Real world testing example of Condensing Boiler in older home

[JC22024]

Post purging is interesting.

On the relatively large (50MW) oIl and gas fired boilers that I was very familiar with, we used oxygen analyzers for combustion control, normally 1.5% to 3%, depending on boiler output. Two oxygen analyzers were used. A post purge only occured if the boiler tripped on either flame failure or if both analyzers fell to 0.2% which meant combustion conditions was getting close to fuel rich conditions. If we requested a normal boiler stop, no post purge took place.

I am not so much concerned about the gases etc with enabling post purge, more so the oil side to make sure the boiler is shutting down correctly each time. I think the oil popping issue may be related to that.

I gave the boiler a perm live tonight and it is now shutting down correctly. The post purge is about 15 seconds roughly. Will be interesting to see if the oil popping noise is gone tomorrow morning on first start up.

 

[John.g]

I can't see any difference in the oil pump solenoid operation with either no post or post purge, actually if you suspect fuel nozzle dribble, it can possibly dribble more with a post purge as the oil pump will also be driven for the 15 secs vs a few seconds on a no post purge as the fan runs down pretty fast as its being braked by the fuel pump, obviously you would/will investigate if you suspect the above.
I wonder why the need for a post purge if its not safety related, perhaps there are some increased emissions with this type of burner on a normal stop and the post purge clears them.

Not sure if I asked this previously, does a hydraulically operated damper shut off the fan air intake when boiler off?.

 

[ShaunCorbs]

The fan keeps going

 

[John.g]

The fan only runs for the post purge period and then stops, there will then be a updraft through the hot boiler with consequent heat losses. I know Firebird did have this damper because I replaced the seals in the hydraulic actuator (ram) for someone as it was leaking kerosene. The actuator was driven by oil from the fuel oil pump and would open almost immediately on fan run up and close by spring return when the fan stopped.

 

[JC22024]

I can't see any difference in the oil pump solenoid operation with either no post or post purge, actually if you suspect fuel nozzle dribble, it can possibly dribble more with a post purge as the oil pump will also be driven for the 15 secs vs a few seconds on a no post purge as the fan runs down pretty fast as its being braked by the fuel pump, obviously you would/will investigate if you suspect the above.
I wonder why the need for a post purge if its not safety related, perhaps there are some increased emissions with this type of burner on a normal stop and the post purge clears them.

Not sure if I asked this previously, does a hydraulically operated damper shut off the fan air intake when boiler off?.

The fan only runs for the post purge period and then stops, there will then be a updraft through the hot boiler with consequent heat losses. I know Firebird did have this damper because I replaced the seals in the hydraulic actuator (ram) for someone as it was leaking kerosene. The actuator was driven by oil from the fuel oil pump and would open almost immediately on fan run up and close by spring return when the fan stopped.

From what I have seen the post purge operates for closer to 10 seconds. THis just counds like the fan motor only running.

Didnt cure the popping noises. The popping only ever occurs for the first 10 - 15 seconds of running of the day. No other time. I will clean the pump's internal filter next.

Does post purge really make no difference(?) It didnt seem optional in the instructions but I know what you mean that there would be additional heat losses too. If I knew it made no actual difference could disable it again

 

[John.g]

The fuel pump is (or at least was) directly driven by the fan motor.
I might contact Firebird at their parental HQ here (Co Cork) and ask them a few questions.

 

[John.g]

A very experienced "oil" man thinks that it probably is a feature to help with the sooting that was a feature of the original enviromax/silver series of boilers and the post purge with the elco burner is more of a clean out of the combustion chamber after each cycle run. Also said the the air intake damper was a feature of the Riello 40 burner and Firebird no longer use the damper.

 

[JC22024]

A very experienced "oil" man thinks that it probably is a feature to help with the sooting that was a feature of the original enviromax/silver series of boilers and the post purge with the elco burner is more of a clean out of the combustion chamber after each cycle run. Also said the the air intake damper was a feature of the Riello 40 burner and Firebird no longer use the damper.

Got it, thanks for that makes sense. The Elco runs at 0ppm for the most part but all the same I'll leave it wired as intended from the factory. It only runs on for about 10 or 15 seconds at most. Nothing to worry about heat loss wise etc.

 

[JC22024]

An update on the Vector BEM

Whatever exactly the counters are counting the difference between them is indicating 16.27% so far. So I guess that's the saving in run time / oil very approximately. There are a number of factors at stake there including pre and post purge times but good to see a clear aparant difference with it installed.

I think i'll run it like that until this Saturday then go ahead and fit the other one with delta sensors and carry out Same test. I can borrow these same counters and wire in for a week and carry out pro rata approx yields.

 

[Ric2013]

Apologies for having skipped part of this thread, and if it turns out I'm repeating what others have said:

1. Possibly the boiler has a a maximum temperature drop and, if you throttle the flow to get a higher differential, it responds by cycling to try to reduce the differential down to below its maximum

2. Thinking about condensing. If the flow temperature is high and the return is below condensing, it makes sense that condensation will only form on that part of the heat exchanger that is coolest. So, if condensing starts below 2°M (M = a made-up temperature scale), say, and stops at 4°M, a boiler running at 4/2 will always condense well, whereas a boiler running at 6/2 will only give 2/3rds of the condensing capacity because 1/3 of the heat exchanger is likely to be above the dewpoint which is 4°M. So there is likely to be more to it than just the boiler return temperature. Not my original idea, but heard it somewhere and it makes sense (although probably a slight simplification of the actual physics at work).

