Low temp UFH with ASHP

[DefCon2]

In the process of trying to have a ASHP installed after complete refurbishment (actually ended up demolishing most of my 1920's house).  I've installed UFH throughout and plan was to maximise efficiency by using the low temp features of a Heat Pump.   I have an preferred MCS installer but been having a few debates on the installation. 

 

Points of debate are:

 

1. do I need a mixing valves for a low temp UFH design ?

2. should there be pumps on each of the (three) manifolds or only one pump for all manifolds? 

3. and linked to question 2 above - Is a buffer tank the best/simplest option ?

 

Any other thoughts, suggestion, advice (see  graphic) would be greatly appreciated and thanks for reading this.

 

Cheers Ash

 

 

[JohnMo]

You are likely to have 3 different floor buildups, so three different flow temps. So would suspect mixers are required. Not sure you need all the pumps, 7 in all including the one in the ASHP. The ones on the mixers should be good enough to pull and push the water from the buffer without intermediate pumps also.

 

Mixers do not entertain any other mixers than IVAR (mechanical) or electronic driven by Vaillant controller.

 

Buffer - really depends on how you intend to operate and the flow rates and min engaged volumes.

 

I would only do a 2 port configuration, not a 4 port.

 

[SteamyTea]

38 minutes ago, DefCon2 said:

I've installed UFH throughout and plan was to maximise efficiency by using the low temp features of a Heat Pump

First things first, what are the U-Values below the UFH pipework?

Especially the ground floor.

 

[DefCon2]

Thanks JohnMo.

 

One question: as its a low temp UFH, wouldn't having manifold mixing cause contention such as the heat pump to short cycle ?

 

 

[MikeSharp01]

Not sure, others can probably give you a better steer EG @JohnMo, but I wonder if the 3 port valve in front of the buffer might lead to short cycling and defrost issues if the coil in the DHW tank does not have sufficient volume for the defrost cycle, the ASHP is not in control of the 3 port and it, the coil, is the only thing engaged when a defrost happens. 

[JohnMo]

9 minutes ago, MikeSharp01 said:

Not sure, others can probably give you a better steer EG @JohnMo, but I wonder if the 3 port valve in front of the buffer might lead to short cycling and defrost issues if the coil in the DHW tank does not have sufficient volume for the defrost cycle, the ASHP is not in control of the 3 port and it, the coil, is the only thing engaged when a defrost happens. 

Definitely the 3 port diverter before buffer, the buffer is there for the heating system only - but only if you need it. I would have the ASHP controlling the the export diverter - it will have an output to control it.

 

Good thing about a 2 port buffer is water only flows through it if required. Otherwise it's just bypassed. You set up the ASHP to control buffer temperature, any room thermostats would control the UFH pump, or secondary circuit circulation pump. Heat pump continues to run even if no call for heat until buffer temperature is satisfied. As soon as a zone has a call for heat warm water is drawn from the buffer by either the UFH mixer pump or secondary circulation pump. Once the buffer temp is lowered the thermostat on the buffer calls for heat from the ASHP.

 

LLH and buffer are the same thing as drawn, just the buffer is a bigger volume. Both can lead to primary and secondary circuit distortion and mixing between flow and return leading to a drip in efficiency.

 

Mixers need or not, depends on the design flow temperature and variances in flow temperature between floors, if they are all the same maybe no mixers are needed. If they are different you may need mixers.  The other thing is number of loops and required volume flow and system pressure drops - can your ASHP circulation pump handle it. If not you do need hydration separation and a secondary circuit with its own pump.

 

Snippet of information is all well and good, but solid facts are really needed. If you are going for a grant via MCS, your hands are somewhat tied anyway. You need to do a whole system design as a one off task, a bit of and bit of that really isn't good enough.

 

There seems to be some misinformation out there, that heat pumps shouldn't cycle.  All heat sources cycle, it what they do to manage loads below minimum turndown. There's a difference between controlled cycling and uncontrolled cycling.  If you heat pump runs for about 10 mins or more the cycle is controlled, running for a couple of minutes or less it is uncontrolled (or short) and a piss poor system design.

 

Minimum information required

Total house heat loss 

Floor by floor heat loss

Number of zones per floor (how many thermostats)

Number of loops per floor - ideally with loop length or flow rate and each floor design flow temp.