Heat loss and radiator sizing are necessary, but they’re only the opening chapter of a proper design.

A competent heat pump design should also include the following, but I’m not sure how many installers will do this all, let alone share their calcs. 

Hydraulic design. This is where most installs fall down. The designer should calculate required system flow rate at design conditions, based on the chosen delta T, typically 5K for a heat pump. From that, they should confirm the primary pipework can carry the required kW without excessive velocity or pressure loss. A heat pump sized correctly on paper will still underperform if the pipework throttles flow. Simply shrugging at 22 mm and 15 mm without calculating flow rate and velocity is not design… it’s assumption. This was something that was never factored in our own install. 

Pressure loss and pump capability. Once flow rate is known, the total circuit resistance should be estimated… emitters, valves, pipe lengths, bends, manifolds, etc. The circulation pump  must be capable of delivering the design flow at that head. Otherwise you’ll see flow errors, cycling or poor heat distribution.

Domestic hot water design. An undersized coil limits transfer rate and increases compressor stress. The coil should be matched to the heat pump’s output at realistic flow temperatures, not boiler-era assumptions. Some of the guys on the forums will probably disagree. Recovery time and legionella strategy should also be defined.

Controls philosophy. Weather compensation curve setup, room influence strategy, zoning approach and whether the system is open loop or thermostat-driven. Poor control logic can undermine an otherwise good hydraulic design.

System volume and minimum water content. The online litres per kW rule of thumb (15-20 L/kW) is conservative and largely aimed at ensuring stable operation and preventing short cycling in low-load conditions.

Defrost management and bypass strategy. How is flow maintained during defrost? Is there a buffer, volumiser, low-loss header or direct system? Each approach has consequences for efficiency and stability. If there’s talk of a buffer, run!

Finally, optimisation margin. A good design considers part-load behaviour, not just peak heat loss. Your house will operate most of the season at 30-60% of design load. Modulation range of the chosen unit matters as much as its peak output.

What you should be asking installers:

What flow rate are you designing for at design temperature?

What delta T are you assuming?

Have you calculated pipe velocities and pressure loss?

Can the internal pump deliver required flow at that head?

How are you ensuring minimum run times and preventing short cycling?

How will weather compensation be configured?

If those questions are met with numbers rather than reassurance, you’re dealing with a designer rather than a box-swapping installer IMO.

You don’t need over-engineering. You need quantified decisions rather than gut feel. That’s the difference between a bodge and a properly designed system.

Posted by: @petch

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The only other thing I have a basic understanding of is system volume, but again, every installer seemingly goes off gut feeling for this (or worse, just always do/don't install a buffer/volumiser). Even this topic seems to have wildly different opinions where you look, this forum I read you should have 15-20L of system volume per kW of output on the ASHP, but if I look at a manual for a 12kW ASHP it says a minimum of 16L and a recommendation of 64L, which is just over 5L per kW - significantly less than what you might read online.

@editor has said a lot but let me address this one head on and specifically explain the disparity.  Heat pump manufacturers typically specify a minimum system volume sufficient to ensure that defrost will work, ie there is enough heat in the system to defrost the fins.  Thats relatively small and is the absolute minimum.   However the other 'system volume' related consideration is cycling.  Heat pumps, just like boliers, will inevitably cycle at mild temperatures and its more efficient if the cycles are long rather than short.  Length of cycle is determined firstly by the minimum output of the heat pump relative to the demand (which is why oversizing is generally to be discouraged) and secondly by system volume.  The latter matters because the greater the system volume the more excess energy can be absorbed by the system before the flow temperature gets too high and the heat pump shuts down.  Basically it acts as a smoothing function.  Recommendations for larger system volume than minimum are targeted at this and yes its a good idea.  

I hope that helps.

A few other things:

• Dont accept a design with a buffer, plate heat exchanger or low loss header between the heat pump and radiators, it should be directly connected.  A volumiser in the flow only (or even in  the return only) is fine
• As regards hydraulic design, I hope you have 2 pairs of 22mm pipes (eg upstairs and downstairs), plus separate feeds to the DHW.  If there is only one pair then its unlikely to be sufficient and the hydraulic design needs a specific focus.
• It would be good if you could narrow down that loss uncertainty.  Is there any useful evidence available from gas /oil consumption?  11.5kW is big unless your house is well over 200sqm, as the scattergram below of measured consumption vs floor area from the 250 or so systems on heatpumpmonitor shows

Hope that helps.  As @editor says asking the installers questions is a good approach.  Dont get too overwhelmed though, some things are not knowable without taking up the floorboards, and in these cases it may be best for the installer make a reasoned risk assessment that what they cant know is unlikely to be a problem, but if it is turns out that it is then there is a backup plan.  Thats probably true with the pipework for example,  2x22mm primaries plus 15mm to the rads is very likely to be enough given the loss estimate, unless the primaries immediately downsize to 15mm before any rads split off.  Without taking up the floorboards you cant be certain what they do.  So a reasonable approach, at least in my view, is to measure the flow rate early on in the install, check its sufficient, but be prepared to deal with it if it isnt.

