Gas combi boiler

moneytalks

What timer and room thermostat do you currently have?

It’s on of those EPH wireless 

ThisIsWeird

moneytalks said:

What timer and room thermostat do you currently have?

It’s on of those EPH wireless 

I think these are 'Opentherm'. Whether wired correctly for this, I don't know. I'd have thought that you would have noticed the boiler's flow temp falling quote dramatically as the room temp approaches the 'set' temp. The boiler should modulate right down, and that includes lowering the actual flow temp, afaIk.

I'd contact Baxi, give them the exact model, and that of the controls, and ask how best to have them set up - and what you should observe as a result.

https://www.youtube.com/watch?v=b3ulvuZVegk

FreeBear

Even with OpenTherm connected, the flow temperature could be swinging up & down due to the boiler short cycling. Many Baxi boilers won't modulate down to under 6kW and with 4 TRVs set to 1-2 and the rest on 3, quite likely there is insufficient heat demand.

My boiler can modulate down to ~3.2kW, and just one TRV activating can be enough for the boiler to start short cycling.

ThisIsWeird

FreeBear said:

Even with OpenTherm connected, the flow temperature could be swinging up & down due to the boiler short cycling. Many Baxi boilers won't modulate down to under 6kW and with 4 TRVs set to 1-2 and the rest on 3, quite likely there is insufficient heat demand.

My boiler can modulate down to ~3.2kW, and just one TRV activating can be enough for the boiler to start short cycling.

Not sure I fully follow what Opentherm and its ilk do.

All modernish boilers will automatically 'modulate' its output down - ie reduce the burner flame - in order to maintain a set flow temp (say 60oC) to match a CH demand. When demand is high - the house is heating up from cold - the flame will be at max or near so, in order to provide that 60oC. As room temps are reached, and TRVs begin to shut off, then clearly less flame is required in order to maintain that 60oC flow to the reduced number of rads. That's the gist of 'modulation', yes?

But, Opentherm goes one better - it will actually reduce the flow temperature in order to maintain the heat requirement of the house? Ie, as CH demand is reduced, the boiler won't just keep chugging out water at 60oC, turning its flame down to suit, until it finally goes 'click' off, but will instead alter the set water temp, turning its flame down to allow the flow temp to fall, and it does this in order to match demand - ie, it'll try and keep the boiler running for longer, at a kind of tickover, with a cooler water flow, so that the rads emit to the room the amount of heat it needs to maintain the room temp. Is that it? So, you can expect the flow temp to actually fall as the demand is approaching being met.

The cooler the water, the more efficient the boiler. The less heat strain on it too.

FreeBear

Yup. Your understanding of how modulation works is pretty much spot on. Adjust the gas flame to maintain a constant boiler temperature regardless of how many (or few) radiators are "on".

OpenTherm and its ilk adjust the flow temperature to vary the heat output from the boiler with the aim of balancing it to match the heat loss of the building. At this point, the boiler should be running at peak efficiency without short cycling and the room temperatures are not swinging wildly between hot & cold.

Here, you can see 60°C is being called for (orange trace), and the flow temperature quickly reaches it (green trace). As the return temperature rises (blue), the boiler starts to modulate down (red trace). After 30 minutes, the room temperature starts to increase, so the thermostat slowly commands a lower and lower flow temperature. In turn, the boiler reduces its output to keep pace.

With this plot, the green trace shows how much the boiler is actually adjusting the flame in relation to commanded temperature and heat output.

In both plots flame (pale blue) is there to show when it is on, not the rate of burn.

moneytalks

FreeBear said:

Even with OpenTherm connected, the flow temperature could be swinging up & down due to the boiler short cycling. Many Baxi boilers won't modulate down to under 6kW and with 4 TRVs set to 1-2 and the rest on 3, quite likely there is insufficient heat demand.

My boiler can modulate down to ~3.2kW, and just one TRV activating can be enough for the boiler to start short cycling.

Should I have all the TRV at a higher setting?

FreeBear

moneytalks said:

FreeBear said:

Even with OpenTherm connected, the flow temperature could be swinging up & down due to the boiler short cycling. Many Baxi boilers won't modulate down to under 6kW and with 4 TRVs set to 1-2 and the rest on 3, quite likely there is insufficient heat demand.

My boiler can modulate down to ~3.2kW, and just one TRV activating can be enough for the boiler to start short cycling.

Should I have all the TRV at a higher setting?

Up to you.

I have all my TRVs set to max as i don't want the boiler short cycling. It doesn't appear to have increased my gas consumption.

moneytalks

FreeBear said:

moneytalks said:

FreeBear said:

Even with OpenTherm connected, the flow temperature could be swinging up & down due to the boiler short cycling. Many Baxi boilers won't modulate down to under 6kW and with 4 TRVs set to 1-2 and the rest on 3, quite likely there is insufficient heat demand.

My boiler can modulate down to ~3.2kW, and just one TRV activating can be enough for the boiler to start short cycling.

Should I have all the TRV at a higher setting?

Up to you.

I have all my TRVs set to max as i don't want the boiler short cycling. It doesn't appear to have increased my gas consumption.

Do your rads ever cool down when you have them to max or are they always very hot?

ThisIsWeird

Interesting.
Rads would be expected to remain hot as long as their TRVs are set higher than their respective room temps. This would be the case with your boiler, because as long as it's running, it'll be delivering 60oC water = hot.
FreeBear's setup is different, tho', 'cos he does have an Opentherm-type setup, so when the 'stat in his main room tells the boiler "Nearly there...", the boiler will cool the flow down, so all his rads, regardless of TRV setting, with them start to cool. 
For you, keep each TRV at a setting that suits each room. And 'off' when not actually required - bedrooms during the day, for example.

moneytalks

Sorry to all of I’m asking stupid questions. I had oil fired heating for 30 years. No TRV just on or off.
The age old question, is it cheaper to run heating on low or turn on when you are cold and turn off when warm?