Questions relating to pipe centres

[Post and beam]

New thread as i have questions on the pipe centres point. Extracted from the  Costs to install 8.5kw Mitsubishi R32 ASHP

 

'100mm centres and minimal zoning (ideally open loop) will work best for maximum efficiency.'

 

 

'Once to get close 10W/m2 there is little difference. So much so that both have to run the heat pump at min flow temp for a heat pump anyway for both 100mm and 300mm centres. Which in its self can be problematic with a high mass UFH system as return temperature doesn't drop quickly enough.'

 

For a few months while learning all i can about UFH via ASHP i have come to understand that close centres and as low a temperature as you can is the received wisdom. On that basis i have been planning on 100mm centres for my upcoming build with one global zone. 

 

Is this not a settled idea? It appears not.

 

 

 

 

[joth]

@JohnMo posted the following graphic on the other thread, I'm responding here to keep it on the UFH rather than ecodan 8.5kW thread. (This topic is posted in the wrong forum section btw, should have been https://forum.buildhub.org.uk/forum/136-underfloor-heating/ but nvmd)

 

Where are this figures taken from ? Not doubting them, would just like to read the full context they are made in, and be able to make my own citation to the OG author 🙂

 

(parts of them are reproduced on https://www.pandhengineering.co.uk/advice/low-carbon-ready-under-floor-heating-ufh but again without citation. But that's a generally silly article with statements like "My first choice for a low carbon ready underfloor heating system type is a low profile screed system with its high heat outputs and quick response times"  -- wtf does high heat output and quick response have to do with low carbon heating?? Well OK unless designing UFH for an occasionally used wood frame shed or something).

 

So yeah would be interested in the source for the above.

 

Edited July 5, 2023 by joth

[joth]

As an aside, I've been pointing to this document for an easy to read guide on self-balancing system in a low energy homes. It has a guideline of 30W/m2 max output which is in line with my expectation for a ~10W/m2 heating load home, as the UFH is often less than 100% of floor covering (often, only one floor out of 2 or even 3 has UFH) and you also want some headroom to allow for reheat the property from cold, DHW duty cycle, future changes in usage/heat loss, and potential TOU shifting of heating demand.

It doesn't have much to say on pipe centres (pipe spacing) other than stating that a low temperature heat source like a heat pump will call for closer spacing to achieve the same W/m2 for a given deltaT

 

 

[JohnMo]

As you may have noticed from a lot of my replies I run 300mm centres, in fact in a 194m2 floor, I have less than 600m of pipe.  My floor U value is 0.09 and the pipe is within 100mm of concrete. It takes a long time for a temperature change in the house, but when running weather compensation that doesn't matter.

 

Generally my flow temp down to zero outside is less than 30. When cooling my flow temp is 12.

 

I run as a single zone, no actuators on UFH manifold, no pump or mixer. The engaged water volume is enough to not short cycle without a buffer.

 

100mm centres isn't the easiest thing to do, if your risk averse something like 150 to 200mm centres is easy to bend etc.

 

So to show how little pipe you can get away with follow my piping. One loop does, main bathroom, hall and kitchen and is 100m long, the living room has two loops, three bedrooms have short loops in each and ensuite another short loop.

 

 

[JohnMo]

The chart is one have seen in a few places on line, I think it originated from DIN standards.  The flow correlate well to loopcad. And experience on my floor is similar, although I did have to extend the W/m2 line down to 10.

 

300mm centres works well with time shifting as I was doing this in March April as an experiment. Flowing a modified WC curve running for 7 hours, kept house at a very stable temp until the next night.

 

Having tighter centres does give more wiggle room and quicker recovery times, but 100mm leave that to a houses that require much more heat per m2.

 

Although there are plenty of low energy (heating) houses on this forum, in general there isn't many in the UK as a whole. So you tend to have to find your own way, plumbers have no clue in general. No right wrong solutions as each house is different. But when you drop to about 10W/m2, the heating world is just different from the normal heat sieve.

[Post and beam]

2 hours ago, JohnMo said:

100mm centres isn't the easiest thing to do

 That is one of the things i have considered. If only because i expected to have to string the pipes myself. It appears the industry does not like to do it as its a lot of work.

I do not yet have full structural data from the TF supplier so do not have SAP calcs but the target U values are .15 for walls, .12 for the floor (150mm kingspan under)  and i think .12 or .11 for the roof. House will be 200 square metres with UFH at ground floor only.

It was my belief that to avoid short cycling i needed close pipe centres, as low a flow temp as possible, 24 hours/day on and one single zone. The second sentence in my initial post at the top of the page throws this into doubt.

[saveasteading]

12 minutes ago, Post and beam said:

the industry does not like to do it

From our experience " the industry" doesn't like laying pipes at any centres, and sends out their , let's say   most junior, employee out to do it....badly.

[JohnMo]

I have not seen an issue with short cycling, since I started operating as a single zone. For about 6kW heat input you need circa 40 to 50L of water engaged.

 

If you were operating lots of zones, close centres might be more important.