It does not know the difference - either will cut off flow through the rad. The purpose of the single lockshield valve on rads with manual or TRV controls is to set the maximum flow through the rad. This allows the system to be balanced.
TRVs do not respond directly to the temperature of the flow through the rad or even its temperature directly. They are supposed to respond just to the room temperature.
So diverting more flow through a rad may pass heat to that room faster, and cause the TRV to close sooner that it otherwise might.
If you reach the stage where the return temperature of the water to the boiler exceeds its internal thermostat's set point, then the boiler will cease firing. However it is still in a "call for heat" mode, and will be running its pump, and waiting for the temperature to fall. At which point it will fire again. Hence it will continuously be using energy (electric pump etc), and periodically firing for short periods just trying to sense what the actual demand for heat from the house is. Heat that much of the time the house may not need.
This is a very inefficient way of running a heating system.
ISTM that is exactly your situation. You in effect have the thermostat "call for heat" input to the boiler hardwire to "on" permanently. So your only controls on it are a timer, and its internal stat.
That means any time the heating is on, it will spend a period of time "idling" while it waits for the house to need heat again.
Proper controls will ensure that does not happen, and save fairly significant amount of energy, while also possibly improving comfort in the house since you don't get so many temperature overshoots in the bypass paths when the boiler fires for a few mins, gets the system water up to temp, and then shuts off again.
In effect you are running manual weather compensation, which is fine. With updated controls you could have the system do that for you (and with finer grained control) if you wanted.
What are the options? Is the house well enough insulated that it could work with a lower temperature heat source? If the answer is no, and you are not planning on increasing the insulation to that level, then you are looking at another boiler of some sort.
A new boiler will also *require*[1] updated controls since what you currently have does not meet building regs and an installer would not be able to sign off the install.
You are highlighting that I don't understand what should be happening. By way of background (and I'm now into my '70s), I was brought up in a coal heated house, I went to Australia and the tropics where CH was not needed nor installed, I moved back to a country cottage which had coal fired CH (extended back boiler). Gas boilers are something new (last 6 years) to me. Going to the utility room to turn up/down the temperature control on the boiler is a doddle compared to not having to go out and get a bucket of coal in the rain and snow, or clear the ashes with wind blowing it back in your face the minute you open the door.
So with that understanding of my ignorance let me check please - is the boiler intended to be running at a modulated level so that it is always firing?
The mode you have described is what I have taken as normal - boiler gets water to temperature, burner turns off for a while, comes back on again, rinse and repeat with the pump constantly operating. The "call for heat" being presumably the temperature of the returned water as it knows nothing else.
It is an extended, detached, bungalow. Lots of external walls and of course lots of roof, some tiled some felted. Whilst not that old (1968) and all windows/doors pvc double-glazed and no draughts it is hard to keep it all warm. I cannot envisage a heat pump (particuarly air) would be sufficient when needed the most.
I have not problem with updated controls though I do wish the previous owners had not made the decision to have the boiler right at the end of the extension to the garage. Hot water takes ages to arrive to the kitchen and the CH has a lot of pipe to run through.
Indeed it will be a massive improvement in ease of use and control, compared to what you have experienced previously.
ok, here is another layer of complexity to delve into :-)
Old boiler designs were fixed output devices. They basically had two modes, firing or off. When firing, they dumped whatever output the burner was designed for into the water. So a 24kW boiler would lob 24kW into the water (or at least they burnt gas at a rate of 24kW - depending on if the power rating quoted was input or output power)
The way a typical system of the time would operate is that the pump and boiler would be powered up at the same time. It would carry on heating until a room thermostat was "satisfied", and this would cut power to the pump and boiler, until such a time as the stat decided it was cold enough to warrant another heating cycle.
In a system like that the "call for heat" was basically just power being delivered to the pump and boiler via the control wiring - typically a time switch, and stats for room and a hot water cylinder, and possibly some zone valves:
So a pipework layout like:
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(or possibly something even cruder without a valve, and water circulating through a cylinder using "gravity" circulation)
Wiring like:
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More modern versions would allow the boiler to control the pump itself, the boiler would have a permanent mains supply, and there would be a dedicated "call for heat" input on the boiler. That would cause the boiler to fire when a stat demanded heat. When the stat was satisfied, it would cease calling, and the boiler would stop firing. However it cold keep running its pump, for a bit (aka "pump overrun") to shift all the accumulated heat out of the boiler and into the rads, rather than let it waft out of the flue.
