Twin system boilers not up to it?

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Hi all. I've moved into a 3-storey house, with the highest radiator 8m above the boilers. There are 30 radiators including towel rails, served by two 28kW Vaillant 628e boilers connected in parallel, i.e. each across the return to the flow. They're system boilers each with a standard pump that say 2.5m Head. They're in an outhouse and connected to the house by 2-3m of 35mm pipe.

It doesn't work very well. The farthest radiator never gets more than warm even with its lockshield valve wide open and the upstream rads balanced and quite restricted. The second boiler in line cycles off quickly, and even with the first boiler switched off it doesn't stay fired up for very long once the system is hot. Gauges on the main 35mm pipes in the house show typically 65 degrees for the flow and only 40 for the return. This is with one boiler running, with its water temperature turned up to max.

What do you think? I'm eyeing up maybe adding a more potent pump to the Flow after the second boiler, like a Grundfos Magna 25-100? Or alternatively replacing both boilers with a single say 42kW condensing boiler and that pump. Any expert advice gratefully received! It was installed by the local Vaillant dealer and has been service multiple times by professional heating engineers so I'm hoping for some independent expertise, please.
 
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so you want to replace a setup with 56KW with a new boiler at 42KW, because the existing set up is not up to it ??????
 
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Well the heating capacity isn't the problem. In fact they're both turned down to 20kW, the latest engineer said. It's the flow.
 
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Can't see that setup ever running effectively without a low loss header between heat sources and heat sinks. Two pumps in parallel without backcheck valves is asking for trouble as well.
 
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Some photos of the current setup would help, but yes it does sound like it's overpowered and under-pumped, or possibly just incorrectly balanced. Do you know what the total kw output of your radiators is, or alternatively has a heat loss calculation been done for your home?

Don't confuse pump head with pressure head. Pump head is what is required to overcome the resistance of the system. It's got nothing to do with how tall your house is.
 
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Can't see that setup ever running effectively without a low loss header between heat sources and heat sinks. Two pumps in parallel without backcheck valves is asking for trouble as well.
Yes, it seems weird to me, but I'm just a kind of DIY engineer. As far as I can see, the second pump is trying to pump into a flow that's already been pressurised by the first pump, so the system is a big heat system with a little pump, in effect.
 
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Some photos of the current setup would help, but yes it does sound like it's overpowered and under-pumped, or possibly just incorrectly balanced. Do you know what the total kw output of your radiators is, or alternatively has a heat loss calculation been done for your home?

Don't confuse pump head with pressure head. Pump head is what is required to overcome the resistance of the system. It's got nothing to do with how tall your house is.
Okay good point about the head, thanks. It's quite a rambling system as well as tall. No I have no numbers for the heat loss I'm afraid. It was not balanced at all, one turn on each lockshield! Bit of a disgrace really. But I have since done it to some reasonable extent. Here's the pipework under the boilers, and the pump (if they link):
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nEZU7nSKpxq7mkFp7
 
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Take 2 with the photos.
25d41e24002a049083ea66184ee9ffb075da67dd.jpg

Vaillant twin boiler pipework. Working upwards it’s Return, Flow, Gas.
 
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Yeah that definitely needs some work! As you have identified, the residual pump head at 28kW is next to nothing, so a header and system pump are required to compensate. I suspect upon fitting those and setting them up checkout you'll find that the system runs quite happily on just the single boiler. Worth getting someone in who understands system hydronics though to take a proper look and specify everything correctly rather than just lobbing any of pump at it. Over-pumping can create noise problems and reduce fuel efficiency
 
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Yeah that definitely needs some work! As you have identified, the residual pump head at 28kW is next to nothing, so a header and system pump are required to compensate. I suspect upon fitting those and setting them up checkout you'll find that the system runs quite happily on just the single boiler. Worth getting someone in who understands system hydronics though to take a proper look and specify everything correctly rather than just lobbing any of pump at it. Over-pumping can create noise problems and reduce fuel efficiency
Thanks, yes this is definitely steering me away from this twin boiler concept. It's like parts of the system have been done professionally and some haven't. Or professional/cynically! It is a big house and so I think I'll want the power to warm up in a reasonable time, a bit more than 28kW. It's been hard to tell with it as it is.

Yes over-pumping, I can imagine. I'm not confident about all the hidden pipework diameters, is part of it, and that Magna can be adjusted down to find the sweet spot. There is an extra pump in the cellar in fact, for a Conservatory radiator!! That's how weird it all is. So I'm feeling it really wants a powerful pump, but they didn't like the extra cost, as they are disproportionately more expensive aren't they. But hydronics, thanks, is what I have to look for then.
 
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What's your nearest town? Might be able to recommend someone.

I quite like the DAB Evoplus range of pumps for large domestic applications
 
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What's your nearest town? Might be able to recommend someone.

I quite like the DAB Evoplus range of pumps for large domestic applications
I'm in Norwich. If you did know someone that would be great. This system has had at least 3 'qualified' heating engineers on it and apparently none of them really clued up. It's been in 20 years and never really worked properly I think - never been balanced because it just won't! I like the look of that DAB pump, thanks. Any thoughts on a heating-only boiler? I have a plinth for floor standing or plenty of wall space.
 
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Norwich seems to be a bit of a wilderness for good heating guys but I'll see what I can do! If you're minded to change the boilers for new then it would be worth having a proper heat loss calculation done using www.heat-engineer.com to find out what size you actually need. It could well be less than you think!

As for boilers, I'm an Intergas fan - very robust simple designs with good warranties. The Xclusive 36 in System boiler mode, or the SB40 might suit. When set up to provide two flow temperatures for fast hot water recovery and high efficiency heating using OpenTherm controls, they're really excellent
 
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Your system flow return temps. where 65 flow and 40 return with one boiler running, this suggests that one boiler is not sufficient to meet the heat demand. I'd have thought 40 deg is a bit low for the return temp. Could possibly be down to very low flow too?
 
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