Question regarding Control of a Buffer tank for UFH using ASHP

[Blynchy]

I am currently trying to design my space heating. I am  putting in a buffer tank on my primary heating side. I haven't decided the size yet but it will be somewhere between 60 and 80 litres. I'll add a sketch below. Through the excellent advice I've read here, I'll have the room stat call for the UFH pump and the buffer will call for the ASHP when required.

My question is what turns the ASHP on and off? Are two stats required?, one at the top of the tank to let you know that the hot side is not warm enough which will engage the heat pump. The heat pump then runs until the stat on the lower (cold) side reaches the required temperature of the tank. Is there a way of doing it with one stat? Am I over complicating it? 

I pan on running the UFH on at 32 deg and off at 27 deg. 

One other question regarding the diagram below, do I need a stand alone pump just after the buffer tank circulate the pumps or will the manifold pumps be enough. Thanks in advance.

[joe90]

I have a buffer tank (80 litre) which calls for heat with a tank stat set to 35’ when the heating is on (winter), a room stat (hallway) starts the manifold pump to draw that heat into the slab, the manifold blender winds it down to 25’. The only downside is the buffer is permanently heated in the winter but insulated well and any heat leak stays within the house.

[Nickfromwales]

If go with a slightly bigger ( 80-100L ) buffer tank, and a single stat. That’ll be simple and effective. 
The waste heat will be advantageous, so not a worry. Possibly install this in the airing cupboard? 

[Blynchy]

Thanks a mill for your help folks. Regarding the single stat, does that mean run the temp of the buffer tank higher than the under floor heating? What would you recommend the delta T of the stat being? Where would be the best placement then around the middle of the tank?

[joe90]

58 minutes ago, Blynchy said:

does that mean run the temp of the buffer tank higher than the under floor heating?

I ran the buffer at 35’ , UFH at 25’ and the ASHP runs at one temp, set for DHW at 48’ so there were temp gradients to aid efficiency. My tank stat is halfway up as that’s where the socket was.

Edited June 5, 2022 by joe90

[markocosic]

Why is there a buffer tank at all?

The UFH controls are controlling the slab temperature in order to control the room temperature. You want this to be zoned. Ok.

 

You can supply 35C into the UFH the deepest darkest depths of winter. 30C when it's mild out. 25C when you need virtually no heat etc. If this is "tuned" correctly then the "runtime" of the UFH (how long the zones call for heat) should be about the same throughout the heating season. 

 

Allowing the heat pump to set the slab temperature by changing its flow temperature, rather than always filling a buffer to 35C and then using the runtime of the UFH to set the effective temperature, would improve efficiency.

 

If you fed the heat pump from the "pump" call on the UFH controls then it is "on" whenever a zone is calling for heat. When the heat pump is "on" it will decide how hot the supply water is based on the outdoor temperature. (or it could/should) It will run until the target flow temperature is reached then modulate down to maintain that target flow temperature; before eventually it cycles whilst the UFH pumps run continuously.

You just need some open system volume to allow for defrost. Is there a single large zone that can be "always on" but throttle back a little so that it always takes longer to warm up than the other zones? 

 

e.g. in my (regular gas boiler house) the open plan downstairs is 3 rads all without TRVs, then upstairs are 3 bedrooms with TRVs. The rads downstairs are setup so that downstairs always takes longer to heat than upstairs. Therefore any time the boiler pump runs it can access the water in the downstairs rads to avoid cycling.

 

You could do the same for your main living space to allow the heat pump to access that water for defrost purposes.

[Nickfromwales]

21 hours ago, Blynchy said:

Thanks a mill for your help folks. Regarding the single stat, does that mean run the temp of the buffer tank higher than the under floor heating? What would you recommend the delta T of the stat being? Where would be the best placement then around the middle of the tank?

Yes, about 10oC higher would prob be ok. Just enough to start running the system and then you can check how the ASHP behaves, duration of heating on / off cycling etc, to decide if it needs tweaking.

 

Most modern sealed cylinders have a fixed stat ‘pocket’, or two, so the location will be defined. Unless you buy an open vented cylinder with foam insulation, which would be quite poor in efficiency vs a sealed and pressurised system. A dumb buffer will not have a T&PRV so go for one of those. 
 

If you require cylinders, ( UVC and buffer ), give Trevor ( Cylinders2go.co.Uk ) a shout and mention my username and the forum and he will look after you. 

[Chanmenie]

How do you calculate the required size of a buffer tank ?

I’m planning on using a 7kw Vailant heat pump with 150 or 180 litre Mixergy tank, Vailant recommend their 45litre buffer tank! Will that be big enough ? 
 

@Nickfromwales Thanks for the Cylinders2go recommendation 

[JohnMo]

There are three ways to plump the buffer, 2, 3 and 4 pipe, each has its own advantages and disadvantages.  The 4 pipe as drawn, has to be hot before it will deliver heat to the system hence the need to have a thermostat on the buffer.  You heat buffer as Joe90.

 

Or you could do it like this as a 2 pipe without buffer thermostat.

 

https://www.kensaheatpumps.com/installer/tech-focus-part-2/

[joth]

2 hours ago, JohnMo said:

There are three ways to plump the buffer, 2, 3 and 4 pipe, each has its own advantages and disadvantages.

And there's more variations above that, e.g. for 2-pipe, you can connect it up in parallel or in series with the emitter.

 

I've not heard or seen an example of the 3-port schematic, do you have an example?

 

[JohnMo]

Can see schematics below but basically the hot side is an oversized tee as per 2 pipe, the bottom cold connection goes into cylinder and back out same as a 4 pipe.  Has all the advantages of a 2 pipe and 4 pipe.

 

[SteamyTea]

On 06/06/2022 at 09:17, JohnMo said:

There are three ways to plump the buffer, 2, 3 and 4 pipe

Buy a 4 port one and you can plumb in any combination.

[JohnMo]

More ports good, flexibility and don't have to use them just blank off