Pumps needed with ASHP and can a area be to small for UFH?

[Bruce]

So we are about to order the materials for the UFH system on the ground floor in our 1930's semi and some of the suppliers add pumps to the manifolds in their quotes and I'm a bit confused about that. My understanding is that when run on ASHP's with as low flow temps as possible, there is no need for mixing or extra pumps at the manifold, is that correct? We have not installed the ASHP yet so that is part of the dilemma we have. Id rather not pay hundreds of pounds for pumps that are not needed.

My second query is regarding our first floor bathroom, the area in there after we install a walk in shower will only be about 1.7m2 (marked on red), is it stupid to install UFH in there? In my head I like heated floors especially with tiles. The heating will be complimented with a 1800mm tall towl rail that will also be on the heating circuit.

 

I'm sure I've missed some important information please bear with me as I'm still learning. 

[SteamyTea]

Have to be a bit careful in the design, and the design terminology.

 

There is a difference in zoning with an individual temperature control on that circuit, and zoning for a room with the pipework connected up to other room zones and all controlled by 1 temperature controller.

With wet heating systems, there is always a minimum volume of water that is needed to make them work.  Even if the minimum is achieved, then the heating system, whether a heat pump of a gas boiler, may still short cycle.  Ideally you want the heat source on all the time it is needed, not switching on and off.

 

If you connect that bathroom zone to another zone (basically you want one zone for the whole house, maybe two if two storey), then the volume is increased by default.

With a properly designed heating system, the mean flow temperature is the same across all heat emitters, then you just change the size of the heat emitters to control the room temperature.

 

Have you done a room by room heat loss calculation?

If not, it is time to do one before you carry on.

[Bruce]

Thank you for your response. The total volume will initially be 30 litres and after we have added the utility room and the conservatory it will go up to 40. The first floor will have 3 radiators, one in each bedroom, and one large towel rail in the bathroom. Not sure how the first floor much that and all pipework adds up to in volume. The idea is to have the heat pump installed and it will be running on weather compensation 24/7 as they are intended.  

I have had a play with Jeremy Harris heat loss calculator and the total heat loss is about 6.55kw at -3C. That is before we add any internal wall insulation but including some insulation works that will take place in the suspended floor before we add the UFH. I had a play with another one online a couple of weeks ago and they seem to come to about the same numbers. It is a standard 1930 semi so it is a bit leaky but we will install an MVHR system as I cannot stand any damp, it drives me crazy, so that is why I have chosen a bad MVHR efficacy to compensate for the leakiness of the house.  

 

After some thinking, it seems to make sense to leave the bathroom with a towel rail and we will just an electric UFH in there to boost the heat a bit when we are taking showers in the winter.

I'll take your answer as if we size the first floor radiators correctly so that they can run at the same temoerature as the UFH system, no blending should be needed on the ground floor UFH system and therefore no pump at the manifold is needed either? 

 

[JohnMo]

Without any zoning you are going to short cycle a 6kW heat pump with only 40L, so you need to add volume. Best way is a volumiser or a two port buffer across supply and extract. You will need a volume of circa 60L engaged volume.

 

If you want heat out of you bathroom do the centres as close as you can, so a spiral pattern is best. Add an electric towel rad also.

 

Running direct from heat pump can be done, just need to make sure the you are comfortable with getting some hot water in the floor when yiu have finished DHW heating. To run on the heat pump circulation pump, you need the buffer/volumiser as mentioned above not a 4 port.

 

Some info on buffers attached 

 

 

Kensa Buffer.pdf

[Bruce]

Hi John,

 

Thank you for your comment. As we also have, and need to keep, the heating on the first floor with 3 large radiators and a towel rail, surely that volume is added to the total volume? Quick estimate give that the first floor would add about 30 litres. The towel rail will be dual fuel so to ptimise running cost and flexibility on when we can use it.

Not too worried about some of the heat from the DHW cycle getting into the UFH, I think we can live with that.

[JohnMo]

You mentioned you will have a total volume of 40, but initially 30L. Assume the 30L is the upstairs, you will keep on. You really need about 60L engaged water to keep the heat pump happy and not short cycle when it's not that cold. If your chosen heat pump says different all good; they will normally specify a min flow rate and min water volume.