Central heating facts and myths?

[ericmark]

The first house I lived in with central heating was gas fired hot air, in the street we all felt it was rubbish, and the latter houses in the estate had a water distributed system, which cost less to run, however this was late 70's, so no double glazing and not even sure if the ducts for the heating were sealed well, after fitting it was easy for some one to tread on the duct and as a result cause it to leak.

I say single glazing as when circulating the air there were no cold spots, and with radiators there were many cold spots with glass getting that cold you would see it removing the humidity from air and allowing it to run down the windows.

Our last house had a Myson fan assisted radiator, again no cold spots as air circulated, and fast warm up times, but we had higher bills there than here in spite being a semi rather than detached. Could also be due to boiler.

So the big question, circulate the air or allow it to remain in the main static? There is no question mild circulation results in a more constant temperature which since it is constant can be set lower.

This moves be on to second item, analogue or digital control, it would at first glance seem analogue is far better, the radiators modulate, the boiler modulates so you have a constant heat, however with digital speed can mean it alternates between hot and cold, or it is that fast that in real terms there is no difference between analogue and digital as far as room control goes, so only question is which releases the most energy to the outside.

Clearly the home type will change results, but having a boiler run at most economic output then turn off for a time, or have boiler turn down to match demand, we are told every time a boiler turns off any heat in the boiler is lost through the flue, but heat is being lost through the flue all the time boiler is running, so does running a boiler at 25% output lose less to atmosphere than a boiler running for 25% of the time?

To use TRV's to control temperature to each room independently requires the boiler to run for an extended time with low output, where having every room set as a proportion of whole output will some correction with the TRV's it can be run with a mark/space ratio.

These all clearly impact on each other, when using hot air central heating from turn on to all rooms at temperature looking at around 15 to 20 minutes, that fast we would turn off heating when we left the house, and turn it back on when we returned. But the hot water and radiator system in this house, with a modulating boiler and electronic TRV on 4 of the 10 radiators it takes more like 2 hours to reheat the house, most of that time is due to the anti hysteresis software being careful not to over shoot.

So first house did not need geofencing it heated up that fast turning it on as I arrived home was good enough, and this house geofencing does not work as it takes so long to reheat and stabilise the temperature.

I have placed thermometers around this living room, depending on day, but can be over 10°C difference around the room, as to if good or bad, not sure, if in bay window is 15°C do I care when I am not sitting in the bay window, if my chair is at 20°C does it matter what temperature rest of room is at? Same when sun heats bay window to 28°C if my chair at 20°C do I care?

I move into yet another house soon, so starting all over again, this time oil fired, so have to decide how far to take automation? So hope to start some debate on what is worth doing and what is OTT.

 

[SimonH2]

Ah, good questions - some of which discussed in other thread.
There's no doubt that moving the air will give a more uniform temperature - but you try force moving air without noise or draughts
One of the benefits of under floor heating (UFH) is that it puts out heat where the room is usually coldest, and where you usually notice cold the most (your feet).
As to control, the less inertia in the heating element, the more noticeable the control method will be - but see later on UFH. So for example, with blown air or a fan-coil (eg Myson) then the room will both heat up quckly and cool down quickly - particularly if you use the internal stat in the Myson that just turns the fan on/off.
The very best control for that type of system is a pump on each unit that circulates water round it all the time, and a controlled blending valve regulating the temperature according to heat demand. That needs a two loop controller - an outer loop measuring room temp and generating a heat demand value, and an inner loop controlling the blending valve accordingly.
Probably not too bad is to simply control the valve by room temp. With an electro-thermal wax actuator, a zero hysteresis stat controlling it would probably do a pretty good job. Bit of overshoot to start with (due to initial warm up/cool down times in the head), but then settling down and just modulating.
But you'd have the drone of the fan(s) all the time which I'd find annoying.

Now I mentioned UFH, and that can offer the reverse of what I said about low inertia systems and controllability. UFH is often combined with a big slab of concrete - so a lot of thermal inertia in the heating element. Control that with just a room stat that's got hysteresis and you will get swings in temp due to control overshoot - but they will be slow acting, very different to the near instant effect of blown air.

 

[ericmark]

I noted the new vector myson has a multi speed fan controlled by room temperature, the old one I have does have variable speed, but manual control. For people who go out to work myson is good, from turning on heating to room warm less than 20 minutes, but with under floor heating it takes that long to even get floor warm.

The day warms up the room quickly, when sun comes out, within an hour room from 16C to 24C so any central heating needs to act fast so when sun does come out, it stops heating room, underfloor heating is just too slow, maybe better than storage radiators but not by much.

Even water filled radiators are slow to react, myson however does make a noise, not much, but depends on fan speed selected, did consider a mod so two speed fan.

