Plumbing advice for extra radiators

My heat pump is too big for my house most of the time.

I want to heat an extra room, an insulated timber office of about 20m2

The office is on the back of our garage and the garage is adjoined to the house.

I have access to flow and return pipes from radiators in the house and want to spur from those to create a loop in the office.

The extra pipe run to the new radiators would be about 8m of pipe length including rises and falls.

I am concerned about getting water that far when it is already at the furthest point from the heat pump, a Daikin 9kW Altherma 3 monobloc

Just adding a second circulation pump seems like a bad idea so I have plan to ensure flow to the office radiators.

My plan is to add a 4 pipe buffer tank in the garage fed from the radiator in that corner of the house and then add a circulation pump on the office radiator circuit fed by the buffer tank.

Would this work?

It’s quite simple to do and I think it would get heat to the office without disrupting the house heating circuits or the heat pump.

1 Like

Did you get an answer on this @matt-drummer?

Flow is about pipe diameter and length (and elbows and pump head and balancing), so short the answer is “it depends”.

Buffers generally best avoided if another way is possible.
The good news is that it’s the index circuit that defines the loss in the system, where the index is the line through the network of greatest resistance (taking into account heat being delivered on the way). Or to put it another way, everything in parallel to the index circuit is not part of the pressure loss calculation, as the pump only needs to overcome the parallel circuit of highest resistance. In balancing, the resistance of the other circuits is increased to match that of the index by closing off flow with the balancing (lockshield) valves.

If buffer then pump from the buffer best DT controlled, but a DT pump for 1 radiator is expensive, so manual setup of the pump to attempt to replicate the DT that feeds the buffer.

Hope that helps

1 Like

Hi Phil,

Thanks for the reply.

I haven’t done it yet but I will over the summer.

I knocked a large radiator over onto my leg when planning the job so I have left it until the summer, made quite a mess of my leg, really surprised it wasn’t broken, it stung a bit!

On to my additional radiator.

At the time I just wanted to lose excess heat from my over sized heat pump and wasn’t too concerned about how well heated the office was.

But I think I have come up with a plan to do it properly.

A buffer wasn’t necessary but I do have a low loss header that I am going to use in conjunction with a pwm controlled Grundfoss circulation pump.

As long as the circulation speed is close to the rest of the system the additional radiator will get its fair share of the heat but it doesn’t matter too much.

I now have a smaller heat pump with a smaller circulation pump so the extra pump to feed the office is a good idea.

As far as I see it, the llh will just act like an extra radiator and the size of that radiator will depend on how fast I run the extra pump, if I want to extract more heat from the main system I just run the pump faster.

This heat pump has a monster circulation pump (25-125W Grundfos from memory). So as long as the pipework allows sufficient flow, you could be OK simply adding extra rads. Heat Geek has some good rules of thumb for the heat that can be supplied through various sizes of pipes (bigger is generally better, of course).
Looking to boost your COP? :wink:

Hi Simon,

I no longer have a 9kW Daikin with a monster circulation pump.

This topic was started in December last year, I have moved on a bit.

I have an unheated office attached to my house and the idea was to lose some of the excess heat from the heat pump to this space.

I wasn’t looking to boost anything, I was looking for a solution to an grossly oversized heat pump, I did that by getting it swapped for a more suitable heat pump.

OK, my mistake- didn’t notice the date.

1 Like

It’s OK, perfectly understandable.

I am still going to do it as using the heat pump is better than how I currently heat it if I want to use it.

Makes sense. I’d be careful with maintaining system balance. The LLH could act as a low resistance section of the network as it’s essentially a big vessel. Maybe fit a balancing flow valve as a precaution on the feed to it so you can keep the balance the network with all the other radiators on it. Think of it like a lock shield on a regular radiator.
This sort of thing: Quicksetter™, Balancing valve with flow meter | Caleffi S.p.a.

I’d still see what could be done to avoid this as regards increasing pipe sizes to avoid the complexity of the LLH and pump. You’ve probably already done this though…

Hope the leg got better!

