Cold radiators - not sure why...

A Baxi 105HE is a powerful boiler - it can output 30kW. More the the point its *minimum* output is over 11kW.

Being a combi, it was probably specified to give decent DHW performance, however from a CH point of view it is probably way oversized.

I expect the problem is that you don't have enough heating load - so even with it running at its minimum of 11kW, it rapidly reaches the point as the return temperature rises it can't keep the flow temperature to the limit you have set - so it cycles off - that is its only option at that point.

The low flow temp may promote more condensing efficiency, but the system efficiency will be abysmal because of the short cycling - costing more rather than less.

Set the flow temp to 75, and see how that works.

If that is ok, then chances are the rads were all sized with an anticipation of an 11 drop and a high flow temp. The way to "fix" it would be to increase the heating load with higher output radiators or additional rads. This might mean larger rads, or perhaps double panel where there are single, or finned where they are plain etc.

Thanks John & alan_m. I think this is finally sinking in.

Whilst my earlier concerns were first raised in milder weather I was expecting the boiler to be modulating in this prolonged cold spell. However yesterday I ran some figures with the aid of the Wiser App that I now have.

Daily usage is around 4 cu ft gas which comes out (according to me) at around 126kWh. The app says the boiler (and I assume burner) was on for about 15hrs, so 8.4kW which is below the 11kW modulating power you mention above. So it has to cycle - got it!!

I believe this is the 3rd boiler, this one being installed in ~2006. The first would not have been a combi though it is likely the 2nd was. My experience with 3 service engineers is when asked about replacing they've all quoted like for like, ie 30kW without going around the house which has changed substantially since 1968.

Indeed I have been chasing condensing efficiency. It seems quite a steep slope of gain.

I'll do that for the next 24 hours - presumably still leaving the schedule as is? I suspect that last year, before the introduction of a room thermostat, that is around where I'd wind the temperature dial and set the timer on all the time in this sort of weather.

I doubt, knowing what some of the previous owners have done, that the extended and modified house led to any sort of sizing.

However I'm trying to keep my bills as low as possible and have turned off a large radiator and turned down others. Surely doing what you are suggesting is going to increase my bills. My wife would love it if the house is warmer at less cost!!

Scrub that. I've just never had a thermostatically controlled device before and keep forgetting to factor it in.

Yup with heating systems you have somewhat conflicting requirements; low flow temps are good for boiler efficiency, but that only makes sense where you can also get the house up to the desired temperature in a reasonable amount of time.

In very cold spells, you need to use higher flow temps to increase the

TRVs and a main stat as an interlock will control the total energy being used. You can also adjust the temp in each room based on need and use with the TRVs. It is important that you don't disable the heating in the room with the room stat - since that would also disable the interlock. Once the house is up to temperature, you want the heating to stop. It can't do that if the main stat is never satisfied because it is in a room with the rads off (or with their TRV temp set below that of the main stat).

John, is it usual not to have TRVs on some rads - we don't have them on hallway and bathroom rads, only on all other rooms (apart from one small utility room).

Our system doen't have a central heating temp stat, only a hot water tank stat. There is a stat in the ground floor hallway but not connected to anything. I was thinking of ignoring that and putting in some kind of wireless control - how easy is that to put in a S-plan system with an oldish Honeywell programmer?

ITYM 4 units (where 1 unit = 100 cu ft), rather than cu ft!

But yup, multiply 4 by 32.15 for an approximation to kWh[1]

Often the case with heating installers - they don't like doing sums it seems, and just assume that the person before them did them! It is often assumptions all the way down.

It is a steep(ish) slope - but remember it is chasing a marginal gain. So if a bog standard non condensing modulating boiler can get 75% overall, and with condensing you can get that to 95% seasonally adjusted, a 25% improvement in condensing efficiency is 25% of that 20% gap - so a 5% overall change.

Thankfully my brain was functioning correctly when I set up my spreadsheet ~ 10yrs ago.

There is some deduction and understanding required, and careful attention to where the ft3 is written:

Yup, that is common and and sometimes "by design". You may also have some uncontrolled rads (i.e. not even manual adjustment) This often what you see in the the hall (or other space) shared with the room stat. You would balance them so that they are among the last to heat to heat up. The reason they are often setup with a lockshield valve at both ends it to stop the user from fiddling with them and defeating the interlock function provided by the stat (i.e. they turn the rad down to "save money" and as a result the heating never turns off, costing them more).

As TRVs throttle down elsewhere, more flow goes through the (always open) non controlled rads. That brings the temp in that room up, and ultimately will turn off the heating using the main stat.

(another reason for always on rads is to make sure there is always a path for water to flow through the system. Something you need to think about with S-Plan systems since both valves can be closed)

That basically means the CH runs all the time that the programmer has it "on". It can cycle on its own internal stat, but will never go properly off. That may be what you want in the coldest bits of the year, but quite inefficient during milder parts of the heating season.

Easy generally. A modern prog stat can in many cases can act as both a stat and a programmer in one unit (so you either remove the programmer, or leave it set to permanently "on").

You install the receiver for the stat near the boiler and wire it to the stat input on that. Then pair the wireless stat with the receiver.

The wireless stat will then let you specify the temperature required at various times of the day. The simplest will have two temps "normal" an "set back". The set back one being the baseline it will not let the property fall below. More sophisticated ones will allow multiple spells of time at independently selectable temperatures.

So you could program it to say 21 degrees between 7 and 9am, 19 degrees from 11am to 4pm, then 20 from 6pm to 11pm. With say a setback temp of

Posher versions are optimising - they will learn the response of the house and adjust the time the heating comes on so that it meets your demand. So with a non optimising stat, if you say 21 degrees from 7 til

Link not working at the mo from here.

Yup, by that measure I could heat this house from farts alone :-)

mmm .. I check links that I post from such sites in a Private window to make sure I'm not in the trap of only working when logged in. In this case it works in both Firefox and Edge (Win10).

worked for me, then an annoying popup from flickr blocked out the photo with cookie request information.

The neons on the boiler are every 10 deg so this is a bit rough and ready but the cycle is now:

T0 Temp 70deg burner turns off T+3' Pump turns off T+6' Burner on and fairly quickly up to 70 T+10' Burner off Cycle repeats

So upping the temp seems to have set a steadier 10min cycle but which now includes the pump stop/starting. Pressure obviously increases and is about 2.5 bar.

Is the above an improvement? Do I need to fiddle more?

Additionally, and relating to previous posts and also a recent thread to another poster, I too have turned off the large hall radiator that has lockshield valves. The hall is a large unused space. The hall's current temperature is about 15C. The smaller bathroom radiator also has lockshield valves and is on and hot. All the 9 other radiators upstream and downstream are working. I'm not aware and wouldn't know how to see if there is any zone valves. I'm not sure I've ever satisfactorily had explained all the pipework coming out of the (combi) boiler which is at the end the garage/utility room extension, but I'm fairly sure there is only one out and one return feed running through the attic.

I think it was a temporary Flickr thing... working now...