Not enough Heat

[Wagger]

We have recently had oil fired central heating installed.
All radiators have TRVs. We also have a system thermostat inthe hall, set at 25 degrees.
When the heating is first switched on all radiators becomehot or very hot but our rooms and the main thermostat reach only 19/21 degrees.After around 2 hours the radiators furthest from the boiler start to cool andwhile they warm up intermittently the rooms never get above 19/21. All TRVs areset at 5.
The system has been balanced and we are satisfied that thisis not a boiler problem. Our plumber is stumped. Any ideas?

 

[doitmyself]

They'll close even on 5 and that could well be before the room achieved temperature because they are measuring the air temp immediately adjacent to the rad, not the room temp, hence a need even with TRVs for proper balancing

Most TRVs have a proportional bandwidth of about 2C, so they are full open 1C below the required temperature and fully closed 1C above.

Not knowing what temperature 5 represents (it varies according to TRV make and model), it's impossible to say if it would interact with a room stat set to 25C.

 

[Best]

Room dimensions in metres 2.56 high x 5.0 x 4.2. 2 outside walls 2 double glazed windows total 2 sqare meteres. Radiator double panel with single convection 600 x 1200. Stat fitted in a suitable place.

I haven't worked out a rough output, but that is a fairly big room. I am sure I would have put a bigger rad in. Usually a double radiator for any room approaching that size, IMO.
Depends on what type of house and how good the insulation is. Also if a bedroom or a living room, as bedrooms require less heat output.

 

[Worcester]

21m2
Very old leaky house = 2.75kw
Brand new build < 840W

Average Flow temp 55° (condensing 60 F/50 R) looking for 30° delta T = 30° Radiator factor = 0.515

Therefore output from P+ 600*1200 = 1691 * 0.515 = 870 W

Therefore rad undersized unless new build.

 

[Worcester]

Problem Solved.

That's the difference, I guess your 'installer' isn't a heating engineer. They should have done full heat loss calcs first and sized the radiators (and boiler) accordingly.

Ask to see his calcs

 

[ShaunCorbs]

Problem Solved.

That's the difference, I guess your 'installer' isn't a heating engineer. They should have done full heat loss calcs first and sized the radiators (and boiler) accordingly.

Ask to see his calcs

+1 the only way I can get the existing rad working in a normal house is to have the room temp at 21 dc

 

[Nostrum]

You 'normally' design for at 20 degree F&R TD 70-50 so a MWT of 60 which gives a 40 degree air to water delta T.

I say normally, because getting a 20 degree delta is harder than it sounds when the boilers oversized or the systems small. That's generally the industry standard on a condensing boiler though.

Worcester- your 60/50 is more feasible but leads to a lower output which can cause problems on a retro fit (keeping existing rads)

We're designing systems with a 50/40 now on gas, but it doesn't go down well even though it'd be more economic. Despite the benefits being discussed, we lose out on most gas work because the customer would rather have smaller rads and a system chucked in for half the cost.

 

[JonG1]

What boilers are you speccing Nostrum, some manufacturers flinch at the thought of their HEX running with acid 24-7?

 

[Nostrum]

ATAG (stainless hex) although as said, not won any jobs to actually fit them :wazzzup:

 

[JonG1]

ATAGs should be fine, but not sure if they are staying in the UK market, something about a takeover by Ariston?

We have done a few where we have been able to spec oversized rads, weather comp the heating or opentherm it and prioritise the HW, mostly use Intergas these days but were looking at ATAG before we started with the Gash.