Radiators and wet UFH combination

[nathan77]

Hello everyone. We are renovating and extending a 1920's property (images attached). Ceilings, floors and plaster have been removed so 'straightforward' to repipe everything from the start. On the ground floor we are creating a large L-shaped kitchen/diner and we would like to go with wet UFH just for this part, and then radiators for the rest of the house. This is because kitchen/diner will have a properly insulated/screed floor and cavity/rockwool insulated walls and ceilings (albeit 3x
Velux windows and sliding doors) and we would like this to be tiled, whereas all other areas are suspended timber floors and floorboards.

Now based on some 'simple' maths, I would need about 4KW for the UFH and 11.5KW for the radiators. We will also have 2 showers and 1 bath/shower. Here are some of the questions I am troubled with and I would like your support:

1) I am concerned that UFH alone might not be enough to warm up the kitchen/diner. There is also a log burner in the room, so that might be a solution? Or should I throw in a radiator or two just in case?

2)What type and KW boiler would be advisable for CH/UFH/Hot Water (to connect all onto one boiler)? From asking around my impression is that a good storage combi would do the work and then separate/zone it for UFH with additional thermostat. I am trying to avoid adding cylinders.

3)Am I right in thinking that it's best to go for a 22mm main feed and then split this into individual 15mm branches serving 2-3 rads each (4KW max)? Would the UFH be on a 22mm or 15mm?

4) We have put in a new MDPE pipe from the meter as we replaced the lead pipes and the pressure and flow are very good. Having in mind that 2 showers might be used at the same time, please advise on boiler flow and other parameters necessary to have adequate flow and pressure to both showers. I presume if two showers are running, we are looking at about 28L/min?

Thank you very much in advance!

 

[ironsidebod]

1. Simplistically, if you think you'll need radiators then don't bother with the UFH. Radiators on their own will be fine, but having them destroys some (if not a huge part) of the benefit of having UFH. You need to look at the system you want to use and look at the flow temp, pipe spacing and so on to see if it can produce the heat output you need for the room.

2. Boiler capacity is normally sized around the hot water requirement which means it'll be far too large for the UFH. We have an issue where our ground floor is wet UFH and if it's the only heat requirement the boiler will cycle. Upstairs has radiators and if both are on it works as expected. To facilitate ground floor only without the cycling I think we're going to need a buffer tank.

 

[nathan77]

We have an issue where our ground floor is wet UFH and if it's the only heat requirement the boiler will cycle.

Do you have the UFH 'T'eed at the boiler with its own zone and own thermostat?

 

[reds42]

I put an overlay wet underfloor heating system in my kitchen as a seperate zone to the rest of the house.

After doing some heat loss calculations given the area of floor not under cabinets I could run the pipes I found on a really cold day it would be a bit short of power to keep the room warm enough ( old house with less than perfect insulation and large amount of glazing).

To make up for the shortfall I put in an oversize radiator on same circuit as the UFH (using same lower temperature water) to just add that little bit more heat.

I still get the nice warm floor and even heat distribution in the room from the UFH, the radiator just puts that bit extra heat out so theres no struggle keeping it warm on really cold days. It does also help to quickly get some heat into the air when the underfloor circuit first comes on.

 

[ironsidebod]

Do you have the UFH 'T'eed at the boiler with its own zone and own thermostat?

Yes we do, one we inherited but which was part of the original house build. Learned a lot, put a lot of effort into balancing and getting it spot on but the load isn't great enough so on it's own it won't heat the ground floor properly due to cycling on a cold day. Our house is such that we don't necessarily need to heat the upstairs but want heat downstairs so on a very cold day we have to put the radiators on upstairs to make sure the boiler pushes hot water through the UFH consistently...problem comes when the radiators start to shut down (as the TRVs kick in) the cycling starts and it struggles to keep up with the heat loss.

 

[nathan77]

So in theory If I were to go ahead with only UFH in kitchen/diner and rads everywhere else, I should find a boiler with low enough modulation limit (but still condensing?) to be able to satisfy the UFH alone if needed? (assuming raising the UFH mixer's minimum temperature cannot solve the issue)

 

[ironsidebod]

So in theory If I were to go ahead with only UFH in kitchen/diner and rads everywhere else, I should find a boiler with low enough modulation limit (but still condensing?) to be able to satisfy the UFH alone if needed? (assuming raising the UFH mixer's minimum temperature cannot solve the issue)

Yes, I would have thought a bunch of radiators on the same circuit as the UFH will help and you're right the UFH flow temp doesn't help solve the issue as it still doesn't take enough heat out for the boiler to modulate properly.

As to the boiler requirements I'll duck out of that one as it's not something I'm qualified in, sorry.