old SEDBUK A rating had a limit of 210 gCO2/kWh, AFAIK. Old boilers can be close to 400.

Last year the electricity grid produced 215 gCO2/kWh, and this year to date it's around 190. It's fallen rapidly in recent years and may fall to more like 115 by 2025, according to some projections.

That means that directly heating water with electricity will, within only a year or two, generate lower carbon emissions than the most efficient gas boilers on the market - and much less when powering a heat pump for central heating. And considerably better than that again if you have your own PV panels.

Which is why the Government should be phasing out gas boilers for new housing, soon.
- CommentAuthorphiledge
- CommentTimeJul 11th 2020
Solar via evacuated tubes or PV with a diverter
- CommentAuthorgoodevans
- CommentTimeJul 11th 2020
Are gas installations still being subsidised - I've not checked in the last 3 years but you could get a new connection to a gas main for a few hundred pounds if the gas main was within 22m of the meter location when I was looking. It would be strange if these subsidies are still present given the withdrawal for new builds in a few years time.
- CommentAuthortony
- CommentTimeJul 11th 2020
Heat pump works too
- CommentAuthorgoodevans
- CommentTimeJul 11th 2020 edited
I can confirm heat pump + cylinder works for DHW - in the summer a CoP of around 4 or 5, perhaps a CoP of around 3 or 4 or so in winter. With a large Cylinder it can be heated once a day for Full flow hot water anytime.

EDIT - I have just completed some more back of the envelope calculations - at best the above statement can be considered as 'ambitious' but a better description would be "utter bo***cks".

We are using less hot water than I thought we were - perhaps approx 2.5 2kWh per day at the taps/showers. In the summer it is costing around 1.1 kWh to heat, in winter around 1.5 kWh - this includes all the losses - pipes, cylinder, start/stop losses etc so the cop values of the heat pump are better than implied here. I'll post at a later date to give a more considered calculation but I needed to correct this post asap. The long and the short of it is the heat pump does deliver good COP numbers (not as good as above) - but as a cylinder is necessary to use a heat pump which reduces it's overall efficiency.
- CommentAuthortony
- CommentTimeJul 11th 2020
Thx, any idea on cost of fuel per kW delivered to hot water?
- CommentAuthordjh
- CommentTimeJul 11th 2020
Posted By: tonyThx, any idea on cost of fuel per kW delivered to hot water?

For what? I'm sure you can work out the cost of PV or of E7 when multiplied by a stated COP.
- CommentAuthorWillInAberdeen
- CommentTimeJul 11th 2020
Twiglet, how is the house heated? Do you already have a hot water cylinder?
- CommentAuthorfostertom
- CommentTimeJul 12th 2020
Posted By: Mike1That means that directly heating water with electricity will, within only a year or two, generate lower carbon emissions than the most efficient gas boilers on the market
That is v interesting, game changing in advice I give to clients. And at home it's the end of wondering whether to heat tap water by electicity or from the 70% oil boiler a) when it's on for CH; b) when it's not otherwise on.
- CommentAuthorJeff B
- CommentTimeJul 12th 2020 edited
<blockquote><cite>Posted By: goodevans</cite>I can confirm heat pump + cylinder works for DHW - in the summer a CoP of around 4 or 5, perhaps a CoP of around 3 or 4 or so in winter. With a large Cylinder it can be heated once a day for Full flow hot water anytime.</blockquote>

Goodevans - what make/model is your heat pump please?
- CommentAuthorCranbrook
- CommentTimeJul 12th 2020
Sorry to hijack the thread but I'm literally about to change my Combi boiler to a unvented system boiler with 300l cylinder, and after reading this I'm now unsure to say the least.. what's the best current option to provide Central heating (mostly rads but two ufh circuits) and dhw (house with 3 showers where two will run at the same time on occasions)
- CommentAuthordjh
- CommentTimeJul 12th 2020
Posted By: CranbrookI'm literally about to change my Combi boiler to a unvented system boiler with 300l cylinder, and after reading this I'm now unsure to say the least.. what's the best current option to provide Central heating (mostly rads but two ufh circuits) and dhw (house with 3 showers where two will run at the same time on occasions)

What power do you need for the central heating? I don't remember how well insulated your house is - remember the mantra - insulate, insulate, insulate. And do it before assessing heating demand.
- CommentAuthorCranbrook
- CommentTimeJul 13th 2020
Posted By: djh
Posted By: CranbrookI'm literally about to change my Combi boiler to a unvented system boiler with 300l cylinder, and after reading this I'm now unsure to say the least.. what's the best current option to provide Central heating (mostly rads but two ufh circuits) and dhw (house with 3 showers where two will run at the same time on occasions)

What power do you need for the central heating? I don't remember how well insulated your house is - remember the mantra - insulate, insulate, insulate. And do it before assessing heating demand.

