Boilers (again) - Weather compensation

I think there's two ways to look at it:

1. TRVs and room thermostat are in charge. The boiler is just a dumb on/off heater.
2. Boiler is in charge, adapting its flow temp to meet the heat losses of the house. Radiators are balanced such that TRVs aren't necessary.

In scenario #1 you have a dozen or more thermostats all fighting against each other. Maybe the far bedroom is cold and its TRV is desperately calling for heat, but the thermostat in the hall hasn't noticed. Maybe the hall stat is telling the boiler to fire but all the TRVs have turned off their radiators.

As a control system it's a mess - you burn gas to circulating heat around pipework that doesn't actually want heat, or you need heat without a way to request it. Additionally there's no way to call for a small amount of heat

- it's a bit like driving a car by jamming your foot hard on either the accelerator or the brake. When you do have the accelerator on, each wheel can then independently disconnect from the drivetrain if it feels like it. It makes it very hard to control.

In scenario #2, you balance the radiators to deliver the right each for each room. For example maybe the far bedroom has 3 exterior walls and a flat roof, so has lots of heat loss, whereas the loo under the stairs has no exterior walls so has less. You set the flow in each radiator to heat each room appropriately, so that when the boiler is on on the coldest day, everything is at the right temperature. You can still have TRVs but you would set them high so they aren't actually triggered (ie unless things go very wrong the TRV is always open). Then weather compensation adjusts the flow temp based on how cold it is today: if it's cold outside, the heat input needs to be more, but if it's mild the flow temp is throttled back.

Because all the radiators are balanced then the heat is delivered evenly across the house. The weather comp also notices when the sun goes down and increases the heat going into the house before the house has had time to get cold, unlike the thermostat which has to actually see inside drop before it'll kick in.

In #2 it's more like cruise control in a car: go downhill you need less power, go uphill you need more. The heat is more even, and the weather compensation allows it to be more efficient. But what you really don't need is each wheel deciding their speed independently, because then you can't control the overall speed any more.

Scenario #1 works if you are likely to want to change room temperatures independently - eg a room you only go in once a week, maybe you turn down the TRV the rest of the time. But if you just change the whole house at a time (eg cooler at night, warmer during the day) you can do that with the central thermostat.

I'm not sure the efficiency gained by lower flow temps in a gas boiler, but in a heat pump design, it actually works out more efficient to keep everything at a constant temperature than to micro-zone using TRVs, even if that means heating rooms that aren't ordinarily used. This sounds counterintuitive but is explained here:

That's deliberate to release in blobs - by avoiding dribbles it stops icicles forming and blocking the outlet. If you can keep everything inside it avoids any freezing troubles.

Theo

I had a modulating boiler installed, with TRV's added on all but one rad., but reused my original cylinder stat and wall stat - on/off switching. I found boiler ran flat out, only modulating based on it return and flow temperatures, with much creaking of pipes, as they heated and cooled, just as it had always done before.

Within a few weeks, I decided to upgrade the controls, to include outdoor sensor/weather compensation, plus actual sensing of room temperature and cylinder temperature.

The difference was night and day - no more creaking pipes, a much steadier, better controlled temperature, and the boiler modulated right from the start of lighting up - no racing up to the stop line.

I've no figures to back it up, but it certainly gives the impression of having saved fuel.

Probably more likely the installers get confused...

The main complication with weather compensation comes with heat only or system boilers rather than combis, since you need to be able to run a high flow temperature for DHW recovery, and not the weather compensated flow temp.

The boiler should still load match and modulate. However weather compensation should gain you some extra efficiency and fuel savings - especially in the bits of the heating season where it is not yet properly cold. (also less temperature swing, and less creaking and clanking of boiler pipes)

Most decent boilers will have a condensate syphon - designed to dump water in a "flush" rather than a trickle. That makes it far less likely to allow a condensate pipe to freeze. You should also transition to a larger pipe than the 21mm that comes out of the boiler before you take it outside.

It sounds like your existing install was a tad sub-standard if it takes the thin pipe outside. However depending on location and exposure, it may not actually have caused problems to date.

Why so? Bleeding a sealed system is usually easy if the bleed valves work. If they don't, mention which ones don't and have him change the bleed valves at the same time.

That does not preclude the "dumb" heater running a flow temp appropriate for the time of year.

Weather compensation is not really matching the house heat losses, it is just responding to the external temperature and using that as a proxy for the heat losses - making the assumption that lower outside temps will result in higher loss rates (usually true), and that the installer will have selected a response curve that reflects the general insulation level of the house.

These assumptions can be thrown out by factors like wind resulting in higher loss rates than the temperature alone would suggest, and non consistent levels of insulation meaning not all bits of the house lose heat at the same rate.

