heating control for small house

[ajrobb]

I'm going to install GCH in a small 2-bed 1978 Wimpey end terrace. There is no hall; just a porch, kitchen and lounge downstairs with staircase rising out of the lounge. Upstairs are 2 beds and a bath.

The whole house used to be heated well by a single Baxi Brazilia 8000ST gas wall heater running 24x7 in the lounge at the foot of the stairs. The built-in thermostat did a good job of controlling the temperature. Sadly, this passed away after 15 years and was replaced by an inadequate Baxi Brazilia 5000.

Using an on-line calculator, I sized up the radiators and the total load was 9kW. However, with the open-plan staircase, I suspect the lounge radiator would need to be bigger than suggested. With so much spare boiler capacity, I wonder if I should install oversize radiators throughout?

I'm thinking of locating a room thermostat/timer in the lounge and TRVs in the kitchen and bedrooms. This leaves the radiator in the lounge and the bathroom towel rail uncontrolled.

 

[D_Hailsham]

Using an on-line calculator, I sized up the radiators and the total load was 9kW.

Which calculator did you use? The majority of on-line versions are very crude and should not be trusted.

Use a "whole house" Boiler Size Calculator first.

However, with the open-plan staircase, I suspect the lounge radiator would need to be bigger than suggested. With so much spare boiler capacity, I wonder if I should install oversize radiators throughout?

Oversizing rads does three things: helps the house heat up quicker; maintains the required temperature when the outside temperature is lower than the standard -1C; allows the boiler to be more efficient.

Most calculators already include 10-15% for heating up, (including the one linked above), so you can ignore that.

You might want to allow for lower outside temperatures. If so it's proportional, e.g;

Assuming Calculator use a room temp of 21C and outside temp of -1C and you want to size for room temp of 22C and outside of -5C. Then the new boiler size is:

Calculated size x (22-(-5)) ÷ (21-(-1)).

As for boiler efficiency, if the return temperature is below 55C the boiler will condense, which increases its efficiency. The downside, however, is that radiator outputs will decrease, so they have to be oversized. The normal oversizing is about 20%.

I'm thinking of locating a room thermostat/timer in the lounge and TRVs in the kitchen and bedrooms. This leaves the radiator in the lounge and the bathroom towel rail uncontrolled.

That sounds OK. The lounge rad is not uncontrolled; it's controlled by the wall thermostat.

To summarize:

1. Use boiler calculator
2. Make allowance for your required inside and outside temperatures.

That gives the size boiler you need to buy

3. Allocate the heating between floors, say 55% downstairs, 45% upstairs.
4. Size radiators, say according to floor area.
5. Increase rad sizes by 20%

You should add the estimated landing and living room rad sizes together and use that for the living room rad.

 

[ajrobb]

Use a "whole house" Boiler Size Calculator first.

Including its default 2kW for hot water, it came up with a size of just over 5kW. This ties in with the old 3kW gas wall heater heating the whole house well. I've not seen boilers this small! This is much less than the radiator calculator load. Besides, the combination boiler must provide water for a decent shower.

I could double the size of the radiators and the boiler might still be short cycling. I could turn the boiler temperature right down to try to get it condensing.

 

[D_Hailsham]

Including its default 2kW for hot water, it came up with a size of just over 5kW. This ties in with the old 3kW gas wall heater heating the whole house well. I've not seen boilers this small! This is much less than the radiator calculator load. Besides, the combination boiler must provide water for a decent shower.

Shows how unreliable the online rad calculators are! Which one did you use?

I agree that there are no boilers which go much below 6kW, and some combis stop at 9 or 10kW. If you are thinking of installing a combi you need to check your water flow rate at the cold water tap in the kitchen. Use a bucket with litre marks inside and a watch. The boiler has to produce 2.5kW for each litre per minute flow rate, i.e 10 litres/min requires a 25kW combi boiler. A larger boiler will not improve the flow rate but it might make the water hotter. (2.5kW will raise the temperature by 35°C, which is normally sufficient.)

I could double the size of the radiators and the boiler might still be short cycling. I could turn the boiler temperature right down to try to get it condensing.

Doubling the rad size will not make the boiler give out twice as much heat - apart from when it is first turned on and the house is being heated up from cold. A radiator is not like an electric fire, which gives out a constant amount of heat.

Turning the boiler temperature down will help condensing, but it will not stop it short cycling if the required output is below the minimum modulation level.

Boiler manufacturers need to get their act together and produce boilers which can modulate down to 2 or 3 kW, particularly for new build houses which have a much higher level of insulation and may only need less than 10kW to heat it.

 

[ianniann]

A combi boiler will be minimum 20kW, often 30kW or more. That is necessary to provide a sensible flow of hot water. Since this is way more than most houses need, they usually have the ability to drop the power output to a half or less. Some of the more powerful combis never operate at full power for the central heating circuit, and some can also be throttled back further by the installer because 30kW or more is simply crazy for a small house. Some will modulate as low as 1kW, but usually more like 6-12 as described already.

Most modern boilers won't short cycle, they have built in limits so that you'll get at least a few minutes off before it fires again. Most of them will also modulate down from the maximum setting in those situations, so you might be getting cycles of 5 minutes off and 15 minutes on at a low burner setting.

I know my house only needs a steady 4kW even on the very coldest winter days to keep it warm, below what the boiler can modulate to. Of course in practice it runs at a higher power level and not for 24 hours a day, but still it can't modulate anywhere near low enough to burn continuously for a period of hours.

 

[D_Hailsham]

A combi boiler will be minimum 20kW, often 30kW or more. That is necessary to provide a sensible flow of hot water.

The hot water water flow from the boiler is the same as the cold water flow into the boiler. Some people have the mistaken impression that installing a larger boiler than necessary will give you a higher flow rate.

Some of the more powerful combis never operate at full power for the central heating circuit, and some can also be throttled back further by the installer because 30kW or more is simply crazy for a small house.

But many installers do not reduce the CH output to the required level. They think that, because the boiler modulates, there is no need to do this. The result, in extreme cases, is that the boiler never reaches working temperature as it is quickly overheating, cutting out, waiting, restarting and the radiators never reach temperature. There have been several instances of this problem on the Forum. They have usually been solved by reducing the CH output

Some will modulate as low as 1kW

Is that the Geminox? Looks interesting, but it would not fit in the space occupied by my existing boiler and it weighs a ton.

 

[ianniann]

That is necessary to provide a sensible flow of hot water.

How about if I rephrase this as "This is necessary to provide a flow of sensibly hot water"

 

[ajrobb]

Doubling the rad size will not make the boiler give out twice as much heat - apart from when it is first turned on and the house is being heated up from cold.

I disagree. With by far the largest radiator controlled only by the room thermostat, the boiler will have to give more heat (for less time) if the radiator is oversized. Your statement applies more to a steady-state TRV-controlled system.

I have seen a typical figure of 3 degrees/hour for heating up. However, that may mean waiting a couple of hours to reach set point. So making the boiler give more heat means less waiting or less waste (heating up an empty house). The main downside is that the radiators cost more.