Rads and UFH manifolds in series?

[SuperPav]

Hi all couldn't find anything on the topic, so thought I'd post random thought of the day...

 

Our house (4 bed detached ~190sqm) has UFH on GF in 100mm concrete slab. Upstairs is T22 rads in bedrooms but piped via a separate manifold. Upstairs more insulated than downstairs.

 

All will be running off a Samsung ASHP, but having been through 2 winters without *any* central heating in the house since the start of the build, the upstairs rooms don't ever get anywhere near as cold as downstairs.

 

System has no buffer, most of downstairs (50 sqm or so) is on a single zone and would always be open as that's the coldest part of the house. 

 

So my thought was... to avoid having two separate temperature zones for the ASHP, could I simply set the flow temp target on the ASHP (e.g. 35 deg) and feed the upstairs rads manifold from the heat pump flow, then take the return from the rads manifold as the feed into the UFH manifold, thereby naturally getting the UFH to run a few degrees cooler than the rads? or would this start to cause problems with the heat pump seeing an unexpectedly high deltaT? 

[JamesPa]

15 minutes ago, SuperPav said:

would this start to cause problems with the heat pump seeing an unexpectedly high deltaT

You could probably avoid that by water pumping harder!  Depends on the head required/available and pipe sizes.

 

Series connection of radiators is used in some countries, sounds like a nightmare from a calculation perspective but of course doable

 

The problem you may have is that as the oat warms up the DT across the rads will reduce so the ft to the ufh will increase, which you don't want.  

 

Interested to see what others say/suggest.  A mix of rads and ufh seems quite common.

 

Is there any chance they could just run at same ft with high flow thru rads and low flow through ufh thus bringing the outputs closer than they would be if you had roughly same flow through both.

 

Edited Friday at 15:23 by JamesPa

[JohnMo]

40 minutes ago, SuperPav said:

Upstairs is T22 rads in bedrooms but piped via a separate manifold

Not sure I would do series, think you are just adding complications you don't need. The WC curves will be very different and the ability for the system adopt automatically none existent.

 

Run then as a parallel system all one zone. In a well insulated house bedrooms will borrow heat from downstairs, but the radiators will also add heat. If the bedrooms end up too hot just switch off the bedroom rads or balance the flows down to get a comfort balance.

 

The above keeps dT sensible, series doesn't. 8 to 10 dT may cause the ASHP to back off or increase flow rate depending on control logic.

[Beelbeebub]

I have ufh and rads. 

 

Basically the house was originally all ufh, but the pipework failed (long story) so I had to disable the entire system. Now can run part of the downstairs on ufh and the upper rooms have rads (those that have heating at all). 

 

Anyway, everything runs at the same flow temp. The upstairs rads are farily chunky) and I have one fan coil as an experiment). 

 

Bit of balancing of the flows and everything ticks along OK.

 

The main issue would be if the upstairs rads weren't big enough to heat the rooms at the lower ufh temps, but that doesn't seem to be your problem.  

[Dillsue]

8 hours ago, Beelbeebub said:

Basically the house was originally all ufh, but the pipework failed (long story) so I had to disable the entire system. 

Care to share how it failed?? Intrigued to know to avoid repeating the cause, if possible

[Beelbeebub]

9 hours ago, Dillsue said:

Care to share how it failed?? Intrigued to know to avoid repeating the cause, if possible

They were convinced to use a now discontinued (because it failed so often the company went under) system. 

 

Basically it use small bore pipes made of a very flexible synthetic rubber, which they were assured had a 50year guarentee and was super durable - "used by NASA and the medical industry" 

 

Unfortunately the material was degraded when in prolonged contact with warm water with dissolved copper (or possibly iron) ions. Situations that don't occur in space or medical devices, but basically describe heating systems!

 

The pipes started to fail after a few years.

 

You're not likely to come across it, and certainly not if you're putting a new system in. 

 

I favour the speedfit systems that just uses their standard (and very durable) 15mm pipe - either PEX or PB. It's fairly cheap and very robust. Some of the other systems use smaller bore and single layer pipe that i trust less. Plus speedfit are a big player and likely to be around a long time. 

Edited Saturday at 14:52 by Beelbeebub

[JohnMo]

Or use Pert-al-Pert. Then you can also find the pipes if you ever need too, because of the aluminium layer.

 

Should only real 16mm for good balance of energy delivery and limited heat loss, compared to smaller pipes.

[sharpener]

On 03/10/2025 at 16:04, SuperPav said:

Hi all couldn't find anything on the topic, so thought I'd post random thought of the day...

 

Our house (4 bed detached ~190sqm) has UFH on GF in 100mm concrete slab. Upstairs is T22 rads in bedrooms but piped via a separate manifold. Upstairs more insulated than downstairs.

 

All will be running off a Samsung ASHP, but having been through 2 winters without *any* central heating in the house since the start of the build, the upstairs rooms don't ever get anywhere near as cold as downstairs.

 

System has no buffer, most of downstairs (50 sqm or so) is on a single zone and would always be open as that's the coldest part of the house. 

 

So my thought was... to avoid having two separate temperature zones for the ASHP, could I simply set the flow temp target on the ASHP (e.g. 35 deg) and feed the upstairs rads manifold from the heat pump flow, then take the return from the rads manifold as the feed into the UFH manifold, thereby naturally getting the UFH to run a few degrees cooler than the rads? or would this start to cause problems with the heat pump seeing an unexpectedly high deltaT? 

 

@marshian proposed this earlier and some implications were discussed in  

this thread (jump to Jan 24 2025).