CH slow to warm house

[ntl142]

My bungalow heats up slowly, around 0.5-1 degree per hour. I've added a lot of insulation and it's still slow, so now I'm looking at the CH system itself. Vokera combi boiler doing CH and hot taps only. Normal size copper pipe to a manifold and microbore from there to 10 radiators around the house

All 10 radiators are hot, but I've noticed that when there is a demand for heating the boiler cycles on/off quite frequently rather than staying on as I'd expect. As an examle: Room temperature is 15 deg and I've set the thermostat to 20 deg. The wireless thermostat receiver by the boiler goes orange (visual indicator of heating demand). This will stay orange until 20 deg is achieved some hours later, at which point it will go off as the thermostat is no longer calling for heat. However the boiler will cycle on/off while the room temp slowly increases. I would have thought the boiler would stay on permanently until 20 deg is achieved. Is this normal or is something wrong?

All I can think of is the radiators aren't dissipating enough heat and so the boiler is shutting down as its internal thermostat says its hot enough (on the CH return?). I've opened all lockshields and TRVs - no change. I'm just about to drain, flush, and replace inhibitor (needs doing anyway). Is there anything else I can look regarding the boilder/rads/balancing to help speed up warming the house? House insulation/draught proofing has already been done as much as I can, I just want to know if what I've got now is normal or unusual. Thanks.

 

[D_Hailsham]

Room temperature is 15 deg and I've set the thermostat to 20 deg. The wireless thermostat receiver by the boiler goes orange (visual indicator of heating demand). This will stay orange until 20 deg is achieved some hours later, at which point it will go off as the thermostat is no longer calling for heat. However the boiler will cycle on/off while the room temp slowly increases. I would have thought the boiler would stay on permanently until 20 deg is achieved. Is this normal or is something wrong?

Your description is a bit confusing. Does the boiler start cycling before the room reaches the required temperature? If so this could be because the boiler is cycling on and off under the control of the boiler's internal thermostat (which suggests that the boiler output is too high - not unusual for a combi boiler); or it could be the room thermostat doing this. Which thermostat do you have?

I've opened all lockshields and TRVs - no change.

All that does is unbalance the system, so some rads will get hotter than others.

 

[mcmoby69]

Weak pump, system restriction, filter blocked or bypass open somewhere.
Edit
I did go to a job with similar problems last year. Turned out to be the hot water flow turbine was faulty, replaced it and heating was fine after that.

Model of boiler could be helpful.

 

[ianmcd]

as @mcmoby69 asks which vokera and what external controls do you have

 

[ntl142]

Thanks for your comments.

Yes, the boiler starts cycling before the house is up to temperature. Imagine the desired temperature on the wireless thermostat is 20 degrees. The boiler will be cycling on/off long before it gets to 20. It is not the wireless thermostat causing this, as I can see the indicator on the receiver (at the boiler) is showing a constant call for heat. The boiler is causing the cycling itself (verified by the little green light on the front indicating when it is firing or not). The thermostat works fine. Worth pointing out that the boiler behaved the same way before the wireless thermostat was fitted over a year ago.

The label on the boiler says:
Max DHW input 32kW
Maximum heating output 25kW

There are 10 radiators in the house. One is a towel rail, one is a large cast iron radiator, and the other 8 are two panel radiators of varying sizes. The boiler sounds as if it's the right sort of size for that.

Siemens RDJ10RF
Vokera Unica 32 HE
http://cdn0.vokera.co.uk/wp-content/uploads/2009/04/unica-he-installation-and-servicing-manual.pdf


Thank you.

 

[stem]

Normally the boiler has a greater capacity than the radiators. So, if for example; you have a max of 30kW output from the radiators, and the boiler is sized at 40kW then the radiators won't be able to dissipate all of the heat produced by the boiler and so it will cycle on and off. This is perfectly normal. [This assumes that your boiler is not of the 'self modulating' variety in which case the flame will modulate up and down as required]

The boiler thermostat controls the temperature of the water leaving the boiler, so when the set water temperature is reached it will switch it off to stop the water actually boiling which would be dangerous.

All 10 radiators are hot.

Doesn't seem to be a problem with them then. If you have the boiler thermostat set high, and the radiators are too hot to keep your hands on (assuming any TRV's are open) then it would appear everything is working as it should be. If your house isn't warm enough and is well insulated including the walls and floors, then it maybe that the radiators are just too small for the size of the rooms in which they are located.

 

[JohnD]

How many cubic metres of gas do you use per day (1) in summer (2) in winter?

 

[ntl142]

Normally the boiler has a greater capacity than the radiators. So, if for example; you have a max of 30kW output from the radiators, and the boiler is sized at 40kW then the radiators won't be able to dissipate all of the heat produced by the boiler and so it will cycle on and off. This is perfectly normal. [This assumes that your boiler is not of the 'self modulating' variety in which case the flame will modulate up and down as required]

Thanks, that confirms my understanding. Is a 25kW boiler with 10 radiators about right?

Doesn't seem to be a problem with them then. If you have the boiler thermostat set high, and the radiators are too hot to keep your hands on (assuming any TRV's are open) then it would appear everything is working as it should be. If your house isn't warm enough and is well insulated including the walls and floors, then it maybe that the radiators are just too small for the size of the rooms in which they are located.

