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Hi All,

My home is 2 storey - split into 2no zones. 14 rads downstairs and 7 upstairs. Open vent system standard boiler. Hot water is immersion heater.

The old boiler was a worchester bosch dansmoor rated at 26-32kw. Nozzle spec was 1.0 gph. However the enginer had 0.85gph nozzle in it. It always heated the rads pretty quickly. Say within 30 mins the downstairs rads would be hot.

Just fitted a new Firebird Condensing oil boiler. It is currently fitted with a 0.5gph nozzle. That is 20kw output rating but depending on nozzle spec and fuel pressure the same burner can put out 26 or 35kw.

I note that with the 0.5gph nozzle the boiler appears to take well over an hour to heat the radiators up on one zone. Downstairs in particular suffers. Now I know a condensing boiler is more efficient when worked hard but perhaps this is too much of a downsize.

So next step will be to fit a 0.65gph nozzle and get it set by engineer at 26kw output. I hope then it will heat the house faster and get up to temp to cycle on and off with stat quicker. Would you guys agree with the principle of what I am saying here?

In summary drop one nozzle size when upgrading to a more efficient condensing boiler. (Don't drop too far like I did at the start)

Also - I notice since the new boiler is installed the radiator balancing is out of whack. Is this something that can happen when a boiler is replaced? In any case that is something I can address myself I understand the principle of 11 degree target drop across each rad and start with the closest rad and work outwards.

I think first step is to get the engineer back to up the nozzle size on the new boiler from 0.5gpg /20kw to 0.65gph / 26kw and after that then I will re-balance the radiators on both zones and hopefully the house will heat up as quickly as it did on the original boiler but should in theory still be more efficient.
 
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Notes on the Boiler nozzle / output.
Screenshot_20211203-233814_YouTube.jpg


The 0.65gph nozzle size will equate to what the old boiler was putting out at 26kw approx. The 20kw spec appears too weak for reasonable speed warmup and hard winter hasn't set in yet either.

Consider too that when the original boiler went in my house was all one zone. Then some years back I split it into 2no zones.

Therefore we only either run the downstairs or upstairs but not both at once. For the most part it is rhe upstairs zone that gets the most use.
 
What’s the heat output of your rads ? / Heatloss of the property?
 
What’s the heat output of your rads ? / Heatloss of the property?

I'm not sure it's a 1970s house that was renovated in the 80s and 90s. Mix of older rads with no fins and newer double rads with fins for example.

However if need be I could measure them all and produce a schedule.

The house has cavity walls with pumped silver bead insulation, 450mm rock wool in loft and triple glazed upvc 0.6 centre pane u value windows. All upgrades I have done over the years and its not overly hard to heat now. (Used to be freezing cold, single glazing and no insulation except 50mm in loft 🥶)
 
How did he or you size the new boiler or just guessed ?
 
Note that it also used to have just an on off switch as control for whole house!

About 10 years ago I split it into 2no zones and installed independent time and temperature digital programmable stats.

And remaining rooms had quality TRV valves fitted also which work pretty well.

These control measures vastly improved the efficiency of the heating system..
 
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But your Heatloss / load stays the same, just made it more effective for you / can control upstairs / downstairs separate but you will still get heat bleeding through to upstairs passively
 
How did he or you size the new boiler or just guessed ?

This is the old boiler spec:

20211120_113331.jpg


Rated 26 - 32 kW. 1gph nozzle. However many years ago the engineer who services it each year downsized the nozzle to 0.85gph. So it is likely running 26kw or so he thinks(?)

That boiler worked very well house always warmed up quick.

I asked him to spec a new condensing boiler and he advised to go with a 20 / 26kw condensing boiler and he could adjust the nozzle to suit basically. Biggest factor for him was the zones and smaller zone getting most use.

These firebird boilers are all identical at 20/26/35kw. (Except nozzle size and settings. The boiler casing is the same)
 
I put the heat on this morning for the upstairs zone and noticed the burner was up to temperature about 5 mins later and cycling. I think perhaps some rads feel cooler than I'm used to due to a balancing issue.

The new boiler must have upset the system balancing somewhat so probably should address that first before getting boiler output raised?
 
Got home from work and did the unthinkable- read instructions!

More specifically the instructions for the Grundfoss Alpha circulator pump.

20211207_193312.jpg

It was set on the 2nd from right marker. Aka 3rd up the ramped diagram. Whilst this appeared to work just fine with the old boiler it did not work well with the new boiler at all. This is not a recommended setting on this pump for a 2 pipe system.

So I reverted the pump to speed setting 3 on the left. Pump wattage went up to 40 odd VS 7w. Heat is now getting out of the boiler alot better. Return pipe at last has a vestige of heat in it where before it was always stone cold. Just wasn't getting enough circulation flow.

Still though the same radiators that used to heat were dead and others were scalding hot. So I started gradually turning down the hot rads at the front of the circuit and at present ALL downstairs radiators in my house are heating up nicely. I have lived here from a child in the 80s, never recall that being the case there was always at least one or two off at any one time. Probably was never balanced properly.

Anyway, balancing is crucial now so there is a baseline established and cleaner in the system so I'll await a quality Bosch infrared thermometer in the post and go back to fine tune the balancing on all radiators and leave the boiler as is for now. House is feeling toasty - perhaps 20kw is enough after all (if taken as half house at a time as described before) Most likely would struggle heating whole house at once but I don't do that.

May also look for a weather compensator when all is running correctly. Or maybe stop being lazy and just periodically adjust the boiler stat on occasion for seasonal changes 🤔
 
Just checked settings at 20kw still and adjusted. Fuel pressure and Co was slightly off the factory recommended settings but bang on now. (7 degrees ambient temp)

20211209_203420.jpg

(Bought my own good used and calibrated co2 meter)

Just seeing how it copes and will also carry out radiator balancing properly this weekend. (Turn all full open first etc) Then may go to 26kw nozzle and test warm up and cycle times etc.
 
So today the 1.65gph nozzle finally arrived. Got it installed, set oil pressure and co2. Noted the ppm function of meter is definitely working aok. Starts up in teens then as it runs for a few seconds it drops well into single figures. Just before it reaches thermostat temp it is running 0ppm carbon.

The air had to be turned up quite a bit and its near zero c ambient temp tonight. Got it set steady for 10 mins all good.

20211213_193424.jpg


Efficiency dropped just a tiny bit with 26kw nozzle but the house is definitely happier with it. Proper heat coming into the rads after only 15 mins of running.

I got a new Bosch Infrared Thermomebter today so in the next few days will balance the downstairs zone then the upstairs zone by proper procedure. (Will be balanced separately as that's how I run it unless someone more experienced advises otherwise?)
 

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