Underfloor heating and radiators


1 May 2009
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United Kingdom
I have an underfloor heating system downstairs and a radiator system upstairs, the radiators all run from a central manifold. The heat input is from a pellet boiler with internal pump, the UF circuit has it's own additional pump. When the underfloor heating system is on I cannot get heat to go into the radiator circuit. I have tried balancing by turning the flometers down on the UF heating but no go. If the UF is off the radiator circuit works OK but the flow isn't great. Can I add a circulating pump to the radiator circuit. I was told it might work against the pellet boiler pump. I used virtually the same setup 10 years ago and it worked great, the only real difference was I used an oil boiler.
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It's an Extraflame Duchessa Boiler/stove declared output of 12 kw
Probably about 10kw claimed to water.

When sizing a pellet boiler you need to add 30% to your heat loss figures.
Another 10% if you live in an exposed location.
Which is in line with CIBSE recommendations.
And another 10% if its heating ufh. (That's my recommendation)

So that would leave you needing 18kw if you've used the 12kw figure!

Bad quality pellets will lower the efficiency also as will unchecked low return temperatures entering the boiler return.
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My Sap certificate (its a new build) says 8542 kwh/yr space and 2426 kwh/yr water. Can you work out the boiler size needed from that?

Give me the floor area of the UFH section and the floor area of the radiator section and I'll give you a rough idea using a fiddle factor.
I assume its two storey?
The UFH section (downstairs) is ~100 m2 and the rads (upstairs- bedrooms) is 64 m2. Insulation levels are high Sap cat B. To be honest with the UFH working you hardly need heat upstairs! but I do want the system to work together fully. The rads do heat up when the UFH shuts down.
I'm getting 14kw with the fiddle factor.
So with the figures I quoted earlier the pellet boiler size would be 20kw.
Would this be causing no circulation in the Rad circuit? I would think an undersized boiler would just mean stuff not getting as hot. I checked the manual and it says useful output to water 10.8 kw so it seems very undersized from what you say. I'll speak to the installer, he did the calcs. and would installing a thermal store sort the problem out, this would be cheaper than ripping out the stove/boiler and I might be able to negotiate with the installer.
The ufh pump will deliver a percentage of the heat produced by the boiler to the ufh loops.
The loops should have flow regulators and flow meters and the correct flow set up upon commissioning.

The other percentage is delivered to the rad circuit. The primary pump should be sized accordingly and be capable of delivering simultaneously to both.

A buffer cylinder may address some of the short comings but no guarantees.
Pump sizing could be more accurate.

Do the MI's recommend an accumulator receptor?
What return temp protection is currently fitted by your installer?
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A buffer tank is mostly used with systems like this. By it's very nature a pellet boiler will never operate like an oil boiler & pellet quality can never be certain.
Sounds very much like a poor design.
I think the ESBY valve is about 54deg (does that sound right) and the MI does allow for direct use. The UFH and Rad design is really good and I have used it before to really good effect which is why I installed it this time. I relied on the installer for the pellet system but I think this is letting the design down, it looks like a thermal store is going to be the way forward. To be fair the installer said a store would be the best solution at the start, but it would work just like an oil boiler. The information you guys are giving me is helping me get a better picture and I'm really grateful.
With the type of fuel for optimum burn you do need a buffer tank. Oh yes it'll work without it, however it's like driving in first gear. Heat Pump, biomass & boilers that have a slow burn/fire need lots of volume & good circulation.

Most modern homes have limited heat requirement, little water volume & are controlled by simple on/off controls - more off than on too!!
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How do you work out what size thermal store you need. Although the store is vented I believe you can still heat an unvented or vented DHW cylinder from it as well as the CH, is that correct?
50/l per kilowatt.
So you need around a 600/l ish vessel which can be sealed or vented.

Some rather foolishly base the calculation on the minimum output if the appliance modulates.
Which is pointless as you don't need the complexities and inefficiencies of a modulating appliance once you incorporate a buffer.
The heat exchanger is designed for full output and operates most efficiently on full tilt.
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