Sizing radiators for Victorian terrace and future heat pump

[boid]

Hi all,

I'm looking at replacing most of the radiators in my mid-terrace 3 bed Victorian house (in London) and want to check if I'm getting the numbers right and making reasonable assumptions. I've been using this BTU calculator on bestheating.com to figure out the heat requirements for each room (as it seems to be one of the ones that uses more information).

We are considering getting a heat pump in the future, which I understand will have a lower flow temperature and therefore operate at a lower Delta. So, I have come up with 2 options for sets of radiators to buy - one set to work at Delta T50 and another for Delta T30 (which would future proof against the heat pump).

As an example, one room on the ground floor is 2.9m W x 3.2m L x 2.8m H. It has one external wall and a double glazed glass door to the garden. The required BTU is apparently 3404. For Delta T50 I can get a Type 22 500x800mm rad that is rated for 4351 BTU. For Delta T30, I would have to size it up to a Type 22 600x1000mm (the largest that would reasonably fit) which is rated for only 3224.

In general, for Delta T50, it's very easy to find rads that meet the heat requirements of each room and more, so I have chosen a set that meet the BTU requirement + 20%. On the other hand, it's quite hard to find rads for every room that will match that performance for Delta T30 mainly due to the size restrictions from the configuration of the walls, windows, current pipe positions etc. This means that the rads I have selected for Delta T30 are mostly rated for just under the heat requirement.

I wanted to check whether:
a) The way I'm going about this sounds more or less reasonable.
b) Whether, given their flow temperatures, a heat pump is likely to actually need to run at Delta T30 (rather than say Delta T being more like 40˚C)?
c) Is it ok to have rads that are rated a bit less than the BTU requirement for a room. If, so what is the leeway you can get away with?
d) Is there a problem if you have a big differential in the relative rating of each rad?

 

[Notch7]

Heat pump and Victorian semi in the same sentence is scary.

I know FA though so ignore me, but there are some fantastic heating engineers on here and Im sure you will great advice.

 

[polesapart]

Heat pumps work best in very well insulated properties where a lower flow temperature will achieve the comfort levels required.

As well as upsizing the radiators you will almost certainly need to upsize the pipework to get the mass-flow rates needed.

If the radiators are undersized the system will need to run at a higher flow temperature to achieve comfort.

As flow temperature increases the coefficient of performance of the heat pump decreases, reducing or negating the hoped-for cost and environmental savings.

In my opinion a better approach is to tackle insulation levels and, especially in a Victorian house, air permeability (aka draughtproofing). To get to modern new-build standards will not be cheap or convenient!

 

[Bodgedbuild]

You can get 65 degree single stage or 80 degree centigrade two stage heat pumps these days and both will create water hot enough to heat an old house with standard rads. Even the basic monoblock systems run at 50 degrees these days. I personally have an 80 degree system and I've turned it down (you can set the water output temperature not just the room temperature). You could literally make a cup of tea with the hot water when on it's factory setting. When the mother in law decided to go for a heat pump we told her not to bother with a "hot" system and even then her system creates water hot enough that you can't keep your hand on the rads. Her house is solid wall, no cavity and the system easily heats it.
If you stick with big names (Hitachi etc), they work much better than you might believe.

 

[boid]

To clarify, we are replacing the radiators in the house anyway. This is not so much about how to best heat the house, but whether we're taking the right approach to choosing those new rads, given the heat requirements of the house and possible future heat pump (although no concrete plans for that yet).

Heat pumps work best in very well insulated properties where a lower flow temperature will achieve the comfort levels required.

The house is relatively well insulated in the roof spaces and has new doors and windows. All of the fireplaces are blocked up. I've read of others running a heat pump in this kind of situation and it seems fine.

As flow temperature increases the coefficient of performance of the heat pump decreases, reducing or negating the hoped-for cost and environmental savings.

Good to know. Although the environmental benefit of a heat pump comes from the fact that it can be powered from the grid, which in theory could be shifted to cleaner sources of energy (although more efficient is always better!).

You can get 65 degree single stage or 80 degree centigrade two stage heat pumps these days

I thought these were a way off. Will have to look into that.