URGENT: boiler flow piping hot but everything after the low loss header lukewarm. House not warming

Blackhat356

Hi all, I'd really appreciate help quickly. We've just finished renovating an old stone house in Ireland, and the heating isn't warming the house properly. I have two toddlers, and with winter setting in this is getting serious. I need to find the cause and get it fixed as soon as possible, so any pointers are very welcome.

Setup

• Grant Vortex Boilerhouse oil boiler (26–36kW)
• Low loss header between the boiler and the heating circuits
• Nu-Heat LoPro underfloor heating, controlled by a Nu-Heat UH8-C wiring centre
• Radiators
• Hot water cylinder heated by its own pump
• Pumped zoning: separate pumps, no zone valves

Symptoms

1. Radiators are only lukewarm.
2. Hot water is not getting properly hot.
3. The UFH isn't performing as expected.
4. The flow pipe from the boiler is piping hot, but every supply pipe leaving the low loss header is only lukewarm.

My suspicion
I think the issue is at the low loss header. It looks as if return water is being mixed back into the flow. That would happen if the circuit pumps are moving more water than the boiler-side pump delivers.

Questions

• Does this pattern point to a flow imbalance across the header, with the boiler-side flow lower than the combined system-side flow?
• Which pump should be set to what speed? Is there a boiler primary pump, or does the Vortex's internal pump feed the header?
• Could the header be undersized, or piped wrongly (flow and return swapped, or connections in the wrong positions)?
• Could air or a partly closed valve on the boiler side cause this?
• What temperatures should I measure, and where, to confirm the cause?
• Are there any quick checks or setting changes I can make tonight to get more heat into the house while I sort out a proper fix?

I attached a picture of the header pipework. Thanks so much in advance.

GGross

your zone pumps probably not running. the low loss header is supposed to mix some its a normal install. flow from boiler through LLH, but not hot on the outlet from LLH indicates no flow on the space heating side

Ironman

That piping is a mess. I really can’t make heads or tails out of it.

That being said, it could be the radiators are air bound and need purging.

Ironman

Asa far as the LLH goes, there’s really nothing in it to malfunction other than becoming clogged.

It should be piped so that both supplies are on top and both returns are on the bottom with the boiler on one side and the system on the other.

Blackhat356

Thanks GGross. The pumps do seem running. They are all set to constant pressure on high speed. They are also humming.

What I noticed is that when central heating (rads+ UFH) is off (i.e. one of the pumps) the HWC starts working as expected and the supply to it becomes hot

Blackhat356

Thanks @Ironman. Upper Left is boiler supply and bottom left is return + pump. Upper right, there are two supplies, one for central heating and the other for the HWC. Both of their returns are at the bottom with two separate pumps.

rads were installed recently and air purged twice

Jamie Hall

Pumps can hum even when there is no or very little flow… and your symptoms point to no or little flow through the secondary pipes. Closed valve? Failed pump? Air bound?

GGross

If all of the pumps are powered when they should be but the rads aren't getting hot that signifies no flow, air lock. looking at it from this angle I would believe its air locked. a single high point vent really isn't adequate for most systems.