UFH, Rads & New Combi Help Please

[sailorjames]

Hello all,

 

First post, looks like a great forum.  We're part-way through a renovation of our living areas.  Radiators have been removed downstairs (8 remain upstairs and in downstairs bedroom etc.).  

We were planning UFH in the living area, originally electric but our current old combi packed up the other week so are looking at a new boiler/system to run low profile wet UFH (suspended timber floor) with tile over.

I wanted a thermal store but have ruled out on cost of purchase/installation.  I would like to reduce combi cycling so would a buffer tank help with this?  If so how are they plumbed in with the UFH?  Presumably it goes boiler -> buffer -> UFH manifold.  What size?  There is a little 20l one made by AA.

Also any recommendations on control systems for the rads vs UFH?  The UFH will be in 1-2 zones max as it's open plan (except hall).

Also we're wrestling with whether to put 20-30mm insulated tile backing board over the floor or rip it up and 50mm celotex underneath and new floorboards.  Obviously the celotex is the "best" solution from a thermal efficiency and floor build up perspective but has anyone used the insulated tile backing board?

 

Thanks in advance

James

[ProDave]

Hi and welcome to the forum.

 

The first thing is to get the basics right.  And the No 1 thing if installing under floor heating, is to get the floor insulation right.

 

So before you embark on this, what insulation is under your suspended timber floor, or what insulation will you be putting there?  I guess the answer is nothing at the moment.  If so you want a LOT more than the 20 to 50mm you are talking about.

 

Re controls.  Radiators upstairs and UFH downstairs work very differently so you will want separate controls.  So use a separate programmer channel for each to allow different times to be set.  Use 2 port motorised valves, avoid that work of the devil, the 3 port mid position valve.

 

Upstairs the rads will be controlled by their timer channel and one or more room thermostats.

 

downstairs it is normal for each zone of the UFH to have it's own room thermostat, and the manifold controller takes car of it, turning on the UFH when one ore more zones calls for heat.

 

I will leave the plumbers to talk buffer tanks.

[Nickfromwales]

Hi James, welcome to the forum.

The first question has to be, “will that UFH system heat my rooms?”

A decent UFH supplier will do heat loss calls for you for a small fee and tell you what heat ( watts per square metre ) the UFH would need to provide to keep the temp at 21 degrees when it’s -5 outside. Anything below that and you’d need auxiliary heating to supplement the UFH. Buyer beware!!

100% take all the floors up. Staple some nylon pallet strap to the underside of the joists to make a cradle for loose ( fluffy ) mineral wool insulation to be dropped in between the joists. Full full those voids and enjoy some much better insulation values for not too much money.

Draught-proofing the ventilated space is a no no, but sealing the floor / joists to the walls at the top to stop that draught getting into the house is a great way to reduce heat loss, and therefore the above measures may well bring you into the realms of UFH being viable. 

If you intend to go this path, many in here have installed Wunda products ( I have fitted the same for many clients ) and if you commit to buy from them they will do the full design and calculations for you and compile the materials list to suit. Takes a lot of the unknowns away then, and you can instantly see if UFH will suffice. 

Even if you can scrape by without the added Insulaton / draught-proofing you’d be mad not to do it ( depending upon how long you wish to stay at the property of course ). 

The room thermostat for the spaces served by UFH needs to be chosen well, with a hysteresis of .2-.5 degrees C maximum.  Do NOT use a standard rotary room stat or you’ll go from hot > cold > hot > cold with the resulting over & under shoots that you’d get from a room stat with even as much as a 1 degree swing. 

You can just fit spreader plates atop the joists, see Winda’s website for examples, but an Insulaton board system will interrupt cold bridging from the floor joists and improve things further.

Forget leaving a draughty cold air blowing around under your UFH as that’ll suck away a lot of heat, and the resulting infiltration could render UFH ineffective.

The downstairs bedroom radiator will need to be on the UFH zone valve & controls ( not on the manifold, just fed with heat from the boiler, teed off the pipes that feed the manifold AFTER the UFH zone valve ) and will require a TRV to stop that bedroom from overheating.

As for a buffer, you could maybe get away with fitting a monster of a radiator in the hallway ( if the hall / stairs / landing ) are open plan, and run that radiator as a bypass rad, and even better if you can do a bathroom towel radiator as a bypass too. It’s mainly to get some extra primary system volume which should deal with short cycling enough to negate a buffer. 

Either that or a small buffer tank in the airing cupboard so the waste heat keeps the Y-fronts toasty. 

Fabric first, heating system design second   

[sailorjames]

Hi guys,

 

Thanks for the replies, well I took your advice and am pulling up the floorboards.  Planning to reuse most of them after putting 100mm celotex between the joists to achieve Bldg Regs floor spec.  

I want a draught barrier on top of joists before refitting floorboards, does this need to be breather or would plastic DPM do?  Plan to seal this around the edges to walls.  Any thoughts?

 

I'll come back to the UFH later, will have a look at Wunda.

 

Thanks again.

James