Heat exchanger for combined UFH/rad system?

Bomber6

Might be overthinking this but would be interested in knowing if anyone has done anything like this previously. I'm renovating a house and have underfloor heating on the ground floor and radiators on the upper floors. The radiators are reclaimed cast iron radiators (pressure tested and powerflushed). We'll also be going with an unvented system and a system boiler.

The plumber has expressed concern that sludge from the radiators has the potential to block the UFH loops over time, and I'm guessing they're rather difficult to flush. I'd considered just buying new rads until someone made the point that any ferrous radiator is likely to rust internally over time (maybe more so!). I'm idly trying to think about solutions that will help. Top of the list is a magnetic filter (or two) and adding inhibitor to the loop, but I'm wondering if a heat exchanger could be fitted to create in-effect two linked but separate closed loop systems, one for the boiler and rads and the other for the UFH.

I've included a rough schematic of what I'm thinking. Has anyone ever seen or done something similar? Am I overthinking this?

(PS: I realise I'm likely missing a second expansion vessel for the UFH loop)

10-10-20

I'd be tempted to put a plate heat-exchanger on the lower demand heating circuit, which ever of the two that is.

Not a too dissimilar system shown here:

https://www.systemlink.ie/plate-heat-exchangers.html

John.G

You could, but you could also install a combined magnetic & particulate filter just before the boiler return and maybe another one on the supply to the UFH manifold. Gas or oil fired boiler??, what is the ~ total rad output and the UFH output, do you think you will run them alone at any time, ie rads only or UFH only.

Bomber6

Doubling up on filters was my initial thought, but I wasn't sure that that would be sufficient. RE: the rest of your questions, it's a gas-fired boiler. There are three floors at circa 50m2 per floor. House is moderately well insulated and air tight (as far as can be done for an old building). Plugging room sizes etc. into the SEAI heat loss sizing doc previously yielded me a total heat loss of ~3500W, though I suspect it'll be higher under real-world conditions. The 7 rads we have have a combined output of 9000W @ delta T50 (they're intentionally oversized to allow for a heat pump down the line). I would imagine the UFH and rads would always run together, though of course some zones could be turned off (more likely with rads since living spaces are all on ground floor).

John.G

What about the UFH, is that heat requirement in addition to the 3500w or part of it? and what is its output?

John.G

https://www.boards.ie/discussion/2058322571/heat-exchanger-for-combined-ufh-rad-system

Your schematic shows, I think, that the rads flow temperature will be reduced if the UFH is also in service, if this is your intention you can go down the filter routes and just install two close coupled tees CCTs (the Tees should be one pipe size bigger than the flow pipe) on the flow line, the UFH will just take whatever flow is required, and mix it in a TMV to give the required UFH temperature. The problem with extracting the heat from the flow is that you have sized your rads as "T24.1" rads to give you 38.8% (3500/9000) of the T50 which means running at flow/returns of 46.7C/41.7C, dT 5C so if you extract the UFH requirement from that you will still further reduce the rads temperature and output?