Thermostatic Rad Valves (TRV)
When these came along, they changed the dynamic a bit. Since now temperature could be controlled by room rather than for the whole building. Some installers used them to do a very quick and easy installation. Not bothering with a room stat, stick a TRV on everything (and either a sprung bypass valve somewhere or leaving one or two rads "open"). Then wiring the call for heat input on the boiler directly to the "CH On" output of the time switch. That basically leaves the boiler running all the time, stopping only when so little heat can be dissipated via the rads (because their TRVs are occluding most of the water flow through them), that the return temperature rises to the point that the boiler's own stat will turn off the burner. In poorly insulated places and with abundant cheap gas it was "ok" and made for a comfortable house. This could be what you have from the sounds of it. It's an easy system to install, and many installers might even use it as an excuse to not even bother balancing the heating system (saving them many hours of installation time) - which can mean some rooms get fully warm before others even get a look in. But in general it will "work", but be quite wasteful to run.
Modulation
Since short cycles of heating are not good from a comfort or efficiency point of view (especially when combined with mechanical stats with poor accuracy and large amounts of hysteresis), boiler makers started making "modulating" boilers. These are more sophisticated, and require electronic controls and fan assisted induction so that they can pre-mix the correct amount of gas and air and inject into the heat exchanger. This allows the boiler to exhibit a range of output powers. The boiler can then be designed to "load match", basically look at the water return temperature and start throttling back the output power as it rises. (rising return temp being an indirect way of sensing that the house getting up to temperature). This allows the boiler to run for longer, but using less fuel. You get a quieter system, less overshoot, and fewer cold spells between heating cycles.
If the heat load of the house falls below then minimum output of the boiler, then it reverts to cycling on its stat like a fixed output boiler. A good modulating boiler might be able to drop all the way down from say 24kW to 5 or 6kW.
So a modulating boiler setup with TRVs and no stat, will be better an more efficient overall than an older fixed output one, but still has the problem that it is never told to stop heating when the house is warm enough. (which in a reasonably well insulated place might be quite a long time).
The call for heat in this case is probably a terminal (or terminals) on the boiler that are designed to be connected to a programmer and or room stat. However they are instead just hardwired together or hardwired to mains voltage (depending on if the terminals are 240V or "volt free" contacts). Lots of boilers come "out of the box" with a loop of wire in the terminals.
Nope, does not sound like it - at least not without loads of extra insulation, larger rads, and/or under floor heating.
Is there anywhere you could locate a cylinder? Although it does not seem to occur to many system installers, you can have a cylinder heated via a combi boiler - that can give the best of both worlds - wholesome mains pressure hot water near the combi, and high flow rate, high mix temperature water for baths etc further away in the house.
With an unvented cylinder you don't need a tank in the loft to feed it:
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(and if the cylinder is not near the point of use either, it can have a secondary circulation circuit to make sure you still get "instant" hot water at the taps)
I suspect it has more intelligence than it has been connected up for. But being 16 yrs old and probably due replacement I don't think connecting room thermostats to it would be worthwhile, and it certainly would not be cheap to run a cable so we'd be looking at some wireless solutions.
I've fitted TRVs to all the radiators except the two that started the thread, the Hall and the Bathroom. The Hall radiator is earmarked for being turned off, or at least down to a dribble. I'm assuming the TRVs on all the radiators will just kick in quicker.
As I read it the Baxi 105 HE is modulated. I'm not convinced the hot water is well modulated and that my be a thermistor issue. Of course the CH is not on at the moment so I can't/not prepared to do checks just at the minute but will when autumn sets in.
When constructed the bungalow had a boiler centrally located in the attic, with a hot water tank. There is ducting under the floorboards and outlets, now papered over, in the main rooms.
Indeed - I would hope to have an improved solution though I suspect the main gain might be that the boiler is more efficent, or is that wishful thinking?
No way. I've just spectaclarly failed to fix the simple potentiometer speed control for my electric golf trolley. Totally lost focus, concentration and patience in the past few years.
The logical place would be around where it used to be in the attic central to the main part of the house and near the kitchen. The boiler is at the end of a flat roofed garage that has been extended. It is a major job to even get cabling fed through so any piping would be best taken off existing feeds which are the hot water and the central heating. In a way it would be beneficial if the CH pipes could coil through the water tank to raise its temperature. We really only bother about hot water in the kitchen when the weather is cold (yes, very spartan). The shower is in the extension and next to the boiler which of course is a good arrangement.
Until the recent energy crisis running gas was a lot cheaper than running electricy but now I'm wondering whether a couple of instant electric water heaters or similar would make more sense.
Mind you as most of the hot water requirement is for washing up maybe I should just get the broken dishwasher (it was left here and has never worked) replaced.
You mean like they have at hotels? That's would be a bit of an extravagance for just the two of us don't you think?
So a modern high power condensing combi. The modulation range is quite wide - althoug it only goes down to 11kW which is quite high.
You will need the stat for the replacement, so no harm fitting it to the existing one.
If yo starve one rad of input, then the others will get it instead if there is the pipework capacity to deliver it. (there is an upper limit on the amount of heat you can shift through a pipe at a given flow temp)
yup 31 - to 11 kw according to the manual linked above.
