That sound like a complete mess.  Are they the original radiators or did they replace them with newer high output ones?

People who know more will be along shortly; we have a Samsung and it works okay but I know what you mean about poor controls that aren't easily customisable.

@guthrie Thanks, edited the original post with that info

Not sure where to start with this so I will start with basics.  Apologies if you already know this.

Heat pumps work best at the lowest possible flow temp, which generally means leaving them on 24x7 with a modest setback ( say 3-5C) overnight.  They also work best with weather compensation, ie the flow temp reduces in warmer weather, a feature which is common on boilers in many more enlightened mainland European countries, but sadly not in the backward 

Almost invariably the valves are organised to heat either dhw or space but not both at the same time, because usually they are at different flow temps.  Most often this is done with a diverter valve but of course an arrangement of 2 port valves works also.

Thermostats and trvs should not generally be used to control room temp, this should be done by getting the flow temp right and balancing rads correctly.  However it's now becoming more common to run downstairs open loop (ie no thermostat or trvs, or if they exist set high) but put trvs upstairs.  The logic behind this can be explained on request.  The heat pump controller should be the primary control system NOT any trvs or thermostats.

Heat pumps are generally of a lower capacity than boilers, more closely matched to the actual demand.  So where you might have fitted a 28kW boiler, which is way more than the actual demand, you will likely fit a 12 or even 8kW heat pump.  Obviously this means there is less reserve capacity for instant heat, but because you run them for longer it doesn't matter.

That's some of the basics.  Ask some more questions/provide more info and I and others will doubtless provide more focussed comments (but understanding the basics is the essential background).  Has anyone got a heat loss survey for the house and which model is the heat pump?  A system diagram would also be helpful.

Assuming that the capacity is adequate heat pumps are perfectly capable of heating leaky properties,  but you have to abandon the idea of rapid heat up, in favour of more gentle control.  Depending on the occupancy pattern this may cost a bit more, or may not, and will likely be much more comfortable.  

PS - for transparency - I'm neither a heat pump engineer nor do I have any connection with the heating industry.  I just have a degree in physics, a background in engineering (and subsequently local government),  good DiY skills and have spent 2.5 years studying the technology in order to facilitate the installation in my own house. 

My background means I like to understand how things work and why I am being told 'how things 'should' be done.  If someone cant explain to me why what they are telling me should be the case, I don't see any reason to trust what they are saying. 

Sadly I don't yet even have a heat pump (except in my car, fridge and freezer) because my local planning authority (run by the Green Party!) imposes unachievable and wholly unnecessary noise requirements.  However I run my gas boiler at the lowest flow temperature of which it is capable and throttled down to 8.5kW (the minimum of which it is capable) to prove to myself and others that I will be able to heat my house with a much smaller capacity heat pump than most (fortunately not all) MCS qualified installers claim, without robust justification, that I need.  As it happens this has reduced the cost of running the gas boiler whilst materially improving comfort, because there are fewer thermal gradients and the boiler now condenses whereas previously it didn't.

Its your choice whether to believe or not anything I say!

Basically unless they are only 10 years old, I have great trouble believing that the original radiators are able to put out enough energy from the cooler water flowing through them from the heat pump. 

Is it running from just the Samsung controls?  You can get into the menu and statistics fairly easily, various guides are out there on youtube etc, see what sort of energy use it has going on as well. 

As James says, a diagram would be helpful for everyone, it sounds like the pipework etc is a mess and not helping things. 

Posted by: @guthrie

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Basically unless they are only 10 years old, I have great trouble believing that the original radiators are able to put out enough energy from the cooler water flowing through them from the heat pump. 

Maybe.  Radiators can 'start off' well oversized, particularly if the fabric has been improved (although that appears not to be the case here).  We need to see the calcs and design FT!

Scientifically, yes, but I've been in a lot of older houses over the years and seen all sorts of older radiators and rarely anything from the 21st century; often driven by oil or wood, putting out a lot of heat but also in the case of the older ones, having a large metal mass to heat up in the first place before they can heat the rooms.  If I have to caveat everything I write, I'll be here all day.

Posted by: @guthrie

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having a large metal mass to heat up in the first place before they can heat the rooms.

Nothing wrong with a bit of thermal mass unless you want to keep varying the indoor temperature, which is not generally how you aim to run a heat pump.  Some thermal mass in the system is, after all, necessary for defrost, which is why volumisers are sometimes added if the permanently engaged system volume is insufficient.

Apparently somebody did the rad calculations and so far we haven't heard that the house is especially cold so at the present time no evidence that the radiators are undersized.  I grant that they may well be, but until we know they are we shouldn't condemn them (yet).

Actually reading the original post again it's not clear what the symptoms are that we are trying to fix beyond the ones he/she has already fixed, other than 'it goes wrong a lot'.  Perhaps @seagrave could clarify this so we collectively know what problem we are trying to fix!

Heck, we never got any heat calculations for our intallation; if anything it is a bit oversized.  I'm not condemning the radiators, I and am pointing to them as a possible issue. 

You are right, we do need a bit more information about what precisely is wrong.

Thanks for the replies, and looking back it is a bit vague on what the problems are.

I'll sit down tonight and try to do a schematic and also a run through as to what my concerns are.