Buffer tank required?

[DCG]

My builder is suggesting a vaillant ecotec 637 boiler. 

 

I haven't worked out the heat loss of the building but I will be insulating as much as possible during the extension and renovation. 

 

My questions are:

 

1. Are buffer tanks a good idea when there's UFH laid in screed? I ask because I've read that short cycling occurs when the boiler is producing heat that ufh cannot use. But if the ufh heat is going into screed, I suspect it can all be used and therefore there might not be short cycling? Fyi, there will be celotex under the screed 

 

2. If I did have a buffer tank, can I have it in the loft? I was planning on locating my boiler and HW tank in the loft and it makes sense to have the buffer tank there? But I read that given the loft is uninsulated from above, the buffer tank wouldn't work as intended. 

 

3. Any problems with having boiler and HW tank in an uninsulated loft? 

[SteamyTea]

For the few quid that a buffer costs, fitting the correctly sized on saves hassle.  You may end up fitting on anyway.

 

Cold lofts are not the best place to fit hot stuff, the parasitic thermal losses can be quite high.  Better to have those losses within the heated envelope, during the heating season they become useful.

[JohnMo]

First obvious question is why do you need a nearly 40kW system boiler, unless you are heating a leaky old mansion.  Size boiler to suit, not what's the biggest you can get.

 

What is your heat loss calculation telling you kW heat loss is a your lowest temp?

 

If you want an efficient gas boiler treat sizing it and running it the same as a heat pump.

 

Install a heat pump cylinder with a 3m2 coil, for fast recovery and a big delta T to ensure return temps are low, for condensing.

 

Install as DHW priority, X plan for example, so you run different temps for cylinder and UFH.

 

 

A few factors you need to consider

 

Although UFH can contain lots of water, it is only useful if it's not in lots of zones.  The smallest zone is your system capacity, not the sum of all loops. Ideally run weather compensation with all loops open all the time.

 

What is your min and max heating requirement in kW compared to min output of the boiler in kW?

 

How much insulation under the screed, 25mm or 300mm or somewhere in between?

 

All cylinders loose heat, the bigger the difference between internal cylinder temp and the surrounding air the higher the heat loss.

 

Getting the correct sized boiler no buffer required.  Get A rated cylinders with 3m2 coil, add more insulation if in an unheated space.  Or find room in the house.

 

[Marvin]

Plus one for @SteamyTea's comment about stuff in the loft.