The megaflows must have a huge share of the market, given how much cheaper the alternatives are.

Has anyone used them and can comment?

Joules
Gledhill
Etc?

Having read their website I cannot for the life of me fathom what is is gravel!

''Having read their website I cannot for the life of me fathom what is is gravel!''

Gravel? Had a quick scan of Sunamp site/residential and can find no ref.

I'm also interested - I'm trying to avoid a combi boiler - the Gledhill units seem to be good value and comprehensive but I'm put off a little because the heat loss seems high (45 watts plus - but at least they publish unlike others) also because of the external heat exchanger there are electrics and pumps that can go wrong. But on the other hand the tank is a simple as can be and all the bits can be repaired/replaced.

All I want is a simple store with a finned/substantial coil for DHW (or two coils), an immersion heater (for if/when I have solar PV and boiler failure) and, ports for the boiler and heating.

I'm avoiding combi boilers because they seem overly complex and don't seem to fit with UFH.

I have to admit it's a bit of a prejudice I have - possibly unwarranted - I wish to run my UFH near continuously at a relatively low temperature to keep the house temperature stable - but I suspect a combi will not modulate down to say 1 or 2 kw so it will have to regularly fire up for a few minutes at a time - (although little heat will be wasted here as the boiler is within the building envelope).

It just feels wrong to size a combi for the peak energy flow you need occasionally and yet run it at the bottom end of its range. I prefer the idea of running a smaller, simple system boiler, having storage for several minutes of shower use and letting the boiler catch up over the next 20 mins or so.

However I suspect that the combi boilers loose less than 45w on average so perhaps this is the way to go. I'll just have to get used to the combi firing up every few minutes when the weather is cold.

Posted By: goodevansI suspect a combi will not modulate down to say 1 or 2 kw so it will have to regularly fire up for a few minutes at a time
[...]
I suspect that the combi boilers loose less than 45w on average so perhaps this is the way to go. I'll just have to get used to the combi firing up every few minutes when the weather is cold.

I have a 10 year old 24 kW Vaillant combi (because that's what was in the house before I took it apart and put it back together again), it will modulate down to 6 kW (where it runs at it's max efficiency of "98%" or so).

Because my UFH zones have quite high thermal mass (all in concrete or high-conductivity screed), the boiler will fire for about an hour when all the zones are on and the heating hasn't been on for a few hours (it's set to cut off at a flow temperature of 44 C - which is the lowest temperature the settings allow).

I think Viessmann do a high efficiency combi about the same size which will modulate down to 1 or 2 kW.

A sub-£1000 air source heat pump, with a heat output of about 3 kW would be nice...

The other thing is to heavily insulate the pipes close to the tank. I have double insulated so there is about 60mm wall thickness of insulation.