 

[JC22024]

Apologies for having skipped part of this thread, and if it turns out I'm repeating what others have said:

1. Possibly the boiler has a a maximum temperature drop and, if you throttle the flow to get a higher differential, it responds by cycling to try to reduce the differential down to below its maximum

2. Thinking about condensing. If the flow temperature is high and the return is below condensing, it makes sense that condensation will only form on that part of the heat exchanger that is coolest. So, if condensing starts below 2°M (M = a made-up temperature scale), say, and stops at 4°M, a boiler running at 4/2 will always condense well, whereas a boiler running at 6/2 will only give 2/3rds of the condensing capacity because 1/3 of the heat exchanger is likely to be above the dewpoint which is 4°M. So there is likely to be more to it than just the boiler return temperature. Not my original idea, but heard it somewhere and it makes sense (although probably a slight simplification of the actual physics at work).

Thanks for that, I will still also try jetting the boiler down next size and see if it makes any difference to the ability to condense. The air flow is greatly reduced at the next nozzle size down so it most likely is kore efficient if it can heat the house with the smaller nozzle.

A quick video of the Vector going through the stages of stat off to economy then firing 🔥

 

[JC22024]

OK So I had an unexpected couple of hours free last night and went ahead and fitted the domestic digital BEM



Observations:

The way it is set up means it does need a perm power supply and also power from the call to stat supply. Not between the stat and burner like the other Vector unit.

This means I cannot do a direct comparison between the 2no readings. So I have added a counter to the burner. And a counter to the heating call wire. Which means I can record the same data between the 2x BEMs as long term tests. Not super accurate but as a pro rata comparison for any reduction in burn time ratio VS heat demand hours. (Aka there will be variables there to account for such as the heating times etc but I will leave the room thermostats programmed and not boost them.

Had this one on the upstairs zone tonight. It's operation is slightly different to the Vector unit.

This one calls the boiler. Then boiler fires if the boiler stat is calling and runs until the boiler stat is satisfied, fully up to temp. Then the BEM calculates heating load and goes into economy mode, interupting the power/ call wire to the boiler stat. Waiting longer than the Vector unit did between cycles. Down to 56.8c is what I just recorded before it fires up again and goes to full boiler stat temp again and so forth.

So it appears to be holding out alot longer than the Vector unit. Perhaps due to it monitoring the pipe temps.

Will see how it goes for the next while.

If it is any indicator of economy, before BEMs the boiler cycling was excessive. The flue pipe felt hot to the Touch at any time. With the Vector unit the flue felt warm. With this digital BEM the flue felt positively cold 🥶

 

[ShaunCorbs]

Any condense tho ?

 

[JC22024]

Any condense tho ?

I'll put the bottle back on tomorrow and see for sure. I think it looked like there were a few drops tonight but will need the bottle to test for sure. Very tempted to downsize the nozzle too sooner than later. 26kw is probably oversized for my house especially the upstairs zone which gets the most use.

 

[ShaunCorbs]

Simple rad Calc will tell you all you need to know

 

[JC22024]

Simple rad Calc will tell you all you need to know

Yes I'll look into formulating that for both zones and posting. Some rads like the new mysons I'll find data for. Some of the 40 year old non Finned rads could be trickier to determine but I'll fully measure up and look for calculators

 

[ShaunCorbs]

Mins half the output for non convectors

 

[JC22024]

Update, I did go ahead and jet my boiler down to 0.5gph nozzle / 20kw with interesting results.....



With the BEM switched off / overrode and on the downstairs zone only the boiler ran for over an hour there and wouldn't reach stat temp of 70! Levelled off at about 62c flow 50c return and stayed there running full time it felt like.

About 10c outside also.

So I put on upstairs smaller zone now and will let it run tonight. I'd imagine it will be just fine for upstairs but for downstairs it's clear 26kw / 0.65 nozzle is the output needed so that will go back in tomorrow and that's the end of that! 26kw it is.

 

[John.g]

A 0.6 USGPM SHOULD GIVE ~ 21kw the 0.5 is probably only 16/17 kw

 

[JC22024]

A 0.6 USGPM SHOULD GIVE ~ 21kw the 0.5 is probably only 16/17 kw

Using Danfoss Nozzle Calc spreadsheet I got the following values based on specific Firebird settings:

0.5gph / 1.5kg/hr nozzle set at 9.8bar actual pump pressure consumes 1.66 kg/hr / 2.1L/Hr / 19.92kw
0.65gph / 1.95kg/hr nozzle set at 9.3bar actual pump pressure consumes 2.81 kg/hr / 2.66L/Hr / 25.23kw

I put it back to the 0.65 nozzle this evening. I also cleaned the pump filter (I think a faulty new paper oil filter element on my oil tank caused temporary contamination) and that appears to have cured the strange popping noises on first startup.

So I guess 25.23 x 92% efficient means roughly 23.2 kw to rads? It does heat either zone well like this.