Posted by: @patrick

↑

What has me a little confused is their recommendation of a 30L volumiser. We'll be running UFH from JK throughout the ~61m2 ground floor. Some quick ChatGPT assisted maths says this will be at least 33L in the UFH loops, before the manifold, other pipework, and 4 rads upstairs are taken into account. So easily above the minimum 32.5L system volume from the datasheet.

So are Grant just being cautious here? Would it be a bad idea to omit the volumiser? Conversely, are there any downsides to including it?

Minimum system volumes are normally enough to cover defrost, which is critical.  However a larger system volume may improve performance when the heat pump is cycling and a volumiser, which will give you that, wont do any harm.  It will also reduce the water temperature reduction during defrost which is no bad thing, particularly as you are in Ireland which like the UK (or Great Britain if you are in N Ireland) is humid.  Make sure it is a 2 port volumiser though not a low loss header or buffer.  It should ideally be plumbed in the flow, but in the return will do.

If you are combining rads and UFH be sure to design them to work at the same flow temperature.  You dont want to be mixing down for the UFH as it simply reduces efficiency.  Make sure the HP native system controller is sited somewhere sensible (eg in an accessible, representative room, not in a cupboard).  Far too many installers hide away the system controller and rely on too many external controls to the detriment of efficiency and frequently also comfort.  Many people find that operating on pure weather compensation with no room influence is the way to go, best stability and best efficiency.  Alternatively weather compensation with TRVs/Thermostat in a minority rooms that benefit from high solar gain, to serve as limiters only not controllers 

Feel free to ask questions about the proposed design and control strategy. 

Bold move, and welcome to the forums @Patrick.

I’ve heard good things about Grant’s R290 units and they do seem to be performing well across a growing number of installs, so that part of your decision feels well grounded.

On the volumiser question, this really comes down to total system water volume and how conservative manufacturers choose to be. I have no idea where Grant stand, but too little volume and you risk short cycling and instability... too much volume and you’re simply paying to heat water you don’t actually need, which increases run-up time and costs. There is definitely a sweet spot.

As a rough rule of thumb, many top installers aim for somewhere in the region of 15-20 litres of system volume per kW of heat pump output, and in practice that tends to deliver very good results. So for a 6.5-7kW unit, you’re looking at a minimum of around 100-105 litres, with some flexibility either side depending on controls, emitter mix and flow rates.

Underfloor heating systems are often assumed to provide plenty of volume, but in reality many UFH-only systems fall short. While your estimate of 33 litres in the UFH loops sounds reasonable, that’s only part of the picture. You then need to account for the rest of the system: typically 4-6 litres per standard panel radiator (more if they’re column rads), around 5-10 litres within the heat pump itself, plus all the flow and return pipework between emitters, manifolds and the unit. When you add all of that up, many mixed UFH and radiator systems still end up borderline without realising it.

This is why manufacturers often specify a small volumiser. It’s not necessarily because your design is wrong, but because they’re trying to protect the unit across a wide range of real-world installations, including poorly balanced or low-volume systems. In your case, Grant may well be taking a cautious, one-size-fits-most approach rather than responding to a fully detailed hydraulic design.

Including a volumiser can improve hydraulic stability, help smooth defrosts and reduce cycling, particularly in shoulder seasons. But if your calculated system volume genuinely lands comfortably within that 15-20 litres per kW range, and the system is well balanced with sensible flow rates, omitting it isn’t automatically a bad decision. The downside of adding unnecessary volume is simply efficiency and responsiveness, coupled with increased running costs.

You’re asking the right questions, and honestly, this level of thinking puts you well ahead of most installs we see. Keep sharing your plans and doubts here... that’s exactly how you turn an anxious install into a really solid one. 

Thanks for the great insights guys!

I finally got around to sizing the rads, which meant I was also able to roughly estimate the system volume. I'm getting about ~85L, so a 30L volumiser does indeed bring us into that 15-20L/kW sweet spot. It's reassuring now that I at least have numbers that back up the inclusion of it.

I have designed everything at 40C, so there'll be no need for mixing. Interestingly, Grant actually specified a mixing pump set if we were to use their UFH system. Thankfully the JK system that we'll use has no mixing on their low temp manifold.

The plan is to run on pure weather compensation with no zones or stats. We'll keep the TRVs on the upstairs rads since they're there anyway, but we'll keep them fully open or only as limiters.

In the shoulder seasons we do get a bit of solar gain in the mornings and evenings, but the house faces east/west so it's uneven. The front is in the sun in the morning while the back is in shade, and vice versa. In practice, will this be an issue with an open loop system? I suppose some of this you just won't know until it's installed and you start to tweak things, but just want to see if there's anything that needs to be taken into account at this stage.

@patrick you won’t fully know until you have it in place. The simple solution to uneven front back heating is leave doors open (our option). The more sophisticated is room stats (set above your normal level, to an overheat level) which will close down one of your ufh loops. BUT that will reduce your system volume and ufh has long response times which might mean the sun has gone when the slab locally cools and the room over-cools. It depends on which room how uncomfortable the sun over-heating is, in March last year the sun warmed our south facing rooms, (including the lounge when the control is) and the ashp went off, but the colder hall on the north is warmed by the open lounge door. The other way round wouldn’t be as cosy.

If you can move your sensor control box around you’ll get what’s best for you, so a radio control box is better than a hard wired version. Never leave the box (or a room stat) where it is actually in the sun.

Posted by: @judith

↑

The simple solution to uneven front back heating is leave doors open (our option).

Closing blinds/curtains is another option, standard practice in sunny countries.

Our 12kW Samsung heat pump install would have a system volume without volumiser of ~140L and our installers still included a 50L volumiser bringing total system volume to ~190L, and putting us in that 15-20L/kW range. For reference, the Samsung recommended minimum system volume was 50L, so we are 4 times the minimum recommended value and still at the lower end of that 15-20L/kW range.

@patrick I'm also in Ireland (Kilkenny) and seriously considering Grant for parts only supply. Like your good self I have done most of the design and the heat loss, volumes, flow etc and intend to send that on to Grant for their view under their free design service.
I did speak to a rep a while back and he said they honour the actual HP unit warranty in full even for self install, if done according to their spec. 
I will go on their training if it is free!
Cheers

PS not enough interaction here to be able to send direct message though

@max-abbey so interesting to hear you’re another homeowner doing the design and planning a self install. Really pleased to hear Grant will honour the full warranty. I think the Grant training is free, but please keep us posted and maybe start a new thread dedicated to your DIY install which is of huge interest to me (and many other homeowners out there too I’m sure).

@editor I will do that, it's probably going to be a long process overall as part of a major retrofit, 90's bungalow and mostly DIY so we will be juggling time, quality and cost like everyone has to!

@max-abbey, absolutely, but it’ll also be so interesting to see how much cheaper it’ll be than the standard-ish going rate for heat pumps install.

@max-abbey hey! That's super cool, so nice to hear from other like minded people in Ireland! Do keep us posted, will be very interested to hear how you get on.

Would love to go on the grant course myself as well if they'd have me. Although I chose to leave the installation to a plumber, just because we've got other work going on and I wouldn't trust myself to be done fast enough to get out of the way of the builders.

Great news this week too. The heat pump grant here has increased from €6500 to €12500.