Your opinion on CH Boiler setting

I have a cylinder but my condensing boiler has different temperature control settings for hot water and radiators, so presumably I can set the radiators to a lower temperature without affecting the water.

How low is low in this mild weather as a guideline?

Thanks

Jonathan

Reply to
Jonathan
Loading thread data ...

Dual temperature controls on combis are commonplace. Dual controls on heating only or system boilers are less so (since your simple boiler has no idea what source the call for heat is coming from).

Posher ones have more sophisticated controls. Some even have things like periodic anti legionella programmes to heat the tank to 70+ once a week etc.

Reply to
John Rumm

Thank you for that gem of information. However the simple modulation you describe relies upon the boiler water temperature whereas an intelligent controller also takes account of internal and external air temperatures

Reply to
cynic

But the saving due to the reduced average will be more than offset by the longer burn period. In your example the energy used during the 5 hour "on" period remain the same in both cases and the average temperature during warm up from (say) 14 degrees will be 17 degrees on both cases so the heat losses during the shorter 20 minute warm up will be one third of what they would be in the 1 hour warm up. There'll be a saving from the improved efficiency of the condensing boiler running at a lower return temperature but during the warm up period it won't be enough to compensate for the longer burn. Ideally we need a system that starts to warm up at maximum water temperature and then drops back to a lower temperature as the target room temperature is approached, unfortunately very few domestic heating systems provide this.

Reply to
Mike Clarke

The heatloss is directly proportional to the average temperature. So having a lower average as a result of slower warm up will result in lower losses.

Ignore the length of burn and just think in terms of rate of energy loss. If you maintain an average temperature of 20 degrees over five hours you will lose more heat than if you maintain it at 19 degrees over five hours.

The longer burn (i.e. less cycling, less overshoot, and greater condensing efficiency only help the case)

Pretty much *all* modern boilers do something along these lines. They will typically reduce their input power to try and match the heating load. What varies is how they do this, and how much modulation range they have. Most for example will not exceed the upper water temperature set on the boilers front panel control, even when the circuit is taking the full output of the boiler.

Reply to
John Rumm

And the construction of the building. Typically the installer selects one of a set of response curves when using weather compensation to dictate how fast the flow temperature ramps up in response to falling outside temperatures.

Reply to
John Rumm

Yes, but if the room temperature rises from e.g. 14C to 20C while warming up the average temperature during this time will be 17C irrespective of the duration of the warm up so a slower warm up will have higher losses.

But a 5 hour run at 20C with a 20 minute warm up from 14C equals 5 hours @ 20C plus 20minutes @ 17C which is less heat loss than 5 hours @ 20C plus 1 hour. @ 17C

Yes that helps but I doubt if it's enough to compensate during the slower warm up.

Yes, but what I was suggesting was a way of automatically overriding the set water temperature if the room temperature was well below target in order to get a faster warm up.

Having said this I'm not advocating leaving the water temperature set to a high value. We have ours set as low as possible consistent with maintaining the desired room temperature. This gives us the best condensing efficiency and provides a more comfortable background heat but it does mean a long warm up time with a little bit more heat loss while warming up. But we're normally at home all day and run the heating from 8am to 11pm so the slow warm up time doesn't contribute much to the overall heating costs and the optimum start feature on the CM927 makes sure its warm enough by the time we need it.

Reply to
Mike Clarke

Ah, I see what you are doing. You are analysing a slightly different scenario by moving the warm up period outside of the measured time period thereby arriving at a longer overall heating period. You can't really do that if you want a like for like comparison.

My notional 5 hours includes the warmup time, and hence some time at less than target temperature due to the warm up delay. This reduces the overall average a little. So in this context, the slower that warm up, the greater the reduction.

Well think about it in terms of 24/7 heating since that is what we really care about. Take a simple case with a manual stat and a timeswitch. Heating comes on at some time, and off at another - perhaps twice a day. The demand temperature set on the stat is left alone. The maximum burn time in both cases is the same, the maximum flow temperature is the same. The target room temperature is the same. The percentage of time spent actually at the target temperature will be less with a slower warm up than a faster one. Obviously there is a tradeoff between efficiency and comfort here.

If you start introducing things like optimising room stats that can choose to fire the heating for longer so as to ensure the room stat temperature is actually on target for the entire duration of the set periods[1] then it gets more complex (although an optimising stat is something designed to improve comfort at the expense of energy use)

[1] i.e. you say I want 20 degrees from 7am to 9:30am, then it will turn the heating on a 4am if that is what it takes to achieve your request.