1 Like

Hi Phil,

Thanks for the advice.

I have gate valves to fit that will act like a lockshield on a radiator.

It is probably over complicated but I have it all so will fit as intended.

My smaller heat pump is still a bit of an issue.

My radiators are so big that even at any sensibly low flow temperature I still get more heat than I need at times.

The Daikin heat pumps I have had don’t like really low flow temperatures and just get inefficient.

I am going to plumb the radiator in the office so I can isolate it if necessary but I think it will be fine.

Off topic now, but I’m keen to know more about Daikins at low flow temperature. My underfloor heating is pipe spaced at 50-60 mm throughout my house. I did this with a future heat pump in mind, even though we’re on gas right now.
The thermostat in the upstairs part of the house hasn’t called for heat in weeks, as the separate apartment downstairs and the downstairs bathroom (open zone) seem to be keeping the place warm. I’m in the alps and it snowed the other day, so I’m pretty impressed with what I’ve built!

We had a boiler fault for a few weeks and 2x3kW immersions in the heat store kept the 2 residences in hot water and heat during winter a few years ago!

However as a Daikin installer, hearing that you’ve had trouble with stable and efficient operation at low flow temperatures is a concern, given the building and UFH performance I’ve achieved so far! All ears!

When I say low, I mean sub 30c

My problems with the 9kW are well documented.

The issue with that heat pump is the minimum electrical input is quite high at around 900w.

I changed all my radiators after my install and increased the overall capacity by just over 250%

My heat pumps both did/do really well when it is cold.

When it is warmer, say around 10c I have more trouble.

At any set dT my radiators give out over 2kW of heat even at 27c flow temperature, they are just that big.

I am not complaining though, I win big time when it is cold.

But running lower than 30 c gives me no benefit, I see the best efficiency at around 31c/32c.

I am waiting for Daikin to come and sort out the firmware so that I have the full range of overshoot available in radiators and fan coils, at the moment it is only there in ufh.

What I have found works best is setting the emitters to radiators.

In the other settings the heat pump seems to focus on the return temperature and adjusts the flow rate and flow temperature to maintain this, but it can’t really work out what to do, the flow rate is erratic and rarely settles, it is always overshooting what it needs to be.

I think it is most efficient with the flow rate on the floor in radiator setting and then it is more stable.

It is a little difficult for me to explain without writing an essay!

You can look at my heat pump on the list, it is the Octopus installation in Ipswich.

I had a 9kW until 20 February and now have the 8kW.

Nothing much to complain about but I think it could work even better.


With this being my controllers chart, and my setting being 0.3 (ie below the lowest drawn line of 0.4) then I’m sub 30 most of the time.

Warm day today at 8 degrees:

Another brand of heat pump perhaps?!

Hi Phil,

Being able to compare heat pump performance is good and bad.

I don’t think many people would be disappointed with the performance of my heat pump, it’s pretty good.

I think some others are better, particularly when it is warmer.

The Daikin is pretty good in the cold and works really well in my house.

I am nit picking now, I wish I could get it more stable.

I am fairly confident it is not my house or my heating system, my 9kW was very stable.

I see that all of the 8kW family of heat pumps are not particularly stable in terms of flow temperature and heat output.

I guess it is the design/software.

Maybe I look at it like the gearing in a car, there is a point where they are most fuel efficient and it is not necessarily at the lowest speed.

I think my heat pump is happiest at about 2,500w of heat and 31c flow, when I say happiest I mean most efficient.

Interesting comparing your unit to itself. Blue being the pre-20th feb, Yellow being after.

It’s a shame the manufacturers don’t do a re-mapping type of service like with cars.

I had a look through all the Daikins on the list and didn’t quickly see any outstanding examples of one running smoothly at sub 30 flow temperatures. Whitchurch seemed to run below 30, but @1nc said it seems to struggle with cycling above 11 degrees outside.


