HELP! Boiler choice for UFH. Short cycling?

[bethelhousereno]

I'm having a real trouble deciding on boilers to install in my rennovation project. Gas combi boilers and underfloor heating - are they a suitable match?
 

My ground floor has recently had 100mm concrete slabs put down with wet underfloor heating within them. about 80m2 in total and I spaced the pipe at around 150mm.

 

First floor still has rads.

 

It's a stone cottage that's been extended a couple of times. It's not the most energy efficient but te extensions have cavity insulated walls and double glazing.

 

I am concerned that installing a combi might lead to short cycling of the boiler if the draw from the underfloor heating is lower than the minimum output of the boiler. I have read online that it is better to have a buffer tank in between boiler and UFH manifold but all of the Gas safe heating engineers I've had around don't think this is an issue. They all say combi will be fine. 

 

I am particularly drawn to ATAG's boilers which have a 10:1 modulation range, meaning a 35kw could range down to 3.5kw. I think...

 

Can anyone offer some guidance? Am I worrying unnecessarily or is a buffer tank necessary in my situation? Thanks in advance!

[Adsibob]

Have a look at Veissman. Highest modulation range on the market (mine is 1:17 so 35kw modulates down to almost 2kw). I have UFH throughout the house and never had an issue with short cycling, even though I don’t have buffer tanks, which is the general recommendation in this forum for UFH with a gas boiler. It’s not necessary with a high modulation boiler.

Edited June 19, 2022 by Adsibob

[JohnMo]

I have an Atag, they seem to a good boiler.

 

Need for a buffer is down to heat output of boiler compared to heat required for the building.

 

You need to know the heating requirements of the building to make that call.  You can get an idea of the heat required from an existing EPC or use the tool on here (boffins corner) to calculate yourself.

 

The worst case for short cycling is when its 10-12 degC outside, as the heating required is very low.

 

Try not to have to many zones and certainly make sure you have no really small ones.

[Gone West]

8 hours ago, bethelhousereno said:

My ground floor has recently had 100mm concrete slabs put down with wet underfloor heating within them. about 80m2 in total and I spaced the pipe at around 150mm.

Has the slab got a lot of insulation under it?

[bethelhousereno]

6 hours ago, Adsibob said:

Have a look at Veissman. Highest modulation range on the market (mine is 1:17 so 35kw modulates down to almost 2kw). I have UFH throughout the house and never had an issue with short cycling, even though I don’t have buffer tanks, which is the general recommendation in this forum for UFH with a gas boiler. It’s not necessary with a high modulation boiler.

That's good to hear. The ATAG boilers claim to be one of the only boilers that have "bottom up" modulation too i.e. they fire up at a low output and work their way up from there, which should make them more efficient.

[bethelhousereno]

5 hours ago, Gone West said:

Has the slab got a lot of insulation under it?

I put 150mm celotex under the main concrete slab (extension) and 200mm foam glass under the limecrete slabs (old cottage). so heat loss downwards shouldn't be too bad.

[bethelhousereno]

5 hours ago, JohnMo said:

I have an Atag, they seem to a good boiler.

 

Need for a buffer is down to heat output of boiler compared to heat required for the building.

 

You need to know the heating requirements of the building to make that call.  You can get an idea of the heat required from an existing EPC or use the tool on here (boffins corner) to calculate yourself.

 

The worst case for short cycling is when its 10-12 degC outside, as the heating required is very low.

 

Try not to have to many zones and certainly make sure you have no really small ones.

What thermostat system have you got installed? The installers are recommending a smart opentherm system so that the boiler output temp can be controlled. I'd much rather have a simple on/off type of system. I don't like relying on wifi for heating control and would rather keep things simple.

 

[bethelhousereno]

Saying all that about ATAG, looking on their online manual they say the minimum output for the 36kw combi is 6.9kwh. That's a far cry from the 1:10 modulation ratio they suggested on the phone...

[JohnMo]

I use an Atag Wize thermostat as the main controller - I have A325ECX and Wize is better suited to that.

 

If you are going Atag use the Atag One.

 

My turndown is to 6kW, but I am using a 160l buffer (also used for summer solar water heating upstream of the combi).  My heating demand is circa 3 kW on a cold day.

[Temp]

3 hours ago, bethelhousereno said:

What thermostat system have you got installed? The installers are recommending a smart opentherm system so that the boiler output temp can be controlled. I'd much rather have a simple on/off type of system. I don't like relying on wifi for heating control and would rather keep things simple.

 

 

How many zones are planned?

 

If the whole house is on one stat with no thermostatic rad valves then short cycling is less likely. If you have lots of small zones then it's more likely, especially if just one is calling for heat or the boiler can't modulate down far enough.

 

We have an oil boiler and they can't modulate at all so some kind of buffer is almost essential if you have small zones.

 

 

 

 

[bethelhousereno]

1 hour ago, Temp said:

1 hour ago, Temp said:

 

How many zones are planned?

 

If the whole house is on one stat with no thermostatic rad valves then short cycling is less likely. If you have lots of small zones then it's more likely, especially if just one is calling for heat or the boiler can't modulate down far enough.

 

We have an oil boiler and they can't modulate at all so some kind of buffer is almost essential if you have small zones.

 

 

 

 

We had planned 3 zones. One for rads and two for UFH (2 x 40m2 zone). The UFH area is quite openplan so I could be tempted to make this one large zone if it helped minimise short cycling.

 

 

[bethelhousereno]

5 hours ago, JohnMo said:

I use an Atag Wize thermostat as the main controller - I have A325ECX and Wize is better suited to that.

 

If you are going Atag use the Atag One.

 

My turndown is to 6kW, but I am using a 160l buffer (also used for summer solar water heating upstream of the combi).  My heating demand is circa 3 kW on a cold day.

It seems the turndown for ATAG is about 6kw. That's not particularly low so I'm thinking a viessman might be better for my needs. Unless I want to install a buffer between boiler and ufh manifold