The new extension has good insulation and good air tightness, the existing building has okay insulaton in the roof but has unfilled cavity walls - bear in mind this is a dormer bunglow. I will be adding another extension soon which will help to reduce the percentage of the property that is poorly insulated. I am however looking at options to sort the remaining section out.
- CommentAuthorCranbrook
- CommentTimeJul 13th 2020
Forgot to add.. I will be using MVHR, it's just the design side of things is a bit complicated and design is slow because of the complexity of my layout
- CommentAuthorMike1
- CommentTimeJul 13th 2020
Posted By: CranbrookWhat's the best current option to provide Central heating (mostly rads but two ufh circuits) and dhw (house with 3 showers where two will run at the same time on occasions)
If you insulate well, then you may find that the existing rads can be run at a lower temperature, making a good pairing for a heat pump, if you can find space for one. There will be some things in this thread of interest too: http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16536
- CommentAuthorMike1
- CommentTimeJul 13th 2020 edited
Posted By: fostertomThat is v interesting, game changing in advice I give to clients.
Yes, it's surprising how quickly things have changed in the past few years, even if there's still a long way to zero carbon.
- CommentAuthorCranbrook
- CommentTimeJul 15th 2020
Posted By: Mike1
Posted By: CranbrookWhat's the best current option to provide Central heating (mostly rads but two ufh circuits) and dhw (house with 3 showers where two will run at the same time on occasions)
If you insulate well, then you may find that the existing rads can be run at a lower temperature, making a good pairing for a heat pump, if you can find space for one. There will be some things in this thread of interest too: http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16536

Could you explain what you mean by the heat pump pairing please mate?
- CommentAuthorEd Davies
- CommentTimeJul 16th 2020
He just means that the heat pump would work well with the radiators if the house heat load was such that the radiators could be run at a low temperature - i.e., the combination of the heat pump and the lower-temperature radiators would make a good pair.

In the not-particularly-well-insulated house I'm renting in the north of Scotland which I keep quite comfortably warm the average temperature of the flow to the radiator in the small bedroom I use as a study (the warmest room in the house) throughout January and February this year was 31.56 Â°C.

If you actually heated this house continuously to achieve the same day-time temperature (i.e., didn't let the house cool off overnight) then the required energy and hence flow temperature would be a little higher but not much, I think, as the heating was only off for less than a third of the time and the lowest the temperature dropped to 13.75Â°C compared with the average outdoor temperature of a gnat's over 5Â°C.

In other words, I don't think you need hugely more insulation/airtightness to make it practical to run standard radiators (as opposed to oversized radiators or UFH) from a heat pump as long as you're willing to change your philosophy to heat more-or-less continuously hence allowing use of lower flow temperatures.

You might choose to back off the heating a bit overnight though it probably wouldn't be worth the hassle. On the other, you might choose to turn it up a bit at night depending on your electricity tariff.