Nice, I’ve been looking at just this on spreadsheets and now have extra rads ready to fit.
I go to the conclusion that the emitter area needs to be infinite to be able to emit enough power to match the minimum heat pump power output as the ambient temperature rises; with the double whammy that the minimum output rises with increasing COP at the milder temperatures!

Hi Matt,
Arriving a bit late to the discussion here, so trying to catch up!
Had a look at your performance stats on the list.

What’s the problem when you try to run your HP at flow temp below 30C? Is it cycling? Or does it give lower COP because of another reason?
I saw it’s running at 31C roughly in the past 24h, essentially heating your home from 21C to almost 23C. Is that on purpose or just a result of having to run the HP at 31C flow temp?


Hi Henri,

Thanks for your questions, I will try to answer as best I can.

My indoor temperature sensor is not in the ideal place, it is in the warmest room of the house and is not representative of the whole house.

It’s where it is because it didn’t work in my hallway, no other reason.

So my house isn’t really 23c.

I can run the heat pump at 26c/27c as long as I have 4c overshoot or manage the start up a bit.

But I find no point, it doesn’t really use any less electricity and the heat pump isn’t that happy.

I think we need to go back to basics, the heat comes from the dT and the flow rate, the actual flow temperature is irrelevant in respect of the heat produced.

Running lower flow temperatures should result in lower energy consumption and it does to a point but if going lower uses no less electricity then there’s no real point.

It doesn’t matter if I run at 10lpm and 27c/24c or 10lpm and 31c/28c, the heat produced is the same.

I get very little less heat running at 27c than I do at 31c, the radiators give up all the heat they can.

My radiators are quite large so i get a fair bit of heat whatever I do, and that is because the radiators can cope.

So really it is about finding a flow temperature that works best.

What I see happen is that I just run at 31c, if the house is colder I get more heat as the flow rate increases to satisfy what the radiators can deliver, as it gets warmer the radiators can output less so the heat goes down.

I have picked a flow temperature that gives me the best COP and it also gives a house temperature that is fairly stable and that I am happy with.

The heat pump is the same as all the other 8kW Daikin’s so at warmer temperatures if my heat pump needs 500w to produce 2,500w of heat then so will all the others. The COP’s should be fairly similar once the benefit of my large radiators is removed.

Where I benefit is when it is much colder, I don’t need to increase my flow temperature much, if anything, as my radiators just give out more heat as the heat loss increases.

So my flow temperature will never change much, in fact, as it gets colder I can reduce it because the compressor is most happy at a certain load.

I can easily get all the heat I need sub 5c outside with a flow temperature of 28c to 30c. The radiators work better as it gets colder and so I don’t need such a `high’ flow temperature to put the heat pump under load.

What I need is a reverse wdc where the flow temperature reduces as the outside temperature falls.

For these reasons, I don’t use the Madoka or a wdc, I just run fixed lwt and turn the heat pump on and off as necessary and adjust the flow temperature to suit.

My biggest problem at the moment is I have to run as ufh to get the overshoot of 4c, it is not available in fan coils or radiators. I don’t like either fan coils or ufh with the fixed dT. I want to run as radiators, it has a fixed dT of 10c that I will never get but I end up with the lowest flow rate and I find that works best.

I could try ufh with a higher dT but I haven’t yet, I don’t know what the differences are exactly between the three modes but ideally I want to run as radiators. Hopefully Daikin will fix this soon. Then I think I could run lower in a more stable fahion.

I hope it makes sense but the key is here that flow temperature doesn’t control heat produced, dT and flow rate does.

On the topic, in theory my extra radiator should help.

It is going somewhere with a much higher heat loss and should output around 600w at the flow temperatures we are looking at when it is 10c or more outside. This means that I should be able to reduce the flow temperature, still get 2,500w or so overall to keep that heat pump happy but have 600w or so somewhere else and less than 2,000w going in the house.

The heat pump should be happy at that sort of load, my office will be heated for free and I’ll have a bit less heat in the house. Maybe the COP will improve a bit in the warmer weather compared to now. It should be all good news.

1 Like

Thanks Matt for the elaborate answer!