Whilst you are all fixated upon pipe diameters, you should also consider the valves at either end of the radiators, which may have an internal area that is smaller than the pipe diameter and will therefore be what limits the flowrate.

@robl I appeciate the maths here, but I feel that with all the uncertainties stacking with a real life measurement, it might come out worse than the annual usage calculation, and definitely worse than a heat loss survey. Also, my problem with running at a lower temp is that the radiators in some rooms are really very small. I have two rooms with radiators that would put out less than a mere 140W whereas they need about 400W!

As you say, the house temperature got to about 22C, depending on the room, and has so far not fallen to 20C in most places. It really is feeling very snug in here.

@derek-m

Hi Derek, thanks for the post.

I thought of that, hence why I thought if the radiator that needs to provide the most heat has 15mm pipes and a fully open valve, then the rooms with the next largest heat losses (coincidentally about half) which would have 10mm pipes can also be mostly open, then other rooms can be adjusted as required. Do you think this makes sense in terms of the likely flows and pressures? Of course distance from the pumps and what comes before and after in the circuit makes a difference, but as long as the radiators with the largest need have plenty of head room, could we not get everything else the required flow even with a bit of valve closing?

@oswiu

The point I was trying to make is that you can have the largest pipes possible feeding your radiator, but the maximum flow rate into the radiator will be limited by the internal area of the valves when they are fully open.

The room with the greatest heat loss may not necessarily be the coldest, it is a balance between the actual heat loss and the actual heating capacity of the radiator, along with the size of valves and pipework.

I'm not an expert on balancing radiators, but probably the best way to start would be to fully open all the valves (including any TRV's) on all the radiators and let the system stabilise for a period of time. Measure the temperature of each radiator and each room, then gradually close the valve at the opposite end of the radiator from any TRV, in the warmest room. Then work you way around the system adjusting the valve on the radiator in the next warmest room. Continue the process until all the rooms, including the coldest one are at the desired temperature, with the valves in the coldest room left fully open.

I just got back from a lovely warm holiday in a different part of the world to a broken boiler! I turned the heating up a day before we flew back home so that we didn't get back to a 12C house, but when I checked if it had worked when we landed back in Blighty, I saw the temperature hadn't increased. I hurriedly turned it off and on again thinking the command just didn't send the previous time, but it was still cold when we got home. Thankfully we got someone to look at it the next morning, but to our dismay they said that they would recommend replacing it rather than fixing it, as fixing it would be costly and might not work since the whole thing was flooded and apart from what they knew to be broken, they couldn't tell if anything else was. Bummer!

So it was out the window with going through proposals with a fine tooth comb, and in with ringing everyone we'd ever spoken to about ASHPs to see when they could soonest install one. Not the best approach by any means... The answers were February, January/February, "after the new year", and "2-4 weeks if you're lucky, but more likely after Christmas". Of course we're pretty chilly in the house at this point having been in 30C temps the day before, so my wife says if we can do it in a month then we can get a heat pump, otherwise let's just get a new boiler. I don't think that's unreasonable given the circumstances.

Amazingly, half an hour after telling that last company that we would go with them if they could do it in a month, two guys rocked up at our house to do a short design survey (they said they do full heat loss calcs and measurements after we sign the contract, but to be frank after we've done a few of them now, I didn't really mind). I said I needed a few hours to think about it, then called them again to check when exactly they could fit us in, and they said they would move some stuff around and would start next week on the install (!!), but we might have to wait for all the rads changing etc. At least we'd have some heat coming into the house. They also said that if we wanted to get a different plumber to do it, that'd be fine.

So there we have it, the first company I contacted about a heat pump ends up being the one we went with, and it's a Daikin system. I am uncomfortable about how quickly it all happened (if you exclude the fact that I've read their contract like 5 times in the last couple of months), but not having any heating system apart from a small wood-burning stove in one corner of our house didn't leave much choice. I guess we'll find out if they actually do move on it in the next week or not. I am keeping my shivering fingers very much crossed. One thing I did note and appreciate is that these guys can really talk about heat pumps till the cows come home.

Until then we've been out to buy two oil filled radiators to heat a couple of choice rooms during the day, and we've put a jumper on the dog.

I would be keen to hear anything that others would want to know before having a system installed at this short notice, as whilst we've paid a deposit and waived the cooling off period, we've not paid the full price.

Sounds like the universe gave you a nudge!

im sure it’ll be fine. 

@oswiu

Not sure where you are with this, but go for a manufacturer where the controller can operate either a heat pump or a boiler. My preference is Vaillant.

Essentially a heat pump is just a heat generator, just like a boiler. 

@alec-morrow 

Why would you particularly seek out a controller that can control both? I don't foresee running both at once, and I am hoping that now my boiler has packed in, I'll never use one again

im not suggesting you have a hybrid system, although that may work for you. It’s the controls that deliver a decent heating system not the boiler or heat pump. 

if you got a vaillant boiler with their weather compensation controller  now, you could up grade that to a heat pump in due course.. just by swapping the boiler for the hydraulic module and running pipes to the out door unit.

A sort of plan to apply modular solutions to common problems.

I didn’t realise you are committed to the daikin.

Make sure the installers are installing daikins own controller and are not proposing to use on off or existing thermostats

also postponing radiator changing is good news..you may not need to get the, changed at all..

but note pump controller must go in habitable space and there must be no other thermostats 

@alec-morrow 

Yes the boiler is completely shot, so no point doing a step-by-step change at this point. The installers have confirmed that they'll be installing Daikin's weather compensation controller with a wired thermometer on the exterior north side of the house. Not only have they confirmed that verbally, it's even listed in the contract. That's a good thought on where they put the indoor thermostat, as I remember that issue happening to @editor , so I'll confirm with them on that.

I agree that it could be useful to delay changing some of the radiators, but I am a little concerned as to how that will affect the Boiler Upgrade Scheme application. The installers have assured me that there won't be any issues with it, but there is a stipulation that the kit can't be used until it's "commissioned", and I don't know if that means that it has to all be installed per MCS requirements, i.e. that the heat loss calculations determine that it will heat the whole house, which at the moment I doubt. I'm hoping they can "commission" the system, then we can finish off the radiators when they make the BUS application and no one would be any the wiser. Either way, I want an efficient low temperature system anyway, and my radiators as they are will not provide that.

Weather compensation control changes the way heating works. you probably don’t need to change them but if the newer ones are bigger you will use marginally less energy…as you can do a bit more heating  at even lower temperatures.

(broadly speaking it takes x watts to maintain  a space at a determined temperature, and facilitating more heat in a shorter time with bigger rads only marginally changes that dynamic ) 

i can’t comment on MCS, but if the controls are in the right place I’m confident you will end up with a great system and have a good experience.

I’m a heating installer  by the way…and have been for 20 yrs…all in install is heat pump controls on gas boilers!