But each house I have had is different, first had hot air central heating gas fired, at that time no double glazing and poorly fitted, leaks under the floor, so expensive to run, however as to keeping house warm, it was the best system I have had, fast warm up, single thermostat did all rooms as air circulated, however down side.

House very dry, we put trays of water in air intake, noise, expensive to run, and sound travelled room to room as events in every door, and we lost cat when it decided to crawl around inside vents, lucky not injured when it fell to lower floor, had to remove vent to get him out.

The second house was open plan, main problem heat goes upstairs, upstairs TRV was essential or door closed bedroom very cold, door left open it was too hot, was designed to be centrally heated with a single gas fire, that did work, but no thermostatic control, so central heating which actually was not central, was needed for auto control.

Next house bay windows were the problem, when sun came out you could see the thermometer raise, so heating needed to be super responsive.

 

[Notch7]

Of course house construction has significant impact on how central geating operates.

Most modern house have the insulation internally so there is no thermal store to maintain temperature after the heat is turned off.

Efficiency of houses built with or without thermal storage does depend on how the house is used. Somebody out working all day will need a diff requirement to somebody at home all day.

UFL is radiant so rooms with tall or vaulted ceiling benefit.

Radiators waste heat with tall ceilings because of the convected air.

convection heating creates more dust to be circulated (I think?).

 

[SimonH2]

I've been toying with a combination of UFH and rads or fan-coils (eg plinth heater in kitchen).
My idea was to run the UFH at a lower level than normal for comfort - I'm really peed off with the cold floors in this house, in particular the concrete slab kitchen, but also the laminate everywhere. But because UFH has the control issues you've mentioned, I was going to combine it with something quicker.
My idea was :
Have the rad or fan coil controlled with a conventional TRV of some sort. Tee into the feed to it with two tees. Arrange a single loop UFH setup, with the TMV set at a relatively low temperature, so that it has it's supply and return from/to the two tees. And a pipe stat to turn the UFH pump on when the pipework is hot.
Idea is that whenever the fan-coil or rad has any flow, there'll be a supply of hot water for the UFH which will warm the floor. As the room warms up, the TRV will throttle back and then stop the flow - cutting off the supply to the UFH. It would seem to avoid the problem of multiple heating systems fighting each other as there's only one control - but I can see some other potential issues.
First of these is that when the room is heating up, the UFH will take a lot of heat out of the feed - slowing the warm up rate. Then as the room warms up, there's a risk of overshoot as heat from the floor comes up. And of course, in this weather, the floor will still be cold to walk on.

I noted the new vector myson has a multi speed fan controlled by room temperature, the old one I have does have variable speed, but manual control.

It was the old ones I had experience with a couple of jobs ago. Nice to see they've updated them.

The second house was open plan, main problem heat goes upstairs

We have that problem here. Previous owner built a rear extension with an upstairs lounge - which is not much above garden level at the back so it works well. But it's got lots of windows so is "quite warm" now even if we open the Velux windows. It's part open plan anyway, and we tend to leave what doors there are open, so any heat downstairs comes upstairs.
When we moved in, there wasn't even a single thermostat - so as a quick fix I put one at the top of the stairs next to the boiler cupboard. It's helped a lot, but when it's even moderately warm, the stat turns off the heating and then downstairs gets nothing - in part that's why I've bought the Radbots and intend connecting them to the boiler (it's on my "round tuit" list" The Radbots will provide the zoning so we can heating downstairs without upstairs getting heated when it doesn't need it.

Most modern house have the insulation internally so there is no thermal store to maintain temperature after the heat is turned off.

Not exactly, it's normally in the wall cavity so the inner leaf of the wall is still a heat store. However, the inner leaf is often of a lightweight block so the storage isn't much. Ceilings/roofs aren't really any different to how they've always been - a layer of plaster(board) with insulation above it.

UFL is radiant so rooms with tall or vaulted ceiling benefit.

I assume that's a typo for UFH ? The temperature UFH runs at means that it will not be radiant (much). Principle transfer mechanism will be by conduction to the air layer in contact with the floor - which once heated will rise and allow cooler air to take it's place. Also, as the warm air rises, it will mix with the cooler air and this should help to create a more uniform air temperature.
The converse is true where cooling is provided by cold surface ceilings. The cool surface cools the warm air at the ceiling, and when it's cool enough it starts to sink and cool (by mixing) the air below.

Radiators waste heat with tall ceilings because of the convected air.

Not really. They actually do de-stratify the air by pulling cold air from the floor and creating upwards currents that will circulate the air in the room. But like any form of heating, unless there's something to bring it back down, warm air will always collect up in the top of tall rooms - something of a problem in church where the ceiling is something like 6m up ! We do now have fan coils which are very good at de-stratisfying this - but they are far too noisy to leave on during a service.

convection heating creates more dust to be circulated (I think?).

Dunno about creating it, but it will move it around.