Another step in this direction would be to keep the flow temperatures low all the time and use some direct electric heating to boost the room temperatures a bit during the coldest few weeks of the year.
```
$ ./httpreader.py --integral study/temperature study/radiator/temperature metar-obs/EGPC/temperature -s 2020-01 -e 2020-02
{
  "study/temperature": {
    "topic": "study/temperature",
    "count": 75097,
    "min": 13.75,
    "max": 25.47,
    "start": 1577836800.0,
    "startf": "2020-01-01T00:00:00.00Z",
    "end": 1583020799.999999,
    "endf": "2020-02-29T23:59:59.999999Z",
    "duration": 5183999.999999046,
    "integral": 108408995.56642808,
    "average": 20.912229083033957
  },
  "study/radiator/temperature": {
    "topic": "study/radiator/temperature",
    "count": 90866,
    "min": 13.75,
    "max": 58,
    "start": 1577836800.0,
    "startf": "2020-01-01T00:00:00.00Z",
    "end": 1583020799.999999,
    "endf": "2020-02-29T23:59:59.999999Z",
    "duration": 5183999.999999046,
    "integral": 163578440.01912406,
    "average": 31.55448302838622
  },
  "metar-obs/EGPC/temperature": {
    "topic": "metar-obs/EGPC/temperature",
    "count": 1438,
    "min": -3,
    "max": 12,
    "start": 1577836800.0,
    "startf": "2020-01-01T00:00:00.00Z",
    "end": 1583020799.999999,
    "endf": "2020-02-29T23:59:59.999999Z",
    "duration": 5183999.999999046,
    "integral": 25971299.999996185,
    "average": 5.009895833333519
  }
}
```
- CommentAuthorjms452
- CommentTimeJul 19th 2020
We've got a gas boiler with weather compensation (where you can adjust the level of weather comp) which has let me try different radiator temperatures.

In a moderately well insulated (victorian) house the challenge isn't really keeping the house warm - it's warming it up in the first place. That is, I can keep the house warm with 30C radiators but if the heating's been off all day it takes many hours for it to warm up. It's a totally different way of heating from switching the heating on and expecting near instant warmth.
- CommentAuthorEd Davies
- CommentTimeJul 19th 2020 edited
Exactly, people have the expectation of heating the house quickly so plumbers always err on the side of over-speccing the boiler.

I did a little experiment to see how much more heating this house needs for continuous heating. It's more than for intermittent heating but not as much as you might expect: https://edavies.me.uk/2019/01/continuous/
- CommentAuthorbhommels
- CommentTimeJul 19th 2020
Posted By: jms452We've got a gas boiler with weather compensation (where you can adjust the level of weather comp) which has let me try different radiator temperatures.

In a moderately well insulated (victorian) house the challenge isn't really keeping the house warm - it's warming it up in the first place. That is, I can keep the house warm with 30C radiators but if the heating's been off all day it takes many hours for it to warm up. It's a totally different way of heating from switching the heating on and expecting near instant warmth.

Same here. After making a mistake with the holiday programming over Christmas it took about a day to get back up from rather chilly to liveable, and another to get to comfortable temperatures!
- CommentAuthordjh
- CommentTimeJul 19th 2020
If you adopt a policy of keeping the place continually warm, which is sensible if it's well-insulated, then heating the place from 'cold' is a rather rare event. So you don't need to design your main heating system to deal with it. You can use additional direct electric heating in one form or another, for example, to speed it up.
- CommentAuthorCranbrook
- CommentTimeJul 19th 2020
Slight diversion off original topic but has anyone used an accumulator to increase mains pressure on incoming water?
I'd be interested to know of results if anyone has
- CommentAuthordjh
- CommentTimeJul 19th 2020
Doesn't seem to be a problem where we are now. We did use a shower pump previously, to overcome the most noticeable problem of a gravity fed system.
- CommentAuthorCranbrook
- CommentTimeJul 19th 2020
Shower pumps on gravity fed systems seem to work excellently from past experience. As I'm leaning towards an unvented cylinder I believe I would need an accumulator if the pressure isn't sufficient to run two showers are once
- CommentAuthorWillInAberdeen
- CommentTimeJul 19th 2020
Posted by Ed Davies: "I did a little experiment to see how much more heating this house needs for continuous heating. It's more than for intermittent heating"

AIUI the best strategy depends on the house's two time constants*, for cooling and heating. If these are large, the house cools down and heats up slowly.

If the house doesn't need to be warm for a period (say overnight or during a work day) and that period is longer than the two time constants, then turn the heating off, as heat supplied during that time will mostly have been lost and replaced before the house needs to be warm again.

The time constants are the k in the temperature response T=To + T1exp(-t/k).

You can measure them by turning the heating off and timing the house cooling 'most' of the way down to reach a steady temperature (technically [1 - 1/e] = 63% of the way down). Then turn heating on and time the house 'most' of the way back to being warm again.