It only quotes that power range for output to heating. It probably runs full chat all the time when running DHW (and cycles the burner off if there is insufficient demand to soak up all the heat)
Probably... the SEDBUK rating (seasonally adjusted efficiency) is quoted at 87.3%, which is pretty good. So there is not much extra low hanging fruit to pick just by buying a more efficient boiler.
However you probably won't be getting anything close to that rating due to the lack of adequate controls, so should be able to get abigger improvement than would otherwise be the case.
You can check boilers' ratings against the database here:
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You can up the efficiency with more sophisticated controls - such as having weather compensation with an external temp sensor (so it can automatically do your trick varying flow temps in a continuously monitored way. Data bus based stats can also help by offering much more fine grained control rather than simple "on/off" control of the heating.
Well to be fair, most folks probably would not countenance doing their own. However I get the impression it can be hard finding installers who are prepared to spend the time to do more complicated installs.
(it took me a couple of weeks to do a full replacement of floor standing cast iron lump boiler, and traditional cylinder with a wall hung condensor boiler and unvented cylinder, with eBus controls, two heating zones, external temp sensor, and a few rad swaps. I suspect that might have been prohibitively expensive as a paid for install!)
Yup sounds like it.
Given the amount of gas we use to generate electricity at the moment, it seems unlikely the cost relationship will change much in the short term.
Yup, contrary to expectations they will do a load of washing up with far less water than doing it manually.
yup. Modern control systems often include the timer capabilities to control the secondary circulation pump as well.
Depends on how much you like waiting for hot water I guess :-)
(to be fair I could have done it, but did not bother - mainly because in our place the bathrooms are clustered close to the cylinder, so the wait is minimal. It's only the kitchen and utility room that has a longer wait, but it is not excessive (probably a 15 - 20m run of pipe))
More importantly, according to the tables in Part L, is the high ratio of perimeter to total floor area, which is rather typical of a bungalow. This means excessive heat loss from the 1 metre wide strip of floor all around and adjacent to all the outside walls.
This is something that the fad for external wall insulation seems not to address.
So is the implication that even when the thermostats are down the boiler might be still providing too much heat?
I'll start investigating this, and hopefully find someone who can plug it into the boiler. I guess the connection will probably be wireless or one of those ethernet over mains plugs.
If I'm doing this I'd like a fairly intelligent device though I'm not sure I want remote access via smartphone. Too much complexity I fear.
There are 11 radiators. I don't know the length of the run, but it is
6m from the boiler at the back to the kitchen at the front and the piping for this is in the attic.
Interesting. I do have to bear in mind that the boiler is 16yrs old. I thought this might be close to its limit. The "firebox" insulation has cracking and there are signs of green on the pipes though I'm not currently losing pressure/water. The divertor assembly was replaced a couple of years ago. The xxx engineer used the pressure relief valve to drain (despite there baing a big notice on the inside of the boiler cover to not do this) so I've had to have a new pressure relief valve.
I think the hot water thermister is not working well.
Many thanks.
ok, on the shopping list. It's currently used for storage so that'll be another problem for SWMBO.
I'd be concerned at the energy losses with just the two of us in the house. I use the cold water tap most of the time but SWMBO would prefer something warmer than freezing in the winter!
Can I at this stage thank you for your input. I've copied this post into a document which will help me form a spec/ideas for moving forward.
The connection to the boiler is usually trivial - either a single 240V terminal that you make live to call for heat, or a pair of "no volt" contacts that you connect together.
You can get wireless stats - they have two parts, a base unit that connects to the boiler, and the remote sensing stat end that talks to the base over RF.
I am not aware of any common wireless stats that can use an ethernet connection, so pretty unlikely. (and the wireless they do use may be on
2.4GHz - but it is not WiFi, and not IP based)
Some of the internet connected devices do this. Usually trivial to setup to be fair.
The length has less impact than the pipe diameter. So for example a 15mm pipe can shift at best around 6kW. Hence why many systems have a backbone in 22mm, and only drop to 15mm for individual rad connections.
Its doing ok, although that is not an exceptional age for a boiler.
green on the pipes could just be condensation on the outside leading to some verdigris.
It is not advised (certainly with an older install), but using it to drain does not mean it will fail. (there is just a risk some crud gets caught in it, and it does not reseat properly)
[snip]
two points; if doing a circulation loop, then the pipe loop needs to be insulated.
Also, consider what you mean by "losses". In most cases you are talking about heat that will escape into the house.
A small inline hot water heater near a point of use could work. You can also get very small unvented cylinders for basin use, and if under 15L are not subject to the same rules as a full domestic install. They are well insulated. (although still electric, so not as cheap as gas if comparing like for like. The tricky bit is comparing like for like, since you have to factor in the amount of "dead leg" water you need to drawer off to get water that is warm enough)
No problem. A fair amount of what I have said is in the wiki, so it's worth having a read though some of the articles there.
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