Controlling temperature of water in radiators.

[Doctor Drivel]

Matt, you know you know nothing about this sort of thing.

[Anode]

Thermostat is Potterton PET 1.

[Graham.]

That's OK.

My comments referred to the old fashioned bi-metal strip type.

Graham.

%Profound_observation%

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[Andy Hall]

On Mon, 3 Jul 2006 20:13:18 +0100, Doctor Drivel wrote (in article ):

That might have more credibility if you really knew what it meant.

[Doctor Drivel]

Matt, I know you are not very good at this sort of thing, but I said "Simple, easy, efficient."

[The Natural Philosopher]

FIT TRV's

[Dave Plowman (News)]

Surely in most cases it's the temperature of the room that matters?

Are you forced to be that close to the rads? I've got one so close to this computer I can touch it from the chair, but I'm not aware of heat actually radiating from it - and certainly not hot air which rises.

That's not to say a modulating boiler isn't a good idea - I shall get one when it comes time to change mine. But this will be for efficiency reasons rather than comfort.

[Aidan]

With burner off, the valve gradually opens as the store cools. With burner on, the valve gradually closes as store heats up. Flow temperature stays at set-point.

No. Hall has room temperature sensor, which has a function in varying heating flow temperature . Outside also has an influence.

Store, boiler, pumps, controls, mixing valve.

[Doctor Drivel]

Richard Cranium, doe the warden allow you have a hot radiator on in this weather?

[Dave Plowman (News)]

Is doe the warden a relation of bob the builder? I think we should be told.

[Anode]

It could be just a personal thing, but radiators above a certain temperature I find unpleasant and best avoided; it seems to be something to do with breathing in the hot air from around them so I was trying to avoid this in future alterations. I understand that comfort level is affected by many factors, including relative humidity and radiant heat emission from walls and objects in the room, so personal comfort can no doubt be a complex concept.

Anode.

[Andy Hall]

On Tue, 4 Jul 2006 00:08:06 +0100, Doctor Drivel wrote (in article ):

What are you rabbiting on about now...?

[Andy Hall]

On Tue, 4 Jul 2006 01:01:43 +0100, Anode wrote (in article ):

If that's your only issue rather than having a more or less constant temperature, you could resolve it quite easily.

For conventional boilers, radiators are run at 82 degrees flow and 70 return. Radiators are sized for the room heat loss requirement to work with this.

If you install a condensing boiler, it *can* work at these temperatures - they do to avoid mandating radiator changes. However, it will operate more effiiciently at lower temperatures, so it is more typical to design for 70 flow and 50 return. Heat output is then about 30% less so the radiators have to be larger. There is no reason why you couldn't extend this principle further, use even larger radiators and lower temperatures.

[Anode]

Thank you Drivel for your suggestion.

Anode.

[Aidan]

Yes, ditto. I find continuously warm rads much preferable to intermittently scorching rads.

The radiant heat losses to/from the surfaces in the room have a significant effect on one's feeling of comfort. You can usually drop the air temperature by a couple of degrees in a room with UFH because the radiant component of the heat emission is much higher.

Whilst this is true, the radiators are sized for the worst-case, mid-winter, design conditions, -5 degC or suchlike outside, which probably occurs for only 2% or 5% of the time that the heating is on.

For the other 95-98% of the time, partial load conditions, the radiators could operate at less than the 'classic design' 82 flow 71 return, whilst still putting out sufficient heat to offset the heat losses. Similarly, if the system can vary the flow temperature, they can operate continuously at lower temperatures, rather than intermittent bursts of 82/71.

I was surprised by the amount of time my modified modulating system operates at less than 100%; probably >90% of the time and I didn't change any radiators. You will achieve worthwhile savings with a condensing modulating boilers even without changing radiators.

In the extreme, it heads towards underfloor heating, where the floors are huge radiators operating at

[Doctor Drivel]

I wonder if the warden lets them watch Bob on DVD?

[Doctor Drivel]

Only if it has the correct modulating control system. One that modulates on load compensation rather than maintaining a flow temperature set point. These boilers are not cheap.

A heat bank/thermal store using TRVs on all rads and an auto modulating pump (Alpha) on the CH circuit , will slowly inject high temp water into the rads when the TRVs throttle back. This high temp water then slowly mixes with the cooler water in the rads. So you may have 70C water in the rad flow pipe, but the rads are very much cooler. If the CH circuit demands 0.25kW of heat, as a few TRV are just cracked open, it just takes it from the store as everything, the CH pump and TRVs are modulating.

A condensing boiler mated to a heat bank/thermal store will be very efficient. Using a store set to 70-75C and a blending valve on the boiler flow/return to the store set to give a 22C to 25C drop of setpoint (say

50C), will be condensing the vast majority of the time. The valve also maintains stratification.

So, a simple, less complex, more reliable, condensing boiler can be used.

[Andy Hall]

On Tue, 4 Jul 2006 14:03:55 +0100, Doctor Drivel wrote (in article ):

In terms of total cost of ownership and the required results over the operating life of the system, this is not significant.

In practice this won't happen because very low flow rates will result in very little mixing of the water. The result would be that the top of the radiator will be at full flow temperature and the bottom at return temperature. It may be that the return is as low as 40 degrees but the top will certainly be at 70 or whatever the flow rate is.

I tested this as an experiment some time ago by feeding a radiator normally specified for 4kW output using a length of 8mm pipe. The effect was exactly as described, so it is not what is wanted.

This is pointless. The boiler will not be operating at maximum efficiency because efficiency does not suddenly improve at the temperature (52-54 degree return). All that happens is that the *rate* of increase of efficiency with reducing temperature will increase.

Therefore the notion of dinking around with blending valves to get the boiler just into condensing mode and hence achieving some nirvana is false.

Added to this, if the store is maintained only at 70 degrees, it has little advantage in terms of capacity compared with simple storage of the DHW.

In short, this idea will not work in terms of providing low radiator temperatures, it won't maximise boiler efficiency and it won't do a good job with the DHW either.

That's also bogus since the difference in cost of fully compensated quality boilers is not because of the complexity (a temperature sensor is added and the firmware in the controller does the work). The main reasons for higher capital cost are in build quality, smaller production volume and the why does a dog lick its balls syndrome.

[Dave Plowman (News)]

Perhaps you'd lend him your set? After you've learnt everything you can from them, obviously.

[Doctor Drivel]

He has Mat Monroe LPs you know.