This is one of the features of weather compensation and also some of the higher end boilers with proportional inputs from their controls, comparing target temperature, actual temperature, and outside temperature, and using these with a set of reference curves to set the boiler operating parameters.

Yup the optimising will confuse issues - how much will depend a bit on the construction of the building and amount of thermal mass that needs to slew around to meet changing temperature demands.

Reply to
John Rumm

Hmm. So occasionally you stick your hands under the hot tap, and instead of the usual hot 50 it's a scalding 70.

I wonder if this is more or less of a hazard than Legionella in domestic systems?

Andy

Reply to
Andy Champ

A friend who has a system like that, says it's programmed for the middle of the night.

Reply to
S Viemeister

Depends entirely on the size of your radiators, i.e. how oversized they are for efficient condensing operation. I run mine at 45C/40C down to around 0C outside, but I did oversize the rads for efficient condensing operation. I would run it even lower for current outdoor temperatures, but the boiler's hysteresis is too large for that to work well.

Reply to
Andrew Gabriel

You would normally have a blending valve on the cylinder outlet these days, so it should make no difference to the water temperature at the tap.

Reply to
John Rumm

Our boiler has this and yes that is what happens but you can choose a day and time of the week for the legionella protection cycle which holds the tank at 70ºc for 1 hour. We've grown use to being caution when opening the hot tap.

Reply to
gremlin_95

This is roughly what I did with the replacement front panel control board I designed and fitted in my Keston boiler, driven from a PC.

Steady state heat output is almost directly proportional to difference between inside and outside temperatures. (You need to factor in wind- chill and rain to get more precise, but I didn't bother with that.)

Then you have to calculate what you need on top of that to cope with not being in steady state, e.g. heating house up from cold. There are two separate components to consider when heating up from cold, because they have such different heating characteristics. Firstly, there's heating the air - this can be done quite quickly. Secondly, there's heating the building fabric indirectly from the heated air, and this takes much longer. The calculated steady state heatloss of the house was around 11kW. I could heat the air from cold to room temperature in 15-20 minutes using the full 25kW boiler output (some of which is including a third component - heating the heating system itself). However, it took around a day to get the building fabric heated up so the output could fully drop back to 11kW. At one point, I had a thermostat embedded inside the brick wall between the lounge and dining room, to measure the temperature of the building fabric.

Bringing this all together, the heating profile to bring the house up to temperature from its setback of 11C, with outside temp of -3C looked rather like this...

25kW for 15 mins. Dropping back steadily to 15kW over next 15 mins. Dropping back steadily to 11kW over next 20 hours.

If you get this exactly right, the roomstat becomes redundant, because the boiler modulates its output to the right level to match the heat losses, and the stat will almost but never quite click off. This is the most efficient boiler operation.

The other thing about heating the house up from cold is that you can do it in different ways. With the capability to do it suitably quickly, you can base it entirely on occupancy. Alternatively or additionally, you can say to the controller that you want it heated up by a certain time, and the controller can do so at a more steady pace over a longer time period. This was interesting to me because there looks to be some evidence that my boiler (a Keston C25) probably has a much longer life if you avoid running it near its highest temperatures, so this is what I tend to do now.

Another thought that occured to me (but I didn't try) is that in the case of preheating the house before occupancy, I could deliberately overshoot the air temperature in order to boost the building fabric heating, and then drop the air temperature back down to the expected level at the due time.

Reply to
Andrew Gabriel

Thus leaving you with no immunity to legionella...

This bacteria has only become a problem since we started trying to make water supplies super clean. Exposure generates immunity. The methods we're using to cure increasing numbers with no immunity due to no exposure are rapidly packing up - it's becoming more antibiotic resistant, and it's become chlorine resistant. Your immune system will protect you providing you give it some exposure to the bacteria, as is the case with millions of other harmless bacteria - that's how your immune system works.

Reply to
Andrew Gabriel

It would have been unusual in domestic boilers prior to combi boilers.

It's pretty essential for condensing boilers to modulate, although some of the very early (and largely unsuccessful) system condensing boilers based on modified conventional designs couldn't.

Reply to
Andrew Gabriel

Do you know where you got it from?

Reply to
Andrew Gabriel

Don't know about you but I doubt my cylinder loses 20 degrees overnight.

Andy

Reply to
Andy Champ

Immunity to some diseases (malaria, cholera) is believed to be the source of sickle cell anaemia and cystic fibrosis. And that's only partial immunity.

We're not immune to everything.

Andy

Reply to
Andy Champ

I got rid of my cylinder in July...

Reply to
S Viemeister

HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.