Hybrid Heating System Design: UFH, Radiators, and Combi Boiler

[ricardo100671]

I’m planning a hybrid heating system for my home and would greatly appreciate input from experienced plumbers and heating engineers to validate my approach.
Here's an outline of my setup and reasoning:

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System Overview

• Heat Source: Combi boiler with an integrated pump, set to supply a single flow temperature of 45°C.
• Heating Circuits:
  • Underfloor Heating (UFH): Covers communal areas (living room, kitchen, diner, and snug).
  • Radiators: Installed in bedrooms, bathrooms, and other spaces, all sized to meet heat loss requirements at 45°C.
• Manifold: Single manifold shared between UFH and radiator circuits, with manual flow valves for each loop to adjust flow rates and balance the system.
• No Additional Pump or Mixing Valve: The combi boiler pump circulates water to both UFH and radiator circuits directly through single manifiold.

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Design Considerations

• The entire system is designed to meet the required heat loss for each room at 45°C.
• All radiators are carefully sized to deliver sufficient heat at this low flow temperature.
• Flow valves on the manifold are used to manually balance the system and ensure the correct temperature difference (ΔT) between flow and return in each loop.
• Heat emitter overrides have been accounted for, where excess capacity in adjacent rooms compensates for shortfalls.

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Room-by-Room Summary (Based on my design report)

1. Bedroom 1:

   • Heat Loss: 530W; Emitter Output: 345W at 45°C.
   • Adjacent rooms provide 967W excess, compensating for the shortfall.

2. Bedroom 2:

   • Heat Loss: 468W; Emitter Output: 387W at 45°C.
   • Adjacent rooms provide 782W excess, compensating for the shortfall.

3. Bedroom 3:

   • Heat Loss: 185W; Emitter Output: 173W at 45°C.
   • Adjacent rooms provide 701W excess, compensating for the shortfall.

4. Living Room (UFH):

   • Heat Loss: 976W; Emitter Output: 1283W at 45°C.
   • Heat loss fully met by UFH.

5. Kitchen/Diner/Snug (UFH):

   • Heat Loss: 1347W; Emitter Output: 1773W at 45°C.
   • Heat loss fully met by UFH.

6. Bathroom:

   • Heat Loss: 401W; Emitter Output: 215W at 45°C.
   • Adjacent rooms provide 689W excess, compensating for the shortfall.

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Specific Questions

1. Is my assumption correct that, since the system operates at a single flow temperature of 45°C, I can avoid additional pumps or mixing valves for the UFH?
2. Do the manual flow valves on the manifold suffice for balancing the system, or would you recommend a different approach?
3. Are there any practical considerations I might have missed when combining UFH and radiators in a hybrid setup at this flow temperature?
4. Based on the provided room-by-room heat loss and emitter outputs, do you foresee any operational issues with my proposed system?

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Thank you in advance for your feedback! I’d love to hear any thoughts, suggestions, or potential improvements from those with experience in designing or installing similar systems.

[JohnMo]

1. yes and no see below

2. yes manual balancing is ok

3. I think the flow temp will prove too hot for the UFH. If you try to control with a normal thermostat you will have big temperature swings.  If you are planning to use thermostats to control your boiler is likely to short cycle, and lead to increased gas consumption.

4. Over heating will occur on anything other than the design day.

 

Excess in rooms means those room are super hot, they will transfer to other rooms but not before melting your current room first.

 

Why not Weather compensation?

You need to be careful with boiler flow temps, they will shift and overshoot by a big margin especially if they expect to see a thermostat intervention which doesn't happen.  I first tried my boiler on a fix flow temp and the boiler got to 60 degs before I noticed.  If they operate on WC they operate in a different regime and try to keep boiler flow temp pegged quite tightly.

 

Depending on your boiler make you may be able to run an ESBE mixer on the UFH, this should not need an additional pump.

[ricardo100671]

17 hours ago, JohnMo said:

1. yes and no see below

2. yes manual balancing is ok

3. I think the flow temp will prove too hot for the UFH. If you try to control with a normal thermostat you will have big temperature swings.  If you are planning to use thermostats to control your boiler is likely to short cycle, and lead to increased gas consumption.

4. Over heating will occur on anything other than the design day.

 

Excess in rooms means those room are super hot, they will transfer to other rooms but not before melting your current room first.

 

Why not Weather compensation?

