UFH + Buffer Tank w/Gas Boiler

[Soulfish]

Hi all - we're renovating a 1970's bungalow that we've installed a new slab (PIR 150mm insulation + liquid screed) within which we've run UFH. In total we've got 10 loops split into 7 zones with approximately 75L of water in the system and a heating demand of about 7.5kw according to the Wunda calculations. 

 

Currently we have the 24kw Worcester Bosch Greenstar 24i gas boiler directly connected to the UFH manifold. Having been reading various threads on here with interest it seems the suggested setup is to have a buffer tank to feed the UFH manifold to reduce short cycling of the boiler. GIven our system requirements what sort of size buffer would be recomended? I'm guessing 50L would be too low and we should be looking at 75-100L? Unfortunately we've got a short space of time to (easily) resolve this before plasterboard goes up everywhere, as ideally we'd mount any additional tank in the loft (where the boiler + 180L UVC are located).

 

Does anyone have any links to a simple diagram/spec I can give to our plumber so he can understand what I'm asking for (and how it differs to the current basic setup), and any suggestions for (preferably compact/wall mountable) buffer tanks (and expansion tanks?) that I could use for this?

[SteamyTea]

You can increase the volume by running pipework in series.

It is the minimum volume that the BT tops up.

How low does that boiler modulate down to, while still condensing properly?

Edited January 23, 2022 by SteamyTea

[Soulfish]

Looking at the specs I think the lowest output from the boiler is 7.2kw

[Temp]

There are several ways to add a buffer....

 

https://blog.heatspring.com/2-pipe-versus-4-pipe-buffer-tank-configurations/

 

Figure 1 is what I have...

In my case the room stats control the right hand side via a wiring centre. The wiring centre performs a logical OR function to control the manifold pump.

 

The boiler/left hand side is controlled by a stat on the tank (and an optional timeclock). 

 

The left and right hand sides work independently from each other the only common thing is a Switched Fused Spur that powers both sides. 

 

However there are alternative ways. The two port system in Figure 4 is claimed to be more efficient but I've no experience of this approach..

 

 

The idea with thus approach is that flow from the boiler can go direct to the UFH. The buffer only takes "excess" water that the UFH doesn't need (for example if only one loop is calling for heat). I think this approach is better for ASHP systems as they have lower flow temperatures and this approach helps maintain the flow temperature.

 

 

 

Edited January 23, 2022 by Temp

[Soulfish]

Thanks for this. I think given the locations and current pipe runs Figure 1 is easier for us to implement but will have a chat with the plumber tomorrow. So from those diagrams I take it the only additional things we need are a buffer tank (and expansion vessel of approriate size for the buffer tank). Are there additional pumps that are needed? I had read something about needing more pumps, but I may have been confusing myself!

 

Given the size/requirements of my system what sort of sized buffer tank would people suggest? Does anyone have any suggested manufacturers of well insulated buffer tanks?

[Olf]

2 hours ago, Soulfish said:

we're renovating a 1970's bungalow that we've installed a new slab (PIR 150mm insulation + liquid screed) within which we've run UFH

You've done good job with the floor, but what about the walls and roof? If they leak, you may not be lucky to experience short cycling.

If your happy with larger temeprature swings, then you can try to set up controls for greater hysteresis, that way you heat for longer and then cool for longer (with the slab smoothing that to large extent) without need for extra space for buffer tank.

[Temp]

2 hours ago, Soulfish said:

Thanks for this. I think given the locations and current pipe runs Figure 1 is easier for us to implement but will have a chat with the plumber tomorrow. So from those diagrams I take it the only additional things we need are a buffer tank (and expansion vessel of approriate size for the buffer tank). Are there additional pumps that are needed? I had read something about needing more pumps, but I may have been confusing myself!

 

Given the size/requirements of my system what sort of sized buffer tank would people suggest? Does anyone have any suggested manufacturers of well insulated buffer tanks?

 

 

In Fig 1 there are two pumps.

 

1). One on the left circulates water from boiler to/from the buffer tank. 

 

2). One on the right is on the manifold and circulates water around the floor loops. On some manifolds this pump also draws water from the buffer. In brief - the mixer controls how much new hot water is drawn from the buffer and how much is recycled from the return loops. However not all manifolds work quite the same way. Sometimes a other pump is required to circulate water from the buffer to the manifold and back. You will need to check with the supplier of your manifold to see if another pump is required. 

 

Not sure what size buffer to recommend. Probably 100-150?

 

Tables on the web suggest the expansion vessel should be around 8% of the total system volume. 

 

[PeterW]

@Temp that is a really useful image - fig4 is my go to design (without the spring non return valves ..!) and is dead easy to do. @Soulfish not sure why you thing fig1 will take up less space ..? It’s easy to plumb and very simple. 
 

i would go with 100litre buffer - not expensive and all pretty standard so not much to choose between them tbh

[Soulfish]

1 minute ago, PeterW said:

@Temp that is a really useful image - fig4 is my go to design (without the spring non return valves ..!) and is dead easy to do. @Soulfish not sure why you thing fig1 will take up less space ..? It’s easy to plumb and very simple. 
 

i would go with 100litre buffer - not expensive and all pretty standard so not much to choose between them tbh

It's more that the fig1 option more easily slots into the existing plumbing with where we'd be able to locate the buffer; however I'll have a chat with the plumber and see what he says about Fig4 and if we could make it work

[PeterW]

Just to add … always put the strainer / filter etc on the return to the boiler or ASHP so you catch the crud before it enters the heat exchanger. 

[Temp]

2 minutes ago, Soulfish said:

It's more that the fig1 option more easily slots into the existing plumbing with where we'd be able to locate the buffer; however I'll have a chat with the plumber and see what he says about Fig4 and if we could make it work

 

I don't think it will be any harder. Both approaches need similar number of pipes between the various parts. 

[PeterW]

Just now, Soulfish said:

It's more that the fig1 option more easily slots into the existing plumbing with where we'd be able to locate the buffer; however I'll have a chat with the plumber and see what he says about Fig4 and if we could make it work

Ok but fig1 needs pipe breaks and flow through  - fig4 doesn’t ..?? It’s pretty much the tank and two tees and two stubs of pipe to fit it - flow doesn’t go through it as in fig1.