[Solved] Central heating pump on the return. The question is how to install a mixer with

Question 1

Will all radiators heat if both pumps run together?

Answer

They can, but only if flows are balanced. A second pump may divert water toward the easier path, starving radiators. Ensure hydraulic resistance is comparable across branches or add balancing on radiators/manifold. “Two pumps are not installed because they can disturb the water flow.” [Elektroda, 762302, post #16789813]

Question 2

Is it better to keep the boiler pump on return or move it to the feed?

Answer

The thread began with the pump on return and planned the floor mixer/pump on supply. That arrangement can work if balanced. One user confirmed success after installation without moving the boiler pump. Adjust speeds and balance rather than relocate by default. [Elektroda, Radekwladek, post #17237951]

Question 3

How big were the floor loops in the working example?

Answer

Two loops were reported: about 70 m and 80 m, serving rooms of roughly 10 m² and 12 m². This sizing helps estimate head loss and set pump speed I. Use it as a reference, not a rule. [Elektroda, Radekwladek, post #16782995]

Question 4

Do I need a mixing valve for the floor heating?

Answer

Yes, the plan added a mixer with its own pump on the supply to protect floors from high boiler temperatures. This separates low‑temperature floor loops from the radiator circuit. It worked in practice at speed I on both pumps. [Elektroda, Radekwladek, post #17237951]

Question 5

What’s the risk of running two pumps in one system?

Answer

Mismatched heads, pipe diameters, and circuit resistances cause one pump to pull flow away from the other. The water goes where hydraulic resistance is lighter, which can unbalance heating. Balance circuits or use valves to control distribution. [Elektroda, 762302, post #16789863]

Question 6

How can I balance the system so both circuits work?

Answer

Use manual balancing on radiator branches and the UFH manifold. Aim for similar pressure drops. One commenter even asked about adding balancing valves to equalize resistances, which is the typical fix when dual pumps compete. [Elektroda, 762302, post #16790041]

Question 7

What if relocating the boiler pump isn’t ‘according to the art’?

Answer

One installer noted the layout was not per textbook standards, yet said, “I assumed it three times… and it works everywhere.” If constraints exist, prioritize functional balance and protection of floor loops. [Elektroda, roman 18, post #16787618]

Question 8

What pump speed should I start with after adding the mixer set?

Answer

Start with both pumps at speed I, as reported by the original poster after commissioning. Verify room temperatures and adjust only if some circuits lag. Lower speeds reduce noise and hydraulic conflicts. [Elektroda, Radekwladek, post #17237951]

Question 9

How do pipe diameters affect two-pump setups?

Answer

A larger main near the boiler and smaller UFH manifold lines shift resistance. The pump will circulate where it is “lighter,” skewing flow. Account for this by throttling easier paths or increasing resistance in the favored branch. [Elektroda, 762302, post #16789863]

Question 10

What’s a quick 3‑step way to convert for three floor sections?

Answer

Add a mixing valve set with a dedicated pump on the supply to the UFH manifold.
Connect the three loops, set flow meters, and throttle faster circuits for balance.
Run both pumps at speed I, purge air, and fine‑tune flows per room. [Elektroda, Radekwladek, post #16789980]

Question 11

Will this work if the system isn’t perfectly designed?

Answer

Yes, users reported working results despite non‑ideal layouts. The key is balancing and modest pump speeds. Quote: “it works everywhere.” Plan for adjustments after start‑up to fix any cold radiators or over‑heated floors. [Elektroda, roman 18, post #16787618]

Question 12

What should I ask or measure before buying parts?

Answer

Confirm floor area and loop lengths. In the thread, the helper first asked for UFH area before advising. Knowing lengths (70 m, 80 m) and room sizes (10–12 m²) guided decisions on pump speed and mixing. [Elektroda, 762302, post #16782733]

Question 13

What did the original poster finally report after installation?

Answer

They installed the mixer and pump as planned and ran both pumps on speed I. The system heated successfully, confirming the approach when balanced. This is the clearest outcome provided in the discussion. [Elektroda, Radekwladek, post #17237951]

Question 14

If radiators cool when UFH runs, what’s the first fix?

Answer

Throttle the UFH manifold flows or slightly increase radiator branch resistance to share head evenly. This addresses the edge case where the UFH path is hydraulically “lighter,” stealing flow from radiators. [Elektroda, 762302, post #16789863]

Question 15

Where should I place the new UFH pump relative to the boiler circuit?

Answer

The discussed setup places the UFH pump on the supply with the mixer, while the existing boiler pump remains on the return. This separation worked when both pumps were set to speed I. [Elektroda, Radekwladek, post #16782483]

Question 16

What should I do if my question gets overlooked?

Answer

Share exact loop lengths, areas, pump locations, and photos. The thread moved forward once the OP added areas and lengths, enabling targeted advice. Specifics unlock actionable recommendations. [Elektroda, Radekwladek, post #16787072]

[Solved] Central heating pump on the return. The question is how to install a mixer with

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