how hot do you run you CH boiler

[tim...]

I had a new Combi fitted today (in the to-be-moved-to house).

Fitters told me that I should run this at 74 degrees.

Which I thought far too high, as

1) it makes the radiators too hot to touch 2) basic thermodynamics suggest that a better temperature profile will result from having the radiators at the lower temperate for longer period than a higher temperature for a shorter period

I tried to explain this but was met with

"The recommended temperature is required for the condenser to have any effect"

and the completely bogus "the temperature of the water in the radiators is set by the TRVs not the boiler temp. I couldn't persuade the guy that he was taking bollox, he played the "I'm the experience heating engineer card and I know better than you" card. Twat

Anyhow, at my current house it is 55 and works perfectly well

what do you guys/galls do

tim

[John Rumm]

Indeed...

It might be required in particularly cold weather, but most of the time you will be able to use less.

You will most heat recovered from the condenser when the return temperature is below about 54 degrees (the dew point of the flue gasses).

I have weather compensation on mine, and so it chooses its own temperature based on the outside temp. Basically that means its runs as cool as it thinks it can get away with and still be able to reach the target set point temperature in a reasonable amount of time. (the relationship is set by choosing a mapping curve that reflects the rate of heat loss of the building).

Currently the external temp is 6.5 deg C, and the flow temp is running at 54 deg. If it were to go well below 0, then it might push the flow temp up into the 70s. When its milder it might run flow temps down in the 40s.

[nothanks]

I thought the advice was to heat DHW to 60 degrees to avoid the risk of legionella, and that's not going to happen if the boiler's running at a lower temp.

[Tim+]

That may be true, but in practice I don?t think Legionella has ever proved to be a problem in domestic systems.

Tim

[Max Demian]

The instruction manual for my Ideal says that the 'e' setting is best for condensing, which is quite hot. I don't understand this as I would have thought that the cooler the returning water temperature the more condensing.

I use a much lower temperature as I don't have a room thermostat and the TRVs are fiddly and don't seem to keep the air temperature constant.

[Harry Bloomfield]

tim... pretended :

I have an opened vented system, with stored HW and the house is heated by the one boiler, installed new last March. A Vailant ECOfit Pure 418. We like the stored water to be good and hot, the boiler only has one temperature setting available for its output temperature - so I have it set at its max, of 75C, in order to meet our HW needs. The house heating would be satisfied by a lower boiler temperature around 60C and it would be more gas efficient, but unless I manually manage the temperature setting I am stuck with the less efficient 75C.

I even tried to find a way to fool the boiler into working at a lower output temperature, by adding a separate bi-metal stat on the boiler output pipe to CH only, set at 60C, to shut it down at that temperature

- as heating satisfied. That was a miserable failure, in that the boiler constantly cycled on/off via the stat, whilst ever the room stat indicated a demand, rather than modulating its output down - as it would on its own as it neared its 75C setting.

If anyone knows a workaround, to allow split boiler output temperatures on this boiler - do let me know please.

[RJH]

snip

I don't follow how that can work properly, as for most homes different rooms will have a different 'curve'. Or does tweaking the TRV compensate?

[RJH]

My understanding of the 'e' mark on my Ideal boiler's central heating temperature control is that any setting above is non-condensing. Any setting below and the boiler's condensing.

[harry]

Oh yes it has. Plenty of people have died.

[harry]

TRVs don't work. They are more influenced by the nearby radiator temperature than the room air temp. You can get them with remote sensors. They work.

[John Rumm]

It seems to work well enough in practice. Each room also has a TRV, and I have the place split into two zones; upstairs and downstairs, so that will account for some variation.

It needs a little bit of experimentation to set the profile the first time - basically waiting for colder days and seeing if the system still heats the place comfortably and quickly enough. If it doesn't, then you just tweak it up to a steeper curve. It also helps if you have appropriate rad sizes for the rooms, and the system is balanced.

The response curves look like:

IIRC the system defaults to the 1.2 line. If you live in a super insulted place / particularly sheltered location then you would tweak down. In my case (exposed location - solid wall construction), I needed to go up. I found the 1.8 curve worked well.

The system is also smart enough to automatically shift the response curve vertically based on the currently demanded internal target temperature[1]:

So if you tweak the room temp up or down during the day (or have different times programmed with different set point temps), then it can vary the flow temperature to match the requirement.

[1] Note that all the system temperatures sensors (internal downstairs, internal upstairs, DHW cylinder, and external) are digitized and processed as actual temperatures, not just as on/off "call for heat" style demands.

[David Wade]

If your boiler allows external timers consider getting a TADO or Drayton Wiser (Or just wireless TRVs and a Reaspberry PI if you are the adventurous type). Lets you see what temp the TRVs think the room is at.

Dave

[John Rumm]

IME many of the Vaillant boilers do support split temperature operation, but it does depends on what controls you have them paired with.

The 400 series Vaillant I did (probably previous model range, since this was >= 12 years ago), actually had separate knobs on the front for CH and DHW flow temperatures. Although that was retrofitted into system with traditional controls that could not distinguish between the source of the call for heat - hence the DHW control never came into play and it ran like your system does.

The blurb on their web site:

Seems to suggest its compatible with the VRC 700 weather compensating controls, so that ought to allow split temperature as well.

[Harry Bloomfield]

Max Demian explained :

That is at odds with my own findings, since installing TRV's last March, but I do have a room stat, in the hall and no TRV on just that one rad.

I tried setting the TRV's up in March for each room, but there wasn't much need for heat at all then. I found it better to set them once the cold weather arrived this winter. Having gradually tweaked each to a comfortable temperature, they work pretty well. Working on just a room stat, the temperature variation was quite noticeable as the boiler kicked in and out on just the room stat..

[Harry Bloomfield]

John Rumm pretended :

Balanced?

I have always assumed there was no need to balance a system, where TRV's are installed.

[Harry Bloomfield]

It happens that John Rumm formulated :

Which is what I have done. The TRV's prevent the rads getting hotter than needed for each room, except when the heating is first put on with the house 'cold'. In quotes, because it never gets truly cold.

To mitigate turning the heating on from cold and the room temperature over shooting, I tend to nudge the room stat up in a series of steps over several minutes.

[tim...]

not something that is a consideration with a combi

[Harry Bloomfield]

They work absolutely fine here, once the heating has been on for a while and the system has settled down. I am able to log/graph room temperatures and once temperature is achieved, I only see variations of around 0.5C in the graphs. What TRV's or any type of stat cannot cope with is the overshoot, when the heating is suddenly turned on, or the stat suddenly turned up to a much higher setting.

[tim...]

if TRVs can be set to reliably keep a room at, say 20 degrees, then it doesn't

but if they can't, then it does

I'm about to find out if the first line is true, because it is clear than my new systems is not balanced.

As it is currently set, the lounge is like a sauna, and the rest of the house still cool.

I actually don't have a problem with the latter, it's the former that's the issue

tim

[Harry Bloomfield]

John Rumm formulated the question :

I have studied it and not found any obvious way to split the temperature. I even spoke to Vailant and they confirmed there was no way it could be done.