Underheating microbore with condensing boiler

[smallbore]

I’ve never been too happy with the condensing Vaillant EcoMax Pro 28E that replaced our original cast iron boiler heating our vented microbore system. The house didn’t seem to get any warmer than before the changeover about 5 years ago, but I’ve been told everything was OK so I did nothing about it.
When sorting paperwork out I found some information produced when the system was checked out to get the right boiler output. It indicates the rads and pipework create a 2.5m head pressure loss.
Looking at the boiler information online I see the boiler has a 2.5m head pressure loss while the original had almost none. The 28E spec says it needs a minimum flow of nearly 1400 litres/hour for the maximum output of 28kW with a flow/return differential fixed at 20C. The flow is set to 80C.
The installer didn’t change the pump, which was presumably selected to suit the original system of 8 rads off one 22mm manifold upstairs with the same setup downstairs.
Could the low heat be due to lack of pump head as the current one can’t move that much water through the pipes even at top speed.
Standard 5 or 6m head pumps can’t achieve the minimum flow at a 5m head either. However there is an 8m head one that can pump 1400 l/h at a 5m head but it’s for a 25mm internal pipe not 20mm.
Am I tackling this problem the right way?

 

[Loge]

I am not a heating engineer I’m a retired mechanical engineer. Recently I decided to get a quote to replace my 1980 boiler with a new condensing heat only boiler. The system is basic micro bore, vented and manually controlled.
I didn’t go ahead because I had concerns similar to what you are experiencing. I too noticed the marked difference in the boiler resistances and wondered would a boiler of similar power output do the job? Also what would be the effect on the existing pump? That started my interest in the subject so I though I would try to model my existing system. From that experience this is my take on your problem.
Firstly, calculate the flow necessary for the boiler heat output.
For 28kW this works out at just under 1200 litre/hour (or kg/hour assuming water density of 1000kg/cubic metre).
Now reading off the Vaillant boiler resistance curve you get approx 2.4 metres resistance head in the boiler with that flow. (Remember in this context head values are meaningless without an associated flow value). Your installer probably assumed a design point for the system of 1200 litres/hour with a resistance of 2.5 metres for the system plus 2.4 metres for the boiler equal to a total circuit resistance of 4.9 metres.
Now the actual volume flowing in the system is dependent on the pump characteristic.
Pump suppliers provide details of this in the pump manual or leaflet. It now gets a bit technical. What you need to do is to plot the design point on the pump chart. Draw a square law line through the design point and where it crosses the pump curve is, in theory, where the system will operate.
I have an old Grundfos 15-50 pump.
I plotted your design point and found the operating point on curve 2.
It was 215 mbar at 0.8 cubic metres/hr or 2.195m at 800 litres/hr.
On Grundfoss curve 3
It was 390 mbar at 1.08 cubic metres/hr or 3.981m at1080litres/hr.
The amount of heat the boiler can transmit is directly proportional to the volume flow.
For example if you have a Grunfos pump set to setting 2 the heat output from the boiler would be 28*800/1200= 18.8 kW.
It depends what type of pump you actually have. You may be able to set it to a higher setting. If there is an automatic bypass fitted check its setting. However remember on a vented system the system pressure has to be less than the level above the pump of the expansion tank. Otherwise there will be backflow into the tank.
Hopefully there will be an expert on this site who may be able to further enlighten us.

 

[Ermintrude]

Fit a viessmann v100heat only the dP is low due to large cross sectional hex.

Use some form of hydraulic separation to ensure boiler and system deliver ultimate performance

 

[dancinplumba]

System probs full of crud

 

[Ermintrude]

No inhibitor [emoji12][emoji12][emoji12][emoji12]

 

[kris]

Milward will tell u how to get the best hydraulic seperation , with his close coupled tees lol

 

[smallbore]

Thanks very much for the insight.
The original, and now current-again, pump is a Selectric 4 operating on speed 4, 2600rpm. I changed the pump with a new, borrowed, unintelligent, DAB 6m-head one, temporarily, operating at its speed 3, 2100rpm. The pump is said to be equivalent to the Grundfos 15-60 but, although I have tried to find the head/volume curves of the 3 pumps I've mentioned I can't locate them, probably as they are all out-of-date. I did find a datasheet for the Grundfos 15-50 and 15-60 but both curves were labelled 15-50!
The DAB resulted in no noticeable improvement although the system is clean!

 

[IDCHAPPY]

Milward will tell u how to get the best hydraulic seperation , with his close coupled tees lol

And PHEX and Weather comp man