A system boiler has the pump and other parts integral to the boiler. ( a system in a white box )
A heat only boiler has pump, valves etc external.. hence easy to get at for repair / replacement.
Combi or system boiler for new build with UFH?
- Thread starter Ruggers
- Start date
A system boiler has the pump and other parts integral to the boiler. ( a system in a white box )
A heat only boiler has pump, valves etc external.. hence easy to get at for repair / replacement.
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Always get system & heat only confused 
Thanks old but not dead for the detailed reply.
1.I thought with UFH the idea was to have room stats with motorised valves on the manifold so each room had its own temperatures & were more efficient, the same as TRV's on radiators, otherwise all UFH rooms would share the temperature of the room with the downstairs stat in, ie: hallway.
2.The build hasn't started yet so no water meter, only a couple of houses being built, some are completed so I could ask the flow & pressures. The meter will be 28 to 35m from the connection in the road if it's where i think it is, not sure how big the outlet will be, needs to be bigger than 25mm from what you suggest.
3.Unvented i take it, is just the traditional tank system with 2 header tanks gravity feeding the supply and radiators but with a newer modern boiler?
4.Do all system boilers have this rapid cylinder recovery. I was expecting a tank to take up to an hour to heat back from empty so that's quite a while with the heating off if it takes priority to heating the tank over C/H?
5.Yeah muggles explained this but said 50 deg would be a closer match to the lower UFH temperatures. I thought i was going to have to take a flow and return temp, not realising the 20 degrees were avg room temps you minus from the flow temperatures.
1.I thought with UFH the idea was to have room stats with motorised valves on the manifold so each room had its own temperatures & were more efficient, the same as TRV's on radiators, otherwise all UFH rooms would share the temperature of the room with the downstairs stat in, ie: hallway.
2.The build hasn't started yet so no water meter, only a couple of houses being built, some are completed so I could ask the flow & pressures. The meter will be 28 to 35m from the connection in the road if it's where i think it is, not sure how big the outlet will be, needs to be bigger than 25mm from what you suggest.
3.Unvented i take it, is just the traditional tank system with 2 header tanks gravity feeding the supply and radiators but with a newer modern boiler?
4.Do all system boilers have this rapid cylinder recovery. I was expecting a tank to take up to an hour to heat back from empty so that's quite a while with the heating off if it takes priority to heating the tank over C/H?
5.Yeah muggles explained this but said 50 deg would be a closer match to the lower UFH temperatures. I thought i was going to have to take a flow and return temp, not realising the 20 degrees were avg room temps you minus from the flow temperatures.
So a system boiler is similar to a combi with the heating pump and other components all within one white box.
Heat only boiler is like the traditional boiler from 30 years ago with external 3 port valve, pump and tank using header tanks?
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1 Radiators can get a room up to 21 degrees or so from ambient quite quickly. UFH is a low and slow approach- you are warming a lot of mass with a fairly low W/sq metre so rapid on/off control with motorised valves etc won't work all that well. You can go that road if you want to but it'll add significantly to system cost and complexity.
2 Well worth knocking on doors and asking, bear in mind that a new estate with a couple of houses completed will probably have excellent flow/pressure from a main specced for the full estate. As more demand is placed on the main the available flow and pressure will drop.
3 Unvented hot water has no header tank, the cylinder is at mains pressure. If you have good (20 litres/minute + at 2 bar +) then your shower experience will be excellent. If mains supply is at minimum (10 litres/minute at 1 bar) it'll be disappointing. Unvented (sealed) heating has no header tank either.
4 No they don't. This is where your UFH may score highly with its low and slow approach- the mass of floor being heated will take a long time to cool off. With your modern well-insulated house you won't notice the heating dropping out when hw priority chimes in, a 300 litre cylinder will take 25 kwh to heat from stone cold (15 degrees) to piping hot (65 degrees). You won't be dong that very often.
5 Return temp of the actual pipework at the boiler is the key to getting anywhere near the efficiencies quoted by boiler manufacturers. Yes you want to be measuring temperature drop across each radiator- get a non-contact thermometer, they're cheap & easy for the job.
2 Well worth knocking on doors and asking, bear in mind that a new estate with a couple of houses completed will probably have excellent flow/pressure from a main specced for the full estate. As more demand is placed on the main the available flow and pressure will drop.
3 Unvented hot water has no header tank, the cylinder is at mains pressure. If you have good (20 litres/minute + at 2 bar +) then your shower experience will be excellent. If mains supply is at minimum (10 litres/minute at 1 bar) it'll be disappointing. Unvented (sealed) heating has no header tank either.
4 No they don't. This is where your UFH may score highly with its low and slow approach- the mass of floor being heated will take a long time to cool off. With your modern well-insulated house you won't notice the heating dropping out when hw priority chimes in, a 300 litre cylinder will take 25 kwh to heat from stone cold (15 degrees) to piping hot (65 degrees). You won't be dong that very often.
5 Return temp of the actual pipework at the boiler is the key to getting anywhere near the efficiencies quoted by boiler manufacturers. Yes you want to be measuring temperature drop across each radiator- get a non-contact thermometer, they're cheap & easy for the job.
A modern heat-only boiler can sometimes have a "header tank" (feed and expansion) but more often it will be sealed and pressurised.
it can, but need not, have a cold water cistern feeding a vented hot water cylinder. More likely it will have an unvented cylinder, which (if water flow is adequate) can give unsurpassed hot water.
edit
too slow again.
instead it has an expansion vessel.
Not really
A combi boiler is like a system boiler but with extra components for heating hot water.
When mains water is flowing through the combi to hot tap the boiler fire up and a divertor system diverts hot CH water from the radiators and instead through a second heat exchanger that heats the mains water flowing to the hot tap(s).
Some combi manufacturers avoid the use of the divertor system by having the mains water flow through a second water way in the main heat exchanger,
I think this is what my baxi platinum does, because if the radiators are on at 75 and i call for DHW which is set at 45 degrees, the first half litre to a litre is very hot probably due to the heat exchanger temp being so hot still.
1. Individual room stats downstairs was just a thought, having some individual room stats might be a waste of time with MVHR because it's distributing the air around the home anyway and might provide an average temperature. Because a stove will be fit in the living room, without an individual stat in there, the UFH will keep warming the living room while the stoves on.
2. Only a couple can be built due to room so hopefully it's decent and it will stay this way.
3. I was supposed to type vented for this not unvented, meaning are the unvented tanks just the same as the older system with header tanks.
4. I'm thinking a 10 minute shower at 20 L/min would use a fair amount of hot water thats why i was wondering how long it would take to reheat. I'll look into the rapid heat ones. 65 sounds hot for tap water?
65 is hot for tap water but it kills Legionella and other beasties so it is recommended as circulation temperature. New builds and refurbs should be fitting mixing valves in bathrooms to prevent scalding.
Think we've done the vented/unvented bit already
Think we've done the vented/unvented bit already
A good bit of 40 odd degree water never done anyone any harm from when i was a kid few drinks from a hose pipe on a hot day and your immune to everything for life.
Thanks for the tips, So one of these mixers for every sink and bath to be fitted?
They look a fair size to try and conceal somewhere behind each sink, like for a bathroom with half pedestal.
few drinks from a hose pipe on a hot day and your immune to everything for life.Thanks for the tips, So one of these mixers for every sink and bath to be fitted?
They look a fair size to try and conceal somewhere behind each sink, like for a bathroom with half pedestal.
In a compact bathroom you might get away with 1 serving bath and basin but essentially yes, check the Approved Docs.
Another system to ponder in your new build (if its going to be large) is circulating hot water. This reduces time lag from tap on to hot delivered (you have a return leg from the farthest away tap and a circulating pump). Doesn't work with combis, has some running costs (heat loss even from insulated pipes) but is another option to consider.
Another system to ponder in your new build (if its going to be large) is circulating hot water. This reduces time lag from tap on to hot delivered (you have a return leg from the farthest away tap and a circulating pump). Doesn't work with combis, has some running costs (heat loss even from insulated pipes) but is another option to consider.
Another thread has started up about when TMVs are needed:
https://www.diynot.com/diy/threads/requirement-for-thermostatic-mixing-valves-tmvs.551603/
I've two options for locating the meter and the tank location is quite central to the house for everything except the kitchen tap which is unfortunately the most commonly used. in a straight line it will be 6.5m from the tank, but by the time it comes up from the tank into the roof space along and drops down the wall it will be around 12m.
Is there a regs book I can buy to read up on a lot of this like the 18th edition electrical regs. They used to be about £20, were very handy when so much changes.
Is there a regs book I can buy to read up on a lot of this like the 18th edition electrical regs. They used to be about £20, were very handy when so much changes.
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