Balancing rads on oversized boiler with varying flow temperature

[hard-work]

In reply to hard-work: the system is balanced, subject to debate as in my original post, but consensus seems to be that it is balanced.

Funny that you should mention option 2: a thermal store is actually part of a long-term plan that involves a woodburning stove, and possibly solar thermal, but not the short-term plan.

Not sure what you mean by option 1. The system already has a programmer, if that's what you're on about.

Modern boiler looks good on paper for an average house and an average family, but since the gas bills based on accurate gas readings for this house as it is actually used come to less than £300 a year, it would take a VERY long time to achieve financial payback for a new boiler, so not really an option in this case.

A timer in that is prevents the boiler from firing off and on, cycling, too quickly. It needs to be in the boiler controls, maybe on the boiler stat circuit. You could set it to say 8 minutes, or whatever. It is not a normal clock/timer.

Will this inefficient cast-iron job last another 10 years? Probably not. Your £300 per year could be £200. £1,000 in 10 years. A new boiler with a 10 year guarantee may pay for itself.

The thermal store with two interlock thermostats will prevent boiler cycling. It cares not a jot what size the boiler is. It switches it in and then out when the thermal store is up to temperature. The pump then takes the heat to the radiators.

I know of one instance where a thermal store was to be fitted with an oil boiler and solid fuel & solar. They could get hold of a 150kW oil boiler which was way too large for the house. I said but and connect it to the thermal store. It heat up the thermal store in one burn in no time at all and then stays off for most of the day. Hot water is taken off a coil in the thermals store. If a shower and bath are being run at the same time the boiler may cut in and then acts like a combi as it pumps so much heat into the thermal store as the hot water is being drawn off, being so big. The biggest gas boiler on a normal U6 meter is 62kW.

 

[Ric2013]

Hi hardwork - I don't know how many registered gas installers would want to modify a boiler's internal circuitry, but I like your way of thinking. It's only by experiment that we ever get any sort of progress!

Not convinced by your economic argument on new boiler though. A decent modern boiler (Veissmann or similar) would cost at least £800, not sure if this includes the controls and the flue. Add to this a powerflush at, at the very least, £300 (though I would personally charge more for this, so not sure if, for £300, you'd be getting it done right) and you're at £1100, add another day or two for an RGI to fit it. We're now talking nearer £1500. So the new boiler has to last 10 years to break even, even assuming the existing boiler costs £500 to keep it running for the next 10 years, and that isn't a selling point, though you may still have a sound environmental case.

That said, I'd be surprised if this old lump doesn't last another 10 years if they want it to. It is only 29 years old. I have family members with old boilers - one was 38 years old when the house was sold two years ago (in the previous 25 years it had only needed two replacement parts - the thermocouple and the thermostat), the other boiler is still in service, at 44 years old (in fairness, it is an oil boiler, so not quite the same thing).

But we're digressing. However, you seem to have experience of thermal stores, and I am interested in this, if you're happy to talk about it.

Your two interlock thermostats... You'd have a stat on the store itself, the boiler stat (which would be unlikely to come into play), and a room stat or stats. Are you suggesting two stats on the thermal store with some kind of relay mechanism in which the boiler kicks in when the upper store stat calls for heat, runs until the lower stat is satisfied, and then switches off until the upper stat once more calls for heat?

 

[D_Hailsham]

In what way do they not make sense?

1. Why are the three proportional pressure curves so close together? (compare the PP curves on pages 11 and 12 of the attached Grundfos Data book.)

2. My working point is nowhere near any of the PP curves (well above) so I don't see how PP could work.

 

[Ric2013]

My working point is nowhere near any of the PP curves (well above) so I don't see how PP could work.

The UPS2 15-50/60 shown on your attachment page 11 gives a head of about 1.75m at 0.7cu m/hr on PP3. Your desired working point shown in your previous graph was 1.8m head at 0.7 cu m/hr. Surely this is quite close to what you want? Though if you have a differential pressure bypass fitted, my understanding is that that won't work.

Obviously, if you have the UPS2 25-80 or 32-80 (both on page 12), then the pump will be overkill, both in fixed speed and in proportional pressure modes.

 

[D_Hailsham]

I seem to have confused you. The graph in my example was for an Alpha 2L pump and the required working point was just an example to show what would happen with a fixed speed pump. The calculated flow and head for my system is 0.45l/min and 2.5m.

I know the "80" pumps are overkill. I was just contrasting how close together the PP curves are on the 50/60 pump compared to the PP curves on the 80 pump. None of the Alpha pumps have PP curves so close together, which makes me wonder if the UPS2 curves are correct.

 

[Ric2013]

Confused - easily done

Assume you mean 0.45m³/hr.? If so, point taken - the UPS2 50/60 is not a terrible match for this if run in fixed speed mode, but develops far too little head in PP mode - the two modes are very far apart at lower flow rates, whereas the 80 series have the two modes closer together. And, as you say, the 50/60 PP curves are nearly the same as each other.

Which means that, while the 50/60 would probably work for you in fixed speed mode, to use PP you'd need the 80, which, as you say, doesn't really make much sense.

To be frank, I'm not sure what Grundfos is playing at at times - they make the UPS2, as well as the Alpha, but the online pump selector tool always seems to recommend the Magna pump. When I played with the the online tool to select a pump for my own heating system at home, the tool actually said that no suitable pump could be found, despite my system being relatively conventional, and despite the fact that my system has a Grundfos pump fitted and working.

 

[D_Hailsham]

Yes, 0.45m³/hr, not litres/sec.

My UPS2 works fine on fixed speed 1.

The UPS2 never appears on the list of choices when you try using the Grundfos pump sizer. I have my suspicions that they view it only as a direct replacement for the Selectric (UPS15-50) and Super-Selectric (UPS 15-60) in existing installations, not new systems, so nobody will bother sizing the pump.