You need to be careful with boiler flow temps, they will shift and overshoot by a big margin especially if they expect to see a thermostat intervention which doesn't happen.  I first tried my boiler on a fix flow temp and the boiler got to 60 degs before I noticed.  If they operate on WC they operate in a different regime and try to keep boiler flow temp pegged quite tightly.

 

Depending on your boiler make you may be able to run an ESBE mixer on the UFH, this should not need an additional pump.

Thank you, I plan you use a Viesmann Vitodens 50. I think you are saying I best to add UFH to its own manifold with ESBE mixer to bring down its temp when running the rads on separate manifold running at the 45deg flow upsizing them to meet the room requirement instead of relying on UFH from other areas to compensate for the short falls 

[Lofty718]

45c is too hot for UFH, mine rarely gets above 40c (controlled by esbe) but most of the time it's in the 28-35c range

 

If you size all radiators to run at 35c that might work, alternatively get an esbe controlling the UFH but I think this might require hydraulic seperation (CCT) and a seperate pump.

 

Seems risky though and maybe difficult to get things comfortable, UFH all round run off one flow temp would be best

Edited December 1 by Lofty718

[JohnMo]

Here is a video showing how best to split flow temperature.

 

https://youtu.be/LL6YhT_HkIY?si=gMktTxr_SCWROGSW to 

[JoeBano]

I took my mixer valve off my Ufh on Friday, on Saturday I had 26c floor temperature on 32c flow temperature. I had engineered wood flooring glued down, manufacturer recommended temperature limit is 27c. I’ve trimmed my flow temperature and I’m gonna fit a sensor to knock the Ufh off when it hits 26c. Think you might struggle with 45c flow temperature unless you are using suspended spreader plate system 

[ProDave]

With such a hybrid is nobody suggesting a mixing valve on the UFH manifold to regulate the UFH temperature regardless of the boiler flow temperature?

 

And a 3 channel programmer so UFH can turn on and off at different times to radiators.

[Nickfromwales]

15 hours ago, ProDave said:

With such a hybrid is nobody suggesting a mixing valve on the UFH manifold to regulate the UFH temperature regardless of the boiler flow temperature?

 

And a 3 channel programmer so UFH can turn on and off at different times to radiators.

I never omit the mixing valve on an UFH manifold, even more so when it’s a high temp heat source such as a gas boiler.
 

Fitting a pump on it is then a requisite (as mixing valves need to be sucked on vs pushed through by a pump) but again if the rads offer a path of less resistance to a single boiler pump then the most resistive of UFH loops will get ignored when both the rads and UFH are running simultaneously (more so when the system is getting the house up to temp and all require ‘full flow’.

 

45°C may be fine for the UFH flow temp but we couldn’t say much more without knowing the screed thickness / floor insulation thickness and more. 

 

 

Edited December 2 by Nickfromwales
screed not screw

[Nickfromwales]

12 hours ago, ProDave said:

And a 3 channel programmer

2 channel if it’s a Combi. 

[ricardo100671]

Thank you @Nickfromwales, @Lofty718, @ProDave,  @JoeBano and @JohnMo for the very usefull comment whcih makes a lot of sence now.

The floor is ProWarm Profloor Foil faced 16mm pipe at 150mm c/c, 12mm Laminate about 1.4 TOG.

My setup is Rads and UFH will run simultaneously of one stat, so thought 1 Boiler Pump would work, but from the UFH resistive arguments I can see how I will need one on the manifold too.

At 45deg the UFH was overshooting by 1000W and I was using that excess to compensate rad shortfalls in adjacent rooms, which form another post shounded like a bad idea. Have doubled the rads and at 45deg they will just about meet the bedrooms heat loss and can't go any bigger with them, so sound like I need the thermostatic mixer valve on the manifold to temper the UFH output, then sound like I will have a nicely balance system.

Thanks @JohnMo for the video link which was very informative, but, if I understand correctly dont think I need Boiler Mixer Valve control since I only have one State calling for heat ?

Can anyone advise what the temperature differential should be for rads and UFH flow/Return please ?

Again thanks all for the unfull advce.
 

[Nickfromwales]

UFH and rads heat up / cool down very differently so should defo each have their own stats. 
Mixing valve on the UFH will control flow temp into the floors but you may find running at 50°c in winter will brighten up the rads significantly so you will then at least have that control / option to do so. 
If, however, now running the UFH off a controlled setup why not downsize the rads and turn the flow temp of the boiler up to 55/60°C? I’d favour smaller rads obvs, if it was my home. 

[ReedRichards]

Your room heat loss figures must apply to a specific outside temperature.  What happens when the outside temperature differs from the specified value?