Gas boiler for 2 bedroom terraced house.

If you oversize them too much they hold a lot of water and take longer to heat and cool causing large temperature cycles.

The larger the rads are the lower the temperature they can be run at, making condensing boilers more efficient (assuming they are well designed).

What are the water heating requirements? How well insulated is the place?

Oversized rads can work well with condensors - particularly when they have weather compensation since that can boost efficiency of the boiler.

The short answer is I don't know the heat requirement but one would have to make assumptions about the heat loss into the adjacent properties anyway.

I was assuming that any combi would be easily powerful enough since they are generally 25kw or more and the house needs much less than that.

This can work very well. I well-oversized the radiators (partly due to a fault in the beta version of Myson's heatloss calculator I used), but I'm really pleased I did. I can keep the house warm running the rads at 45/40 (flow/return), which means the boiler is running more efficiently than it normally would. If you have young or elderly, this also means they can't burn themselves on the rads/pipes. Also, the energy stored in the rads is lower when running at low temperature, and I don't have to allow for any room temperature overshoot after demand for heat is cut, unlike a system running at more normal higher temperatures.

My heating demand is calculated as 11kW at -3C outside. The boiler can output 25kW max, so I can use this to generate a very fast heat up of the house from stone cold. In theory the boiler can run at up to 83C, but the radiator sizing means the radiators reach 25kW output before the boiler gets to 83C, so my system never runs that hot for full output.

I meant more in terms of showers / baths etc and tank or instant hot water.

If you are planning on a combi, then yup most will be 24kW or better (although many of the "or better" only give full output on hot water heating and quite often limit max rad power to 25kW or thereabouts).

A wide modulation range will let the boiler match the actual heat loss better than a narrow one (i.e. allowing longer lower powered burns and less cycling with more accurate temperature tracking).

(Although this is less of an issue with modern low water content boilers anyway).

Thanks for your reply.

There will be just one bathroom with a bath. It would be nice if the bath was not to slow to fill.

I did wonder whether the heat output to DHW and CH would be the same.

If you use a programmable thermostat which probably cycles the boiler several times an hour, how does the boiler know to modulate down? - and does this modulating down confuse the thermostat?

The boiler can basically sense the load by one of a number of approaches. It can either monitor the temperature of the return water and and modulate to keep this at desired difference from the flow, or it can just attempt to keep the flow at a set temperature.

As the return temp rises (i.e. the heating load is reducing; TRVs are reaching their set temp and shutting down, the heatloss from the primary water is reducing) the boiler needs to reduce the heat input to prevent the flow temperature exceeding the set point[1]. Hence it winds back the power. If it reaches minimum power, and that still does not limit the flow temp, then it will have to cycle off.

The room stat is only really interested in the end result. Once the room is up to temperature then it stops calling for heat[2]. So it won't get confused. The only difficulty is when you have a TRV on the rad warming the room with the stat. Then there is a possibility that the TRV limits the room temperature to lower than the set temp on the room stat. Hence it can't be "satisfied" at its current set temp, and keeps the heating running - even if the house is actually warm enough.

(Some of the posher setups may use integrated room stats and weather compensators that talk to each other to get round these issues).

Both the (other people's) combis that I've anything to do with have a pre-set pot adjustment on the PCB for setting the max o/p to the heating circuit. These are Baxi and Vokera models, so hardly qualify as "better" grades. The adjustment is part of the commissioning procedure

- you decide on the heating o/p required, then look-up the corresponding burner pressure in the manual, then twiddle the pot to set the desired gas pressure - simples. However in both cases the 'professional' installers had left the adjustments at max, making for a rather fierce heating start-up.

Sorry, perhaps I was not being clear there. The "or better" I was referring to was not the boiler quality, but the power output. I.e. 24kW or better (i.e. higher).

Mo there is a surprise ;-)

I think the last one I owned did 35kW to water, and upto 24kW to rads (but could modulate down to under half that)