In our current place that's about 8 hours so it's worth us turning heating off overnight. A previous place had solid floors so no point turning heating off unless we were going away for a few days. We actually sleep better when the house is cooler overnight and warm by the following evening, which works better in a house with faster temperature time constants.

For your experiment, do you still have data that could calculate your time constants, and were your 'unheated' periods quite short or long?

Switching to a lower CH temperature will lengthen the heating time constant, though not the cooling one. This might not be a problem if the cooling time is already quite long, or the heating can come on earlier.

*actually not very constant due to all kinds of weather and occupancy factors
- CommentAuthorbhommels
- CommentTimeJul 20th 2020
@WillnAberdeen
This is the gripe I have with my otherwise excellent weather compensation controller. The time constants of my house are well beyond 10 hrs, and the outdoor temperature is never constant on these timescales.
It would be so much better if the weather compensation could look ahead (by picking up the weather forecast from the web) and anticipate what is coming instead of being reactive. Even better would be if the forecast could be used to estimate solar gain...
There have been attempts at this, the only ones I could find however applied to thermal modelling of the Computer Science department buildings in East European universities.
- CommentAuthorEd Davies
- CommentTimeJul 20th 2020 edited
Posted By: WillInAberdeenAIUI the best strategy depends on the house's two time constants*, for cooling and heating. If these are large, the house cools down and heats up slowly.
Absolutely.

However, the heating â€œtime constantâ€ depends on the heating system you use (the bigger the â€œboilerâ€ the shorter the time constant) which in turn depends on the strategy you've chosen. So, really, the strategy(s) which make(s) sense depend on the house's cooling time constant then the heating time constant depends on the strategy chosen.

You can measure them by turning the heating off and timing the house cooling 'most' of the way down to reach a steady temperature (technically [1 - 1/e] = 63% of the way down).
Less drastically, the time constant can also be determined from instantaneous values; it's the rate of change of temperature divided by the difference between the current temperature and the steady-state temperature.

k = (dT/dt) / (T - T0)

k = time constant.
dT/dt = rate of change of temperature
T = current temperature
T0 = steady-state temperature (approx outdoor temperature for cooling if we neglect solar gain, etc, which reasonable for overnight).

Note though, that for heating the steady-state temperature you should use is the temperature the house would reach if you left the heating running full pelt, not the temperature that the thermostat happens to be set to as that'll cut off the exponential curve â€œartificiallyâ€.

For your experiment, do you still have data that could calculate your time constants,
Yes (I don't delete data - current sqlite3 database is 3.8â€¯GB ), will think some more about extracting this.

â€¦and were your 'unheated' periods quite short or long?
Mostly 8 hours overnight from 23:00 to 07:00 and also when out but for the period in question that was mostly fairly short (it being winter). Here's a typical day (2018-12-06) during the last period of intermittent heating: heating was off overnight and for a few hours in the morning while I went to my site until showers drove me home again.

â€œoatâ€ (purple line) is Outside Air Temperature - actually the temperature reported at Wick airport about 30â€¯km NE of me.
- CommentAuthorWillInAberdeen
- CommentTimeJul 20th 2020 edited
The dT/dt/deltaT method is less preferable because it amplifies noise and because you have to assume a final steady-state temperature, which for our place is somewhat above ambient**, due to people and activities.*

But you have enough data in the graph to conclude:
- the curvy pink lines on the left reduce but don't get down to steady state in 2300-0700 -> cooling time constant is of the order of 8hours, plus or minus some hours as we don't know final steady-state. You could do a curve fit?

- they recover to target temperature by 1100 -> heating time constant is order of 4 hours plus/minus etc

If they had been say 2 hours it would be worth turning heating off when not needed. If they were say 2 days than leave the heating on all the time. As they are between, it's borderline worthwhile leaving the heating on when you go out a few hours or switching off overnight, which is as you measured.

As the heating time is about 2x faster than the cooling time, the average heating power during the 'warm up' was about three times the heat loss to ambient, and that's with the stat regulating the heat as you mentioned.

The green line (boiler temperature? can't quite read) shows the boiler is firing for maybe a quarter of the time at ~55degC when the heating is on, so maybe 15%ish of the whole day. You'd also need the return temperature to work out the deltaT across the radiators, to see if they could emit the same heat if supplied at a constant but much lower temperature, but certainly looks promising.

* 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.

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General: Gas