Feel free to ask us more questions and if you have some specific options to put them to the forum.

Posted by: @jamespa

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Posted by: @petch

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• As regards hydraulic design, I hope you have 2 pairs of 22mm pipes (eg upstairs and downstairs), plus separate feeds to the DHW.  If there is only one pair then it’s unlikely to be sufficient and the hydraulic design needs a specific focus.

Hope that helps.  As @editor says asking the installers questions is a good approach.  Dont get too overwhelmed though, some things are not knowable without taking up the floorboards, and in these cases it may be best for the installer make a reasoned risk assessment that what they cant know is unlikely to be a problem, but if it is turns out that it is then there is a backup plan.  Thats probably true with the pipework for example,  2x22mm primaries plus 15mm to the rads is very likely to be enough given the loss estimate, unless the primaries immediately downsize to 15mm before any rads split off.  Without taking up the floorboards you cant be certain what they do.  So a reasonable approach, at least in my view, is to measure the flow rate early on in the install, check its sufficient, but be prepared to deal with it if it isnt.

Feel free to ask us more questions and if you have some specific options to put them to the forum.

I totally agree with the recommendations from Mars and James. Perhaps our example might help.

We had quotes for mostly an ashp of above 12kW, because our heat loss was just above 10kW according to calculations. Using our annual heating bill we knew we needed ~8kW, (see other threads for the equation). The main difference was the loss due to air leakage and we knew ours was low since we had a man to measure it. The installer we chose then altered his heat loss calculation to take the measured heat loss due to air leakage. 

On pipework we knew our layout had a single 22mm pair feeding both upstairs and downstairs and DHW, because we had the heating system installed on refurbishing 12years before. 22m will not carry 8kW without being at least noisy and since it goes through our ensuite we didn’t want that. (It possibly isn’t possible to carry that power, it’s a gradual limitation). So we knew we needed about a third of the primary pipework upgrading to ~28mm. This was done in advance with the radiator upgrades.

On volume we went down the rabbit hole of the equipment minimum too but eventually stopped resisting the addition of a volumiser, since it was a detail really and having negotiated every other parameter it seemed reasonable to accept the guy’s experience.

I accept we were probably a demanding customer but you are the one paying and have to live with it. It’s very sad that the state of the heating industry means the customer can need to get involved at this level of detail.

I also totally agree with the recommendations from Mars and James 🤣 

All installers who came here wanted a big buffer tank. When I attempted to explain there was no need for a buffer tank, lots of whining ensued, there was simply no way to convince them that 20 tons of underfloor heating slabs should be plenty enough heat storage. So I fired them and installed the HP myself. Right now it is producing 10kW at COP above 5, flow temperature is 28°C, return 24°C, flow 1.8-2m3/h. It starts at 9AM, runs for about 90 minutes, then sleeps until the next day.

Posted by: @jamespa

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• Dont accept a design with a buffer, plate heat exchanger or low loss header between the heat pump and radiators, it should be directly connected.  A volumiser in the flow only (or even in  the return only) is fine

One exception: I have non barrier PEX underfloor heating in some rooms, so the plate heat exchanger is mandatory for these rooms, which significantly complicates things. It wastes about 2°C flow temp and a few decimals of COP, and it requires careful balancing of flow on both sides. It's not optimum but there's no other option. Flow/return temp in these slabs is actually 26/22°C.

All great advice 👍 

Going back to radiators and pipework sizing, once you have a heat loss calculation, broken down room by room, you can start to plan radiators and pipework.

Draw out a branch diagram of the pipework and radiators in each room. You should then be able to visualise how much heat needs to flow down each piece of pipe. Heat Geeks have a handy table of pipe sizing for heat flow. If using copper pipes, you can get 2.75kW down a 15mm pipe, 6kW down a 22mm pipe, and 10kW down a 28mm pipe at sensible (non-noisy) flow rates. For most homes, this normally means 22mm branching off into 15mm for individual radiators. For example, my own home has a heat loss of around 7kW. The system has 28mm pipework from the heat pump into the house (10kW limit) and then immediately branches into two 22mm runs serving each end of the house which require around 4kW and 3kW respectively which each 22mm branch being good for 6kW max. We then drop down to 15mm feeding each individual radiator, so as long as no single radiator exceeds 2.75kW I know we are good to go. 

Next up is radiator sizing. Hopefully your preferred radiator manufacturer will provide output data for their radiators, commonly at dT 50C (meaning 70C flow temp and 20C room temp). This can be corrected by applying a factor for lower flow temps to give the radiator output at your desired flow temperature.