 

[ericmark]

New house now so needs to run a winter to see how it does, however I did look at control of fan assisted, the standard modulating boiler will work with the return temperature of the water, however the TRV restricts the flow, where the Myson does not, so when the Myson fan stops, hot water is returned to the boiler, and when the TRV closes the by-pass opens and again hot water returns to boiler, but what happens with a mixture?

There are add on fans, with that in mind, I did an experiment, standard twin plate rad, and a upright fan one end aimed to blow down centre of rad, the fan was controlled by my brewing thermostat with the sensor on the return pipe, idea was if a lot of flow was called for by the TRV then return would get hot so fan would start, set to lowest speed.

It was a failure, as soon as fan started radiator cooled and return cooled so switched off again, clearly the fan was too large? The problem was in that house some one had fitted most of the TRV's on the return, yes they were bi-directional, but by time the TRV got hot, it was too late, the radiator was already stinking hot, reducing the flow with lock shield valves helped, and I just throttled back a little at a time until the room was at temperature set, it did work, however if the balance was due to lock shield setting or the TRV is unknown. But this fine tuning likely messed up the experiment.

One hears the motor on the Energenie heads running, and it is a gradual thing, faster than wax type, but not fast enough to stop over shooting if lock shield is wide open.

The big thing is, I know set to 20°C so if room goes to 22°C I know the lock shield needs tweaking a bit, but a wax type set to 3.5, is the TRV set too high, or lock shield open too far, how do you know? What is the point of *, 1, 2, 3, 4, 5, 6? why not 16 to 22°C?

Thermals are powerful, I like the idea of duct top to bottom of house sucking up hot air at top and feeding it back at bottom, combined with heat recovery even better, but how much power would it need? I had a 36" pipe which needed heating to 700°C at 100°C per hour, holding for an hour then down at 50°C per hour. On the top of the pipe was a plastic blank, which I had taped on with masking tape, it blow off, and it took three of us to replace it, and loads of blue rope to hold in down, the force of thermals was really strong.

So not sure if a toilet fan and duct would actually be strong enough?

 

[SimonH2]

Basically, if you have a properly set bypass then flow rate is more or less constant and the return temp will vary with heat load - more load, lower return temp.
Lockshields are critical in setting max flow rate through the rad. If too open then, as you've observed, the TRV will struggle to control things - letting too much water through, over-shooting, shutting, and cycling.
Running the system at lower temperatures and/or lower pump power (system pressure) should help - allowing better regulation from the (typically oversized) TRV. I say oversized because if you look at the size of the valve internals, for most applications there's going to be little difference in flow between "a little open" and "full open", but between closed and "a little open" gives a big change in flow. That makes control "twitchy" - a tiny move in the controller (TRV head) makes a huge difference in output. In a perfect system, the TRV would control rad output over most of it's travel with the rad size, flow temp, & system pressure combination matched to the heating demand - I doubt that there are many such systems

One hears the motor on the Energenie heads running, and it is a gradual thing, faster than wax type, but not fast enough to stop over shooting if lock shield is wide open.

This comes back to what I wrote above. With the locksheild wide open, then in most systems the rad will become uncontrollable. The TRV only needs to open the valve a tiny amount, the rad fill quickly with hot water, and the room overheats. In theory, with a fair bit of effort, you could program an electronically controlled TRV head accordingly - only open the valve a tiny amount, only open it slowly - but that would be hard work. Simply setting the lockshield properly should make the rad controllable.

... but a wax type set to 3.5, is the TRV set too high, or lock shield open too far, how do you know?

If the lockshield is set properly then it's the TRV that needs adjusting.

What is the point of *, 1, 2, 3, 4, 5, 6? why not 16 to 22°C?

An electronic head can do that, a wax head cannot. The actual temperature regulated to at any specific setting will vary. Lets say that at setting "3" and the room at 22˚C the valve is 10% open. On a warm day that's going to overheat the room which might need to be a degree or two hotter before the valve is fully shut. But in cold weather, it might need to be several degrees colder before it's wide open. So the actual temperature the room settles at will vary depending on several factors.
It's an inherent limitation of wax capsule heads - they need several degrees (I don't know how many) difference to go between fully closed and fully open. Apart from this being an inherent feature of the wax capsule system, it's also required since if it went between closed and open without this sliding scale in between, then the whole system would be unstable and flip flop between states.
The electronic heads can do this differently. They can settle at different valve positions under different conditions, and so settle at a specific temperature.

Thermals are powerful

They are indeed. As an aside, the revolving door was developed when they started to have air conditioning in high buildings over in the USA. The difference in air density between inside and outside meant that regular doors would either be un-openable or un-closable due to the pressure differential at ground level.

Ah yes, nearly forgot. There is a method to setting the lockshields. Basically, select the design delta-T for your rads etc, open all the control valves, run the system and go round testing each rad. Open/close the lockshield so they are all the same delta-T. Now adjust the bypass valve so that it is "only just" not open under this condition. The control valves (TRVs) should now be able to control the system - when they start restricting the flow rate, the bypass will start opening and maintain the system pressure roughly steady.
In practical terms you'll almost certainly need to tweak the lockshields to cater for things like an oversized radiator where you'll want to set a lower delta-T to match heat input to heating load.

 

[Harry Bloomfield]

I'm fairly satisfied with my latest upgrades of my heating system. We have 1950's concrete raft floors throughout, recent DG and CWI, with 9 foot ceilings. Rads and pipework were all installed 35 years ago, pre DG and CWI. Heating is open vented (my decision) boiler is new modulating to which I added a fancy wireless system with outdoor sensor, plus adding TRV4's on all but one rad. The 2018/2019 winter was its first real test. Previously I just had mechanical stats controlling CH and HW. My old system would tend to run hot and cold - too wide a variation in temperature, which was noticeable - so I would tend to set a higher temperature, to minimise the cold dips. The rads are almost all mounted under windows, to maximise air circulation and mixing. I also have several tons of stone in the living room, which acts as an 'accidental' heat store.

New system works very well indeed, no hot and cold running radiators, just steady gentle release of heat, once it has settled on a temperature. None of the creaking as pipework heated up and cooled which I had become used to.

 

[ScottishGasMan]

If your looking at getting the best out of balancing then have a look at IMI Eclipse TRV's They have hydrualics in them to automatically balance load of each radiator so regardless of one or 20 radiators running at once, the flow rate through the operating radiators will remain constant at what is set on installation.

Technically even balancing a normal system to perfection on lockshields still has quite variable flows through the radiators depending on how many TRVs are open or closed at any one time, this can be countered to some degreee with very precise adjustment of the AutoBypass, but that means more of the heat produced by the boiler is simply recirculated back into the boiler, raising the return and therfor flue temperature, reducing efficiency.

 

[ericmark]

The idea of the combined lock shield and TRV is good, and I like the idea of a calibrated lock shield so no need to sit there with a differential thermometer to set the lock shield, however in real terms all it has done it to include a calibration, it still needs a lock shield on other side of radiator to be able to remove the radiator. Also since not bi directional it means if fitted to the return side the head has to be horizontal.

I would say so much depends on the boiler and what algorithms are already built in. Reading the literature it seems some boilers can actually reduce the return temperature as the output falls, so when it reaches the point where it starts to cycle the water temperature is low so less heat lost through the flue, but others the reverse is true and as the temperature of return water goes up, so the boiler output reduces with no special algorithms. Also boilers have a sweet point, an output where they run most economical, some it minimum and some maximum and some some point in between. And with oil often still it is a simple off/on.

So with a simple pump as each TRV closes the pressure increases so the flow through the remaining radiators increases so the boiler can run longer as the differential in the radiators reduces due to increased flow from the increased pressure, this is good with an oil boiler, however with a modulating boiler it may be different. With a modulating boiler you may want the pump to also modulate and you may not need a large by-pass valve, as by time it has lifted the boiler and pump will be at minimum.

So with a oil boiler, i.e. simple off/on you want different controls to a gas boiler that modulates possibly with very cleaver algorithms. So if I look at the Honeywell Y6630D wireless thermostat, it has anti hysteresis software which means as it approaches target temperature it starts to switch off/on using a mark/space ratio so it will not over shoot, this likely works very well with some boilers, however with others it completely messes up the built in algorithms. Nest seems to work out how long it will take to reach temperature, and remembers how much it will over shoot, so adjusts the point where it switches off to match, or uses OpenTherm. So with Nest the boilers own algorithms are not upset as much.

With a modulating boiler if you have 14 TRV heads all with different programs and temperatures likely when the TRV opens the boiler will be running so it will work well, but with an off/on boiler, if the boiler is not running then when the valve opens nothing will happen. So the boost function on the TRV head when activated will open valve for 5 minutes to 80% which should heat the radiator quickly then it takes time to cool so it gives that boost of heat when you first occupy the room and then returns to set temperature. But 5 minutes is not long, with modulating boiler which is already running great, with an off/on the 5 minutes have gone before boiler runs again.

My hall is in the centre of the house, below it is the flat, above it the bedrooms and either side dinning room, kitchen and living room, at the far end there is a door to outside, but rarely used, and the Nest thermostat is at the far end of hall to outside door. The Nest wall thermostat works well, it does one long burn when the temperature changes from night to day, then it will turn off for maybe 2 hours as the house as a whole retains the heat, so I decide I want to use the bedroom, which has been off as not planned to use it, so I turn up thermostat on radiator, but boiler not running, so I have to turn Nest up so it starts then back down so bedroom gets warm. So much for individual room control.