Replacing immersion cylinder with Condensing boiler, is it possible?

[Johntheo5]

I’ve noticed in several recent threads references to need for a large turndown ratio, even to the extent of implying the system won’t work properly without. I’m sceptical and don’t think it makes more than a marginal improvement. The heat has to go somewhere, and generally things are most efficient at rated power - electric motors, IC engines etc. I haven't seen any figures for boiler efficiency vs turndown.

Cycling, in itself, has no great effect on efficiency.

The biggest problem. especially with Vaillants, is the requirement on (all) gas fired boilers to ignite at ~ 65 to 75% of full firing which entails getting the flow temperature right down to avoid the temperature exceeding the setpoint temperature by 5C which trips the burner and can lead to multiple refiring after the anticycle period(s), its not too bad if the burner is released to modulate say 10 secs or so after firing but Vaillants (used to anyhow) maintain the ignition settings for up to 60 secs, by which time the flow temperature has exceeded the setpoint by 5C leading to burner trip. Some boilers have a settable dT of up to 15C which would be a great help but it should surely be technically quite easy to increase this dT for a minute or two after ignition.

 

[Madrab]

@Fionageko - bet you wish you hadn't asked now

 

[Harry Bloomfield]

The heat has to go somewhere, and generally things are most efficient at rated power - electric motors, IC engines etc. I haven't seen any figures for boiler efficiency vs turndown.

Nor have I, good point! This goes into it, in reasonable depth - https://www.heatgeek.com/what-is-boiler-modulation/

It seems to suggest no loss of efficiency, until they drop below 5% output.

 

[fixitflav]

Nor have I, good point! This goes into it, in reasonable depth - https://www.heatgeek.com/what-is-boiler-modulation/

It seems to suggest no loss of efficiency, until they drop below 5% output.

I'll read your link in more detail, but one point is, my boiler is non-modulating but never turns off on its control-stat in CH mode, so cycling isn't an issue. It just runs till the roomstat is satisfied. In HW mode it cycles 3-4 times before the cylinder stat is satisfied.

 

[Harry Bloomfield]

In HW mode it cycles 3-4 times before the cylinder stat is satisfied.

Has does mine, but probably only 2-3. The cylinder coil, simply cannot dissipate the heat fast enough, so it cycles. The CH is a different matter. It's one of those fancy, Vaillant predictive systems I put in. It works out just how low it needs to modulate, to just hit the temperature set, within a reasonable time, then hits the spot perfectly, with no over/under shoot, minimising on/off cycling.

 

[Johntheo5]

All "modulating" boilers control to the flowtemperature setpoint but when their minimum output is greater than the heating demand then the flowtemperature continues to rise until it exceeds the setpoint by 5C, the burner trips, the circ pump and fan remain running until the anti cycle time has elapsed, the burner then refires. A non modulating boiler will cycle all the time unless its matched to the minimum heat demand which isn't much use, my 20kw oil fired boiler will run for about 30 mins in the mornings while getting the rads and system water up temperature but often only runs for a few minutes when rooms up to temperature and the TRVs have throttled down when the heat load is often less than 3 or 4 kws.