There are two approaches here. You could start with your desired flow temperature (e.g, 50C at -3C) and then size each radiator accordingly based on the heat loss for the room. Or you could look at each room, see the maximum size radiator it can reasonably accommodate and then work back to the flow temp required. We took the latter approach, and because we maxed out the size of our radiators are able to run at a flow temp of 32C which is great for efficiency. Our rooms are not perfectly balanced in terms of radiator sizing as we just put in the largest K2 radiator that would fit, but this doesn't matter too much as heat naturally flows to adjoining rooms. If you are replacing radiators anyway, then it makes sense to just install the largest that will reasonably fit.

All of this is something anyone can do with a simple spreadsheet once you have the heat loss results, broken down room by room.

We have a volumiser, and I'm glad we do. The manufacturers recommended minimum volume for our system is 50L. As others have said, this is an absolute minimum and not a suitable design target. Our actual system volume is ~95L in radiators, ~45L in pipes plus a 50L volumiser (~190L total).

Posted by: @bobflux

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All installers who came here wanted a big buffer tank. When I attempted to explain there was no need for a buffer tank, lots of whining ensued

@petch 

Same with me, I had to insist and taking a leap of faith, undertake the responsibility for not installing a buffer (though I would not even dream of installing the heat pump myself like @bobflux ….). 

Our house is 240sqm, with a large thermal mass, radiator+pipes volume guesstimate is 250lt, heat pump is 16kw, heat loss is 500w per 1 degree/13.5kw @ design point -7,  we operated it 10 to 12 hours daily during February using weather compensation, it did not cycle at all, and we have never been warmer. Located in Greece, in an area with circa 1500 HDD need annually. ASHP newbies, we installed it end of January 2026.

The site below has an on line calculator for radiator volume (beware the “panel” option assumes a height of  600mm, so you need to adjust accordingly the result base on the actual height of your radiators).

https://calculator.academy/radiator-water-capacity-calculator/

If you do opt for connecting the heat pump directly to the radiator network, make sure the controller of the pump is installed in a room that can be used as a checkpoint/control point like the living room (and not in the basement close to the ASHP)

Posted by: @mk4

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If you do opt for connecting the heat pump directly to the radiator network, make sure the controller of the pump is installed in a room that can be used as a checkpoint/control point like the living room (and not in the basement close to the ASHP)

Do that anyway, the main system controller that you use day to day should in almost all cases be the heat pumps own controller and almost never third party controls.

That said, if its set up properly you will hardly ever need to touch it, but it needs to know the house temperature so needs to be somewhere representative.

Thanks everyone!

My current system boiler and pipework has 22mm flow pipe that splits into a T after the valve, with one side going to downstairs and another to upstairs. Same for the return. 

The house is around 180m² and probably somewhere around 10kW heat loss if we average out the heat loss surveys, with the split being 55/45 between upstairs and down due to a ground floor extension with a LOT of windows

So assuming the 22mm pipe doesn't just end after a few feet, would that be sufficient for carrying 5-6kW heat? @old_scientist I can't find the heat geek table you're referring to I'm afraid. Maybe the initial foot or so of pipe before it splits into 2 22mm pipes needs to be upped to 28mm? That's not a big job and something I can push an installer into doing. They might want to re-do most of the existing pipework that's inside the plant room regardless 

@petch Yes, if the flow and return pipes immediately split into 22mm runs for upstairs and downstairs, then each branch should theoretically be able to transfer 6kW of heat, so if your heat loss is 10kW, and that is split 5.5kW downstairs and 4.5kW upstairs, 22mm pipe sizing should be sufficient, dropping down to 15mm for each radiator.

A copy of the table I referred to can be found in this excellent article:

https://energy-stats.uk/what-size-heat-pump/

@petch all good news I think and suggests that a straightforward installation should be possible.  Just make sure you consider follow the advice above and you should be OK.  The only thing from what you have said that would concern me is 

Posted by: @petch

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The house is around 180m² and probably somewhere around 10kW heat loss if we average out the heat loss surveys

This positions your house as a serious outlier on the chart below from heatpumpmonitor, which depicts measured, not surveyed, loss vs floor area.  The consequences, if it turns out that that the loss is in fact lower, can be managed, and certainly (high) system volume - referring to the discussion above - is part of managing it.  Thus suggestions of a sizeable volumiser (not a buffer) should probably be taken seriously unless you can somehow narrow the loss down.  

@petch just wondering how your averaged heat loss survey data compares to actual usage data. Is your existing boiler gas, and if so do you have access to half hour data (or daily data) for gas usage from a smart meter?

Maybe you could look back to the coldest day this winter to see what the actual usage was, as a sense check for how that 10kW heat loss figure stacks up. Estimating the efficiency of the old boiler may be a bit of a guess, depending on it's age and whether it was being run in condensing mode or not, could be anywhere from 60-90% efficient. But it's one more data point to satisfy yourself.

We overestimated our heat loss (mainly due to overly high ACH figure used in older MCS calculations). Besides our heat pump being oversized, it has mainly resulted in allowing lower flow temps which of course is no bad thing.