Yes, TRVs are open. I've done as much insulation as I can in 1900 bungalow, but it doesn't seem to have made all that much difference. Sounds like you might be right about the radiator sizing then...

How hot should a radiator be? I'll borrow a temperature probe from work to check them if I know what they should be. May as well re-balance them at the same time.

Also, where should I have the CH temperature set to on the boiler? It's currently at about 1/3 of the range between min and max, around 55 degrees I think. Am I right in thinking that setting this lower will reduce the cycling frequency of the boiler, as there will be less heat to dissipate through the radiators and so the boiler's own thermostat will be interfering less? If I turn it up the boiler will become less efficient (less condensing), and cycle more as it is generating excess heat. Does it work like that?

I've turned it up higher in the past to help the house heat up quicker, but it didn't seem to make much difference. I don't know if the boiler cycled more or less - didn't think to check back then.

Thanks.

 

[ntl142]

How many cubic metres of gas do you use per day (1) in summer (2) in winter?

From some notes I made a while ago, although I'm struggling to read my own writing...

Typical summer month is 10 cubic metres (taps only, no shower/bath)
Typical winter month is 120 - 200 cubic metres

So per day it is:
Typical summer day: 0.3 cubic metres
Typical winter day: 4 - 6.6 cubic metres

Why do you ask?

 

[JohnD]

It tells us how much heat is being put into the house.

You use less gas than me.

If the radiators are fully hot all over, but the rooms are cold, I agree they may be too small.

Give us dimensions of the rads and rooms please.

Check the tops, bottoms and middles of your rads for temp. I set the boiler to 70 in winter.

Either bigger or hotter rads, or less heat loss, is needed.

 

[stem]

Thanks, that confirms my understanding. Is a 25kW boiler with 10 radiators about right?

Depends how big the radiators are and if they are doubles etc., Their output can range from less than 1kW to several kW's. Work out the output of each one and add them up. You can usually find the output of similar sized rads online.

How hot should a radiator be?

If the system is balanced properly, I would expect to see somewhere between 5 to 10 degrees less than the temperature set on the boiler thermostat. (boiler thermostats are not always that accurate so measure the temperature of the flow pipe from the boiler for the comparison.)

May as well re-balance them at the same time

Instructions for that here.

Also, where should I have the CH temperature set to on the boiler?

At the setting that works for your home.

If I turn it up the boiler will become less efficient (less condensing), and cycle more as it is generating excess heat. Does it work like that?

That's the theory, it depends upon the temperature of the return, not the flow, but of course they are related. The cooler the return water the more the flue gasses condense, but in practice I'm not convinced it makes that much difference. If you want to leave it at a lower setting, then have the heating to come on earlier in the day so that it has longer to warm up, or fit larger radiators so that more heat is lost into the room and the return water to the boiler is cooler.

 

[ntl142]

I can't get exact dimensions just now, but to give you a rough idea:

Living rm/dining rm (open plan) is 6x5m. Two relatively small rads, about 1x1m.
Hall 1.5 x 3m. Two rads about 1x1m
Kitchen 4x4m. No rads
Utility rm 4x2m. One very large rad about 1x1.8m.
Bathroom 2x2m tall towel rail.
Bathroom 2x3m large cast iron radiator 1x1m
Bedroom 1 4x4m. 1.5x1m
Bedroom 2 4x4m. 1.5x1m

According to the floor plan from the for sale advert, it's 90 sq m, which ties in with my guess above.

That's 9, not 10. I can't count properly today!

All are two panels, except the bathrooms.

One radiator (utility rm) is cool at the bottom. It's the lowest point in the system. I'm taking this off to flush in the next few weeks and draining the system and replacing inhibitor. No other cold radiators. When I take a water sample from the bleed screws of any radiator it's always clean/clear.

 

[D_Hailsham]

Is a 25kW boiler with 10 radiators about right?

All depends on how big the rads are!
If you assume an average radiator is 1.5kW, then ten rads is 15kW. So a 25kW boiler is way over-sized.
The outputs of typical 600mm high, 1000mm long flat panel rads are as follows:

Single panel, no fins: 610W
Single panel, with fins: 1000W
Double panel, single fins: 1400W
Double panel, double fins: 1800W

Output is approximately proportional to size.

 

[JohnD]

to warm the house quicker from cold, I changed several of my rads from (the builders' original) singles to doubles.

Builders fit the smallest possible rads, to increase profits. The calculators will tell you how much heat is needed to maintain a room at temperature. It takes more to heat it from cold.

TRVs prevent the room being overheated and wasting energy.

 

[ntl142]

Thanks all, really appreciate the guidance. It sounds like the system is ok, as in no fault in particular except an oversized boiler. I'll flush the big rad anyway, and replace the inhibitor as I have no idea if the last owners did that or not. Hopefully no surpsrises when I drain the system...

I think all my radiators (except bathrooms) are double panel with double fins, so around 1500-1800W (thanks D_Hailsham). Now I've got something to work from.

In a way this is good news - I know I've got extra boiler capacity I can use, the lounge is probably the slowest room to heat up, and I've always fancied a big radiator like this...
https://www.diy.com/departments/xim...r-anthracite-h-1800-mm-w-600-mm/256085_BQ.prd

(Granted that's not all that big in terms of output...)