So in reality you set the compensation to run the flow temp a bit warmer than theoretically necessary, and then allow the TRVs to limit the room temps on the still days, and in the better insulated rooms.

Normal TRVs don't really fight each other - they live in blissful ignorance :-)

As each throttles the flow down they may leave more flow available for other rads - but that is generally a good thing if the balancing is not perfect.

That is why you should have the main stat located in a higher heat loss space so that it can function as the final "interlock" - shutting down the heating when everything else is up to temp.

That is just a installer error! There should not be TRVs on the rads in the room with the main stat. They are at best pointless, and at worst they defeat the concept of a boiler interlock and can result in the boiler firing unnecessarily when the house is already warm enough.

That is what the TRV does - disables flow to rads in rooms that don't need heat.

Multiple zones can help in some cases. Or go the whole hog and use TRVs that can communicate with the boiler and reduce zoning down to a room by room level.

Another argument for weather compensation - modulating the throttle!

Weather comp will often factor the difference between the actual internal temp and the set point temp when deciding on a flow temp - it is not just decided on external temp alone.

(you could say it put the heating into a mode where it behaves the way that many people mistakenly assume heating works - where if you turn the stat up, it will actually cause the place to get warm faster!)

On condensing boilers, you get slightly more heat recovery from the combustion gases.

They are not Engineers, they are Fitters, and dont know enough about Weather Compensation to advise you properly. Using WC has been shown to be more comfortable, and uses less gas to heat the house. You need a suitable internal thermostat, but other than that, it's very simple. If you want to use more gas than you need, then go ahaead, and dont fit WC. Also, that they were going to fit a 30kW boiler shows they have little clue about heat losses and optimal running of the boiler.

I hope you get a good guarantee with the boiler. Check the innards are brass and not plastic. Mine was plastic and it leaked like a sieve. There were so many faults I gave up with it.

BTW I went for an Intergas boiler. In the scheme of things it's not that more expensive than a Baxi and is much simpler in concept.

Either way the boiler sets flow temperature and will modulate according to demand. There still needs an overriding on-off switch, normally a thermostat in a hall way.

They don't fight, but provide their own 'zones' in lieu of zone-valve(s).

The TRVs will create the zones, and the pipework to those valves will be cold if there's no demand.

There needs to be an overriding thermostat, or use RF Smart TRVs

I would still recommend TRVs however well the radiators are balanced.

I would recommend weather compensation as the lower the flow temperature the more efficient the boiler is. It should pay for itself. I use manual adjustment in lieu of compensations!

That assumes an outside temperature of -3C, so a room temperature of difference will make little difference in over 20C. Most days the outside to room temperature difference is far less and the saving will be correspondingly more.

I've just found out that the Drayton Wiser HubR I put in a year or so back has an Open Therm connection which the Baxi boiler I've order also does.

Shouldn't that give a more precise form of modulating/load matching? It surely should stop any short cycling and lead to a more even performance?

Ah, because the rads have those stupid little bleed screws at the back that you can barely get at with a bleed screw and at least three of bleed screws just slip. The engineer was not optimistic that I would find the right replacements and even then it would be likely necessary to remove the rads to get or drill the old screw out. He thinks it easier to just put new rads in but then he's not paying for it.

It will allow for a more nuanced conversation between the stat and the boiler than the traditional HEAT / DON'T HEAT demands.

Rather than fixing the existing bleed screws, do your rads not have a spare tail connection?

Most rads have one at each corner, and normally you end up using two, blanking one, and fitting a bleed valve to the other. If you have an unused one, remove the blanking plug and replace with a "normal" bleed vent:

It sounds like those s**te ones that only have tail connection bosses at the bottom, with the bleed nipple at one of the top corners, tapped into the back of the rad.

I>>> users get confused.

Isn't it still only on/off? It has weather compensation but that just adjusts the time it starts heating to bring a room to temp as required.

No, but if you were paying him a decent hourly rate the cost of fixing the existing rads might be comparable to replacment.

Replacements would have better heat transfer...

Dave

It is the Baxi Platinum, brass hydraulics and 10 year parts and labour guarantee subject to annual servicing and the filters.

That sounds more like an optimising stat rather than weather compensation as such.

I'm not really sure what a "spare tail connection" means. Don't the devices you cite need to be at the top of the radiator?

Here are photos of the radiator types and bleed screws.

The problem is the type of radiator. Instead of a built-in bleed, many radiators have a threaded connection at each of the four corners:

Most rads (but not yours apparently!) have 1/2" BSP female tappings at each "corner" - i.e. top and bottom both sides. So you can plumb them with top and/or bottom entry. That is why most bleed valves come with a blanking plug to fill the unused connection.

A new rad is probably cheaper than the plumbers time to deal with knackered ones