Pipe sizing to supply rads for 30 degree flow temp ASHP?

500pages

We're mid-ASHP install. Initially the installers told me that the SCOP improves with higher flow temps and that they'd spec'd rads for 55 degrees. I knew this was not right so it prompted me to do a bunch of research.

I've now reached a bit of a dead end with my self-directed learning on this stuff and could do with a second opinion on the pipe sizing specifically for the supply to the rads.

We've now clarified that we want a much lower flow temp (30 degrees) and we're happy with very big rads in some rooms (we're having ufh on ground floor and rads upstairs). To be honest, the rads are not insanely big IMO, but yes of course they're bigger than bog standard ones.

We're now on the same page as the installers about this, but I remain concerned about a few things. Looking for a bit of guidance from more experienced ASHP folks on one particular item:

Pipe diameters: they've indicated the legacy 15mm copper pipes will be sufficient for the flow and return for the rads but I'd like some actual calculation to back this up - the sense I get from them is that they go on 'gut feel'.

I've assumed that the lower the flow temperature, and the greater the surface area of the rads, the more heated water you need to push through to supply the system. But the more hot (or in this case, lukewarm) water you push through a smaller pipe the greater the pressure.. so I guess what I am looking for is:

a) a way to calculate the litres per second required to supply the rads based on a low flow temp; and 
b) something that tells me that the AHSP (an Ecodan) will be able to supply sufficient 'head of water' to pump that volume of litres per second around the system

To clarify, the feed from the external ASHP unit to the internal unit is a massive pipe, as spec'd by the manufacturer (I think 28mm). What I am concerned about is from the internal unit to the rads. 

To take an example, we've got two bedrooms on FF fed on a 15mm copper pipe that is forked from a 22mm plastic poly pipe that is coming direct from the ASHP.

The total length of the polypipe is about 5m. The copper pipe is then about 9m - a straight run with only elbows at the rads themselves, and this needs to supply two rads of 6.3kw and 2.2kw respectively.

Those figures are the adjusted figures (i.e. the original power figures for these two rads are 240w and 800w).

Given the wattages and pipe distances in play here, is 15mm copper sufficient? 

Thanks in advance

FreeBear

500pages said: The total length of the polypipe is about 5m. The copper pipe is then about 9m - a straight run with only elbows at the rads themselves, and this needs to supply two rads of 6.3kw and 2.2kw respectively.

Those figures are the adjusted figures (i.e. the original power figures for these two rads are 240w and 800w).

Sorry, but those numbers do not add up. Where are you getting 6.3kW from for a radiator (and more importantly at what flow temp) ?

If you are pumping water around at 30°C to radiators that will emit 240+800W (1040W), 15mm will be plenty big enough. 15mm will quite happily deliver ~2.75kW@30°C. To advise further, we would need a plan of the pipework, radiator sizing (and at what Delta T), and the design flow temperature.

NedS

Take a look at this page from Heat Geek, including the useful 'cheat sheet' table for pipe sizing:

https://www.heatgeek.com/does-my-pipework-need-upgrading-for-a-heat-pump-with-cheat-sheet/

As heat pumps work at low dT, best to use the first column for dT of 5C (between flow and return). This tells us you can transmit 10kW of heat down a 28mm pipe, so this is fine for the primary pipework between the external and internal units.

You should draw out a branch type diagram from the source, branching out to all of the individual radiators, and write the heat output of each radiator. Then add up how much heat will need to be transferred down each branch to get to the radiators.

My system has two 22mm branches coming out of the internal unit, each capable of carrying ~6kW of heat. Each radiator then branches off in 15mm pipe and is capable of 2.75kW from the HeatGeek table. So the maximum output of any individual radiator would be 2.75kW as limited by the 15mm pipework feeding it, and the whole branch is limited to 6kW by the 22mm pipework.

Our two biggest radiators (K2 1800x700 and 2000x600) each output just over 1kW at design temp, so well within the capacity of the 15mm pipework feeding them, and the total heat loss of 7.5kW, split roughly evenly between the two branches is well within the 6kW capacity of each of the two 22mm primary branches. If we had used microbore pipework to feed the radiators, those very large radiators would probably struggle to get up to temperature (the plastic microbore pipework we ripped out and replaced only had a measured 6mm internal diameter).

As a rule of thumb, where using plastic piping, the recommendation is to go up one size as the internal diameter is less than the copper equivalent, and any plastic pipe inserts cause turbulence, reducing the flow rate and amount of heat that can be transferred.

Once you have determined the pipework is capable of transmitting the required amount of heat, you can start to look at flow rates and head pressures.

 

500pages

Hi @FreeBear, sorry my 6.3kW was wrong - it was a typo of 5.3kW.

But I suspect that doesn't change your assertion that the figures "don't add up"!

I'm not sure which figures you're referring to.. but I have, as you suggested, sketched the locations and pipe sizing on this google drawing and included the design temps and wattages for each radiator based on the calculations performed by the installers - flow temp 30degrees. We're on the south coast, in case that is relevant to external temps.

https://docs.google.com/drawings/d/1G9Az4CMFc7qLgEMIGKIXVwaqHsY05WFjIfSjTkMDwK0/edit?usp=sharing 

It's not as clear as the one on the Heat Geek page - I can redo if needed to make clearer but hopefully it makes sense: 

500pages

hi @NedS, thanks for your post - super helpful.

I've reviewed the heat geek sizing page - I've read similar before but it's a better and simpler summary. HOWever, I remain confused about the numbers. The sizing I gave above - the example of, say, a 5.5kw rad - that is based on a coefficient of x/0.1234 being applied to the usual wattage given at DT50. So for example, if the plumbers were to installing a regular rad to this room with a gas boiler operating at DT50, it would need a rad that could output 659w let's say. But to get a rad sized to work with DT10, I've been told to apply that coefficient, producing a total power output required of about 5.3kW for the rad in this room.

But the heat loss for the whole house is <10kW, so that doesn't add up (perhaps what @FreeBear was referring to).

So my question re your comment and the heat geek page is whether the figures relate to the adjusted or non-adjusted wattages. IYSWIM.

Reed_Richards

Were you not given a room-by-room breakdown of heat loss?  I thought that was part of the MCS standard package.

500pages

Reed_Richards said:

Were you not given a room-by-room breakdown of heat loss?  I thought that was part of the MCS standard package.

Yes, I have referred to that above - the figures given in Watts are the heat loss calcs from the supplier

FreeBear

500pages said:

hi @NedS, thanks for your post - super helpful.

I've reviewed the heat geek sizing page - I've read similar before but it's a better and simpler summary. HOWever, I remain confused about the numbers. The sizing I gave above - the example of, say, a 5.5kw rad - that is based on a coefficient of x/0.1234 being applied to the usual wattage given at DT50. So for example, if the plumbers were to installing a regular rad to this room with a gas boiler operating at DT50, it would need a rad that could output 659w let's say. But to get a rad sized to work with DT10, I've been told to apply that coefficient, producing a total power output required of about 5.3kW for the rad in this room.

But the heat loss for the whole house is <10kW, so that doesn't add up (perhaps what @FreeBear was referring to).

So my question re your comment and the heat geek page is whether the figures relate to the adjusted or non-adjusted wattages. IYSWIM.

Thank you. Knowing what the ∆T is for the rated outputs you were giving helps. So your 6.2+2.13kW@∆T50 works out as ~1kW@∆T10 - Well within the capacity of 15mm copper even with a DT5.

My only concern would be the 22mm polypipe where it goes from the DHW tank to the point where it splits to feed upstairs & downstairs. Being plastic, the internal bore will around 19mm compared to ~21mm for 22mm copper. This will reduce the amount of water that can be pumped around - Although, this can be offset by increasing the pump speed a little. If possible, I'd get that short run replaced with 28mm copper.

Reed_Richards

500pages said:

Reed_Richards said:

Were you not given a room-by-room breakdown of heat loss?  I thought that was part of the MCS standard package.

Yes, I have referred to that above - the figures given in Watts are the heat loss calcs from the supplier

Where exactly are these figures in Watts?  If they're in that linked document, the text is too small to read.

michaels

I went down a similar rabbit hole, as we have 15mm upstairs loop following on from 22mm downstairs.  I calculated that based on the pipe length the upstairs section could deliver enough heat (3.5kw) at a sensible flow but only with a 7c temp drop and the the 22mm downstairs section could also deliver the whole house demand of 8kw (the 4.5kw downstairs plus the 3.5kw upstairs).  However that was strictly on pipe length and ignored any corners/fittings giving extra flow resistance.  This was based on a flow temp of 43 at -5 design temp.

Probably to do it properly and get a lower flow temp and lower flow-return dT I would need to split the upstairs and downstairs into separate loops and hack into the upstairs somewhere (through the ceiling) - disruptive but not nearly as much as trying to replace the pipework throughout the upstairs!

NedS

500pages said:

hi @NedS, thanks for your post - super helpful.

I've reviewed the heat geek sizing page - I've read similar before but it's a better and simpler summary. HOWever, I remain confused about the numbers. The sizing I gave above - the example of, say, a 5.5kw rad - that is based on a coefficient of x/0.1234 being applied to the usual wattage given at DT50. So for example, if the plumbers were to installing a regular rad to this room with a gas boiler operating at DT50, it would need a rad that could output 659w let's say. But to get a rad sized to work with DT10, I've been told to apply that coefficient, producing a total power output required of about 5.3kW for the rad in this room.

But the heat loss for the whole house is <10kW, so that doesn't add up (perhaps what @FreeBear was referring to).

So my question re your comment and the heat geek page is whether the figures relate to the adjusted or non-adjusted wattages. IYSWIM.

All radiator manufacturers will quote the rated output of their radiator at a given dT. This is the delta between the average water temp in the radiator and the room temp. They normally quote for dT of 50C, so this may correspond to a boiler flow temp of 70C and a room temp of 20C (hence dT of 50C). 

A standard 1600x600mm K2 radiator may have a rated output of ~2.75kW at dT50 (check your manufacturers website, or if not available I use Stelrad as my source as they provide data for many difference sizes and types of radiator and the data should be broadly similar across manufacturers).

You would now apply a correction coefficient to de-rate the output for the reduced flow temp.

dT30 will give an output of 0.51, so 0.51*2.75kW = 1.4kW at flow temp of 50C

dT20 will give an output of 0.31, so 0.31*2.75kW = 0.85kW at a flow temp of 40C

dT10 will give an output of 0.12, so 0.12*2.75kW = 0.33kW at a flow temp of 30C

As you can see, the radiators will dissipate much less heat as the flow temperatures drop. This is the amount of heat your pipework needs to transfer to the radiators. So you need to work out how much heat the radiator needs to output when it's -2C outside in order to heat the room to the desired temp, and then ensure the pipework is sufficiently large enough to cope on the coldest of days. You will most likely not be running flow temps of 30C when it's -2C outside unless your radiators are absolutely HUGE (or you have underfloor heating or an amazingly well insulated house), you are much more likely to need flow temps of 40-50C under the coldest of conditions, but that will ultimately depend on the size of your radiators and the heat loss of the room in question.