Further tuning CH

[Kostas Kavoussanakis]

Hi,

I have received some help (from Ed and others) tuning my CH in the recent past. The set-up is: a Potterton Netaheat conventional boiler (its stat set to 3); a Honeywell CM907 programmable thermostat at the hall (driving the CH pump); TRVs everywhere but the hall rad; and a hot-water tank with an external thermostat attached to it (set to 70; doubt I am getting that).

The coldest room is my lounge. I struggle to get it to 19.5 in this weather and the heating has been on for over 12 hours (though the room stat has cut in hours ago). According to

I need ~10K BTUs. According to I have approx 13,200 BTUs available from my two, similar, 25-yo rads in the lounge. The TRVs are full on, as are the other valves. When the heating is on, touching the top of the rads is possible but painful.

The target temp is 21 degrees (for the room stat and for the lounge). The hall radiator is tuned well down and the room stat took two+ hours to get there and cut out this morning. While building up heat, I noticed that the boiler was cutting out, but the pump kept going. Is this detrimental to the pump/boiler?

The options I see (bypassing for now the investigation of window insulation; this room has three open sides and 10 m2 of windows) are:

- Increase the boiler stat

- Increase the pump rate (not sure if this will heat the lounge faster?

- Increase the room stat setting and let the TRVs sort out the rooms already well heated. I am concerned about the effect of this on the pump, given the throttled hall radiator.

How would one start?

Thanks,

Kostas

[Roger Mills]

Not the most sophisticated of calculators! If you list the parameters you put in, I'll try a different one. Yours doesn't let you specify the room temperature, or the outside temperature, and gives no idea what K-values it's using for different types of walls, windows, etc.

I have approx 13,200 BTUs available from my two, similar, 25-yo rads

No, that's quite normal. It simply means that your radiators can't dissipate the full output from the boiler, so the burner cycles on and off such that the average (on/off) output then balances what the rads can absorb. If the boiler ran continuously under these circumstances, it would overheat.

What is the current flow temperature? If it's less than 80 degC, you can tweek it up a bit.

What is the current temperature differential across the lounge rads. If it's a lot more than 10 degC, increasing the flow rate would decrease the differential, thus increasing the mean rad temperature and consequently increasing the rad output - and would thus be beneficial.

You may need to do that anyway because if the room stat cuts off before the lounge is hot, it ain't going to get any hotter until it cuts in again. How about moving the room stat to the lounge? The pump shouldn't mind having its flow restricted. It is, after all, a centrifugal pump rather than a positive displacement device, so it's probably even happy to run stalled.

Has any attempt been made to balance your system? If not, it would be well worth doing. Assuming that all the rads are appropriately sized for the rooms they occupy, the system should then heat up more evenly - and the room stat shouldn't cut out until the whole house is warm.

[Doctor Drivel]

If you have sized up the rads and they are man enough then make sure the boilers is on "full". That is what threads are sized at 80C flow temp. If still too cold. The pump needs to be on a higher speed, or the pipes are undersized to the offending rads - the heat is not getting to them.

[Kostas Kavoussanakis]

Ok: H242, L640, W390. Wood DG windows (10m2). 3 sides to the elements. Soil under wood (I guess) under; pitched insulated (50mm) roof above. Insulated brick cavity. 21 Deg target temp. Which currently has been achieved, as the sun is out.

Thanks, good to know.

No idea. All I know is the boiler is set to 3, the Max being 5.

No idea again.

No reason to move the stat (far too difficult and intrusive, plus I doubt there is a good place in the lounge for it). The stat has a displacement setting that I could play with.

Guessing from the facts that the (now almost switched off) hall rad was at full throttle; the other (switched off from very early on in these investigations of mine)) hall rad was on (albeit with a TRV); and that 60% of the TRVs were placed at the wrong side of the rads, all bets are off in this house :-( The only "balancing" was done by me, as I said in the previous sentence, plus a check that the lounge rads were fully on on the TRV and other valve sides.

Thanks for your input Roger.

Kostas

[Ed Sirett]

It strikes me that currently we are at or near the intended design (if there ever was any) of most heating systems. Right now any system that was 'correctly' designed and has little or no extra radiator capacity will need the maximum flow temperature that the boiler can knock out.

It's so easy to put the boiler on '5' and see what effect it has. If other areas of the house are getting too warm too fast then they need throttling back (the TRVs will do this for you).

[Roger Mills]

OK, I'll feed them into one or other of my programs when I get time, and see what they come up with.

The flow temp is unlikely to be anything like 80 on setting 3 - set it to 5!

If you want to sort out your heating, you'll have to measure these things! I know it's a bit late now, but try to get Father Chrismas to bring you an infra-red thermometer (about 30 quid) which makes these measurements very easy to carry out.

You could certainly use the offset, but it's only really the same as cranking up the set-point.

Sounds like balancing should be on your urgent job list for the New Year, then! The IR thermometer is virtually essential for that job.

[Roger Mills]

When I feed that into my Myson Heatloss Manager program (kindly sent to me by Andy Hall) it gives a figure of 2534 watts (8633 BTU/Hr) needed just to balance the heat losses, based on an outside temp of -3degC and 1.5 air changes per hour, and assuming no temperature gradient across the internal wall - so 10k isn't far out, allowing a bit of bunce for transients.

Mind you, if you increase the air changes to 3, you then need 3284 watts (11205 BTU/Hr) just to balance the losses - so if your windows are very draughty, that could explain a lot!

Also, as stated earlier, your radiator will only produce its rated output if you run it at the stated delta-T value - which will only be achieved if you turn up the boiler and, possibly, the pump. Better still if you balance the system properly and make sure that everything's working to spec.

[Kostas Kavoussanakis]

Thank you Ed and Roger for the suggestion. I will try it and see what happens. Some instinct advised against throttling things to their limit. Would one expect a serious increase to the gas bill?

Merry Xmas!

Kostas

[Kostas Kavoussanakis]

I have now read thes results too, many thanks.

Well, Greeks get their pressies on the 1st of Jan (St Basil brings them ;-)).

Would this do?

do I have to shell out?

again and Merry Xmas!

Kostas

[Roger Mills]

> > Or do I have to shell out?

> > Thanks again and Merry Xmas!

The cheaper one *may* be ok - but it doesn't give the full spec - so it's hard to tell. The battery looks a bit exotic, and may bump the price up.

The dearer one doesn't say what batteries it needs, and it has a distance to target ratio of only 3:1 - meaning that you'll have to postion it very close to small or narrow objects - like pipes - to get a reliable reading.

I would tend to go for something like

which is a bit (though not a lot) dearer, but uses standard AAA batteries and has a healthier distance to target ratio of 9:1.

[Roger Mills]

If you make the heating work harder in order to increase your comfort level, it will inevitably mean that the boiler's burner is on for more of the time - so you'll use more gas. The only way of avoiding that is to reduce the heat losses by improving the insulation and draught-proofing.

[Kostas Kavoussanakis]

Thanks Roger, for the thermometer recommendation as well. I pushed the boiler stat to 4 and will monitor improvement before going to 5; looking